Elizabeth Carroll | University of California, Berkeley (original) (raw)

Papers by Elizabeth Carroll

Research paper thumbnail of A family of photoswitchable NMDA receptors

eLife, 2016

NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consi... more NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consist of several subtypes with distinct subunit compositions and functional properties. To enable spatiotemporally defined, rapid and reproducible manipulation of function of specific subtypes, we engineered a set of photoswitchable GluN subunits ('LiGluNs'). Photo-agonism of GluN2A or GluN2B elicits an excitatory drive to hippocampal neurons that can be shaped in time to mimic synaptic activation. Photo-agonism of GluN2A at single dendritic spines evokes spine-specific calcium elevation and expansion, the morphological correlate of LTP. Photo-antagonism of GluN2A alone, or in combination with photo-antagonism of GluN1a, reversibly blocks excitatory synaptic currents, prevents the induction of long-term potentiation and prevents spine expansion. In addition, photo-antagonism in vivo disrupts synaptic pruning of developing retino-tectal projections in larval zebrafish. By providing p...

Research paper thumbnail of An atomic force microscopy and molecular simulations study of the inhibition of barite growth by phosphonates

Surface Science, 2004

The effect of five phosphonic acids (hydroxyethylene diphosphonic acid, HEDP; nitro trimethyl pho... more The effect of five phosphonic acids (hydroxyethylene diphosphonic acid, HEDP; nitro trimethyl phosphonic acid, NTMP; methylene diphosphonic acid, MDP; amino methylene phosphonic acid, AMP; and sodium phosphonobutane tricarboxylic acid, PBTC) on the growth of the barite(OO 1) face has been investigated using atomic force microscopy (AFM). Experimental data have been obtained by in situ measurements of the velocities of barite monomolecular steps growing from solutions with different concentrations of each phosphonic acid. Adsorption isotherms, constructed by plotting individual monomolecular step rates versus inhibitor concentrations, indicate a Langrnuir adsorption mech anism in the range of concentrations from 0.5 to 10 Illliolll. Both affinity constants calculated from adsorption iso therms and measurements of growth rates of barite monomolecular steps as a function of inhibitor concentration allowed us to give the following ranking of inhibitor effectiveness: PBTC > NTMP > MDP > HEDP» AMP. Molecular simulations of the interaction of the phosphonic acids with barite(O 0 1) surfaces indicate that only kink sites along monomolecular steps can be considered as possible inhibition sites. This is in agreement with the AFM observations and measurements.

Research paper thumbnail of A single source femtosecond-millisecond broadband spectrometer

Review of Scientific Instruments, 2009

Time-resolved measurement of population dynamics extending over femtosecond to millisecond time s... more Time-resolved measurement of population dynamics extending over femtosecond to millisecond time scales typically requires a combination of transient absorption techniques involving different laser systems and detection schemes. The spectrometer design presented here facilitates transient absorption measurements over 12 decades with a single ultrafast laser system by picking pump and probe pulses independently from the laser oscillator pulse train. Unamplified pulses seed a photonic crystal fiber to a supercontinuum probe source for spectrally resolved measurements. The utility of the system is demonstrated by measuring triplet state dynamics following photoexcitation of vitamin B(6) in aqueous solution.

Research paper thumbnail of Photo-isomerization upshifts the pKa of the Photoactive Yellow Protein chromophore to contribute to photocycle propagation

Journal of Photochemistry and Photobiology A: Chemistry, 2013

ABSTRACT The influence of chromophore structure on the protonation constant of the Photoactive Ye... more ABSTRACT The influence of chromophore structure on the protonation constant of the Photoactive Yellow Protein chromophore is explored with isolated para-coumaric acid (pCA) and thiomethyl-para-coumaric acid model chromophores in solution. pH titration coupled with visible absorption spectra of the trans and photogenerated cis conformer of isolated pCA demonstrates that the isomerization of the chromophore increases the pKa of the phenolate group by 0.6 units (to 10.1 ± 0.22). Formation of the pCA thioester reduces the pKa of the phenolic group by 0.3 units (from 9.5 ± 0.15 to 9.2 ± 0.16). Unfortunately, a macroscopic cis-TMpCA population was not achieved via photoexcitation. Both trends were explained with electronic structure calculations including a Natural Bond Orbital analysis that resolves that the pKa upshift for the cis configuration is attributed to increased Columbic repulsion between the coumaryl tail and the phenolate moieties. This structurally induced pKa upshift after isomerization is argued to aid in the protonation of the chromophore within the PYP protein environment and the subsequent propagation of the photocycle response and in vivo photo-activity.

Research paper thumbnail of Extracting Information from Adaptive Control Experiments

Israel Journal of Chemistry, 2012

periments can reveal the pulse features responsible for control and also the underlying molecular... more periments can reveal the pulse features responsible for control and also the underlying molecular dynamics. In this article we review attempts to extract information from optical control experiments using adaptive learning algorithms to search the available parameter space, and we discuss how these kinds of experiments can be used to achieve and understand multiphoton optical control.

Research paper thumbnail of Phase control of the competition between electronic transitions in a solvated laser dye

Chemical Physics, 2008

Excited state population can be manipulated by resonant chirped laser pulses through pump–dump pr... more Excited state population can be manipulated by resonant chirped laser pulses through pump–dump processes. We investigate these processes in the laser dye LD690 as a function of wavelength by monitoring the saturated absorption of chirped ultrafast pulses. The resulting nonlinear absorption spectrum becomes increasingly complex as the pulse is tuned to shorter wavelengths. However, fluorescence measurements indicate that the excited

Research paper thumbnail of First demonstration of CdSe as a photocatalyst for hydrogen evolution from water under UV and visible light

Chemical Communications, 2008

CdSe nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na 2 S... more CdSe nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na 2 S/Na 2 SO 3 solution under irradiation with ultraviolet and visible light.

Research paper thumbnail of Influence of linear chirp on non-vertical transitions in a dye solution

Chirped laser pulses can manipulate vibrational coherences in dye solutions. We show that the eff... more Chirped laser pulses can manipulate vibrational coherences in dye solutions. We show that the effect of linear chirp also depends strongly on the power spectrum of the ultrafast pulse. We use a programmable phase mask to control the spectral phase of 25-fs visible pulses generated in a noncollinear optical parametric amplifier (NOPA). Following chirped pulse excitation of the oxazine laser dye LD690, coherent oscillations are observed in the time-resolved transient absorption. When a vertical transition is excited, negative chirp leads to strong ground state vibrational coherences as resonant stimulated Raman processes are enhanced by the frequency sweep of the laser pulse. When the chirped pulse excitation is higher frequency than the Franck-Condon transition, the optical response of the dye is significantly affected by excited state absorption. In this case, coherences are established in excited states but no chirp-enhancement of the ground state wavepacket is observed.

Research paper thumbnail of Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

The Journal of Physical Chemistry A, 2009

Time-resolved transient absorption spectroscopy was used to investigate the primary geminate reco... more Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (<100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.

Research paper thumbnail of Influence of Environment on the Electronic Structure of Cob(III)alamins:  Time-Resolved Absorption Studies of the S 1 State Spectrum and Dynamics

Journal of the American Chemical Society, 2007

Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate st... more Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate state populated following excitation of cob(III)alamin (Cbl(III)) compounds. This state is sensitive both to axial ligation and to solvent polarity. The excited-state lifetime as a function of temperature and solvent environment is used to separate the dynamic and electrostatic influence of the solvent. Two distinct types of excited states are identified, both assigned to pi3d configurations. The spectra of both types of excited states are characterized by a red absorption band (ca. 600 nm) assigned to Co 3d --> 3d or Co 3d --> corrin pi* transitions and by visible absorption bands similar to the corrin pi-->pi* transitions observed for ground state Cbl(III) compounds. The excited state observed following excitation of nonalkyl Cbl(III) compounds has an excited-state spectrum characteristic of Cbl(III) molecules with a weakened bond to the axial ligand (Type I). A similar excited-state spectrum is observed for adenosylcobalamin (AdoCbl) in water and ethylene glycol. The excited-state spectrum of methyl, ethyl, and n-propylcobalamin is characteristic of a Cbl(III) species with a sigma-donating alkyl anion ligand (Type II). This Type II excited-state spectrum is also observed for AdoCbl bound to glutamate mutase. The results are discussed in the context of theoretical calculations of Cbl(III) species reported in the literature and highlight the need for additional calculations exploring the influence of the alkyl ligand on the electronic structure of cobalamins.

Research paper thumbnail of Multiphoton control of the 1,3-cyclohexadiene ring-opening reaction in the presence of competing solvent reactions

The journal of physical chemistry. A, Jan 31, 2008

Although physical chemistry has often concentrated on the observation and understanding of chemic... more Although physical chemistry has often concentrated on the observation and understanding of chemical systems, the defining characteristic of chemistry remains the direction and control of chemical reactivity. Optical control of molecular dynamics, and thus of chemical reactivity provides a path to use photon energy as a smart reagent in a chemical system. In this paper, we discuss recent research in this field in the context of our studies of the multiphoton optical control of the photo-initiated ring-opening reaction of 1,3-cyclohexadiene (CHD) to form 1,3,5- cis-hexatriene (Z-HT). Closed-loop feedback and learning algorithms are able to identify pulses that increase the desired target state by as much as a factor of two. Mechanisms for control are discussed through the influence of the intensity dependence, the nonlinear power spectrum, and the projection of the pulses onto low orders of polynomial phase. Control measurements in neat solvents demonstrate that competing solvent frag...

Research paper thumbnail of Photoactivatable genetically encoded calcium indicators for targeted neuronal imaging

Nature methods, Jan 13, 2015

Circuit mapping requires knowledge of both structural and functional connectivity between cells. ... more Circuit mapping requires knowledge of both structural and functional connectivity between cells. Although optical tools have been made to assess either the morphology and projections of neurons or their activity and functional connections, few probes integrate this information. We have generated a family of photoactivatable genetically encoded Ca(2+) indicators that combines attributes of high-contrast photolabeling with high-sensitivity Ca(2+) detection in a single-color protein sensor. We demonstrated in cultured neurons and in fruit fly and zebrafish larvae how single cells could be selected out of dense populations for visualization of morphology and high signal-to-noise measurements of activity, synaptic transmission and connectivity. Our design strategy is transferrable to other sensors based on circularly permutated GFP (cpGFP).

Research paper thumbnail of Two-photon brightness of azobenzene photoswitches designed for glutamate receptor optogenetics

Proceedings of the National Academy of Sciences, 2015

Mammalian neurotransmitter-gated receptors can be conjugated to photoswitchable tethered ligands ... more Mammalian neurotransmitter-gated receptors can be conjugated to photoswitchable tethered ligands (PTLs) to enable photoactivation, or photoantagonism, while preserving normal function at neuronal synapses. "MAG" PTLs for ionotropic and metabotropic glutamate receptors (GluRs) are based on an azobenzene photoswitch that is optimally switched into the liganding state by blue or near-UV light, wavelengths that penetrate poorly into the brain. To facilitate deep-tissue photoactivation with near-infrared light, we measured the efficacy of two-photon (2P) excitation for two MAG molecules using nonlinear spectroscopy. Based on quantitative characterization, we find a recently designed second generation PTL, l-MAG0460, to have a favorable 2P absorbance peak at 850 nm, enabling efficient 2P activation of the GluK2 kainate receptor, LiGluR. We also achieve 2P photoactivation of a metabotropic receptor, LimGluR3, with a new mGluR-specific PTL, d-MAG0460. 2P photoswitching is efficiently achieved using digital holography to shape illumination over single somata of cultured neurons. Simultaneous Ca(2+)-imaging reports on 2P photoswitching in multiple cells with high temporal resolution. The combination of electrophysiology or Ca(2+) imaging with 2P activation by optical wavefront shaping should make second generation PTL-controlled receptors suitable for studies of intact neural circuits.

Research paper thumbnail of Calcium Niobate Semiconductor Nanosheets as Catalysts for Photochemical Hydrogen Evolution from Water

Journal of Physical Chemistry C, 2007

We have studied the photochemical water splitting activity of individually dispersed, semiconduct... more We have studied the photochemical water splitting activity of individually dispersed, semiconductor nanosheets derived from the Dion-Jacobsen phase HCa 2 Nb 3 O 10 by exfoliation with tetrabutylammonium hydroxide. The dimensions of the nanosheets are 0.001 × 1.2 × 1.2 µm 3 based on TEM measurements and crystallographic data, and their band gap is determined as 3.53 eV based on the absorption band edge at 350 nm. When an aqueous suspension of the nanosheets is irradiated with light from a Hg lamp, hydrogen is produced at rates of up to 2.28 µmol/h with a quantum efficiency (QE) of 0.22%. No oxygen is evolved. Derivatization of the nanosheets with Pt nanoparticles increases the hydrogen production rate to 78.37 µmol (QE ) 7.5%). Restacking of the nanosheets at low pH does not significantly affect the catalytic activity. Transient absorption measurements of the nanosheets reveal charge separation on a nanosecond time scale.

Research paper thumbnail of Ultrafast Carrier Dynamics in Exfoliated and Functionalized Calcium Niobate Nanosheets in Water and Methanol

Journal of Physical Chemistry C, 2008

We have characterized the initial charge carrier trapping and recombination processes in suspensi... more We have characterized the initial charge carrier trapping and recombination processes in suspensions of exfoliated calcium niobate nanosheets derived from the Dion Jacobsen phase KCa 2 Nb 3 O 10 . Niobate nanosheets have previously been found to catalyze hydrogen generation in photochemical water-splitting reactions. Photogenerated charge carriers relaxed with second-order kinetics on a sub-nanosecond time scale that depended on the nanosheet size. Methanol was used as a sacrificial electron donor to separate the electron and hole dynamics, and the photo-oxidation dynamics was found to directly compete with electron-hole recombination. Absorption spectra for trapped carriers were determined and used to separately analyze the trapped electron and hole dynamics in [H 1-x Ca 2 Nb 3 O 10 ] x-(x ) 0.15-0.20) sheets in water. Surface modification by 3-aminopropyl(trimethoxy)silane was found to quench trapped carrier absorptions, resulting in an observed optical

Research paper thumbnail of Rapid Photodynamics of Vitamin B 6 Coenzyme Pyridoxal 5‘-Phosphate and Its Schiff Bases in Solution

The Journal of Physical Chemistry B, 2008

The active form of vitamin B 6 , pyridoxal 5′-phosphate (PLP), is an important cofactor for numer... more The active form of vitamin B 6 , pyridoxal 5′-phosphate (PLP), is an important cofactor for numerous enzymes in amine and amino acid metabolism. Presented here is the first femtosecond transient absorption study of free PLP and two Schiff bases, PLP-valine and PLP-R-aminoisobutyric acid (AIB), in solution. Photoexcitation of free PLP leads to efficient triplet formation with an internal conversion rate that increases with increasing pH. The measured excited-state kinetics of the PLP-valine Schiff base exhibits a dramatic deuterium dependence as a result of excited-state proton transfer (ESPT) of the CR hydrogen in the amino acid substrate. This is consistent with formation of the key reaction carbanionic intermediate (quinonoid), which is resonance stabilized by the electron-deficient, conjugated π system of the Schiff base/pyridine ring. The transient absorption signals of the PLP-Schiff base with R-methylalanine (2-aminoisobutyric acid), which does not have a CR proton, lack an observable deuterium effect, verifying ESPT formation of the quinonoid intermediate. In contrast to previous studies, no dependence on the excitation wavelength of the femtosecond kinetics is observed with PLP or PLP-valine, which suggests that a rapid (<250 fs) tautomerization occurs between the enolimine (absorbing at 330 nm) and ketoenamine (absorbing at 410 nm) tautomers in solution.

Research paper thumbnail of Is the photoactive yellow protein a UV-B/blue light photoreceptor?

Photochemical & Photobiological Sciences, 2011

Research paper thumbnail of Synthesis and Characterization of Manganese-Doped Silicon Nanoparticles:  Bifunctional Paramagnetic-Optical Nanomaterial

Journal of the American Chemical Society, 2007

Mn doped Si nanoparticles have been synthesized via a low-temperature solution route and characte... more Mn doped Si nanoparticles have been synthesized via a low-temperature solution route and characterized by X-ray powder diffraction, transmission electron microscopy, optical and emission spectroscopy, and by electron paramagnetic resonance (EPR). The particle diameter was 4 nm, and the surface was capped by octyl groups. Mn doping at 5% resulted in a green emission with a slightly lower quantum yield than undoped Si nanoparticles prepared by the same method. Mn2+ doped into the nanoparticle is confirmed by EPR ...

Research paper thumbnail of K4Nb6O17-derived photocatalysts for hydrogen evolution from water: Nanoscrolls versus nanosheets

Journal of Solid State Chemistry, 2008

The layered hexaniobate K 4 Nb 6 O 17 is known as a photocatalyst for methanol dehydrogenation an... more The layered hexaniobate K 4 Nb 6 O 17 is known as a photocatalyst for methanol dehydrogenation and hydrogen evolution from water under ultraviolet (UV) light. Here we show that the activity is retained in propylammonium-(PA) or tetrabutylammonium-(TBA) stabilized H 2 K 2 Nb 6 O 17 nanosheets and TBAstabilized H 4 Nb 6 O 17 nanoscrolls that can be obtained by exfoliation of K 4 Nb 6 O 17 followed by cation exchange. The catalytic activity of the exfoliated systems is comparable to K 4 Nb 6 O 17 , with scrolls being most active in water, and PA sheets giving enhanced H 2 rates due to sacrificial electron donor action of PA. Femtosecond absorption spectra for TBA scrolls and PA sheets exhibit broad features between 450 and 700 nm due to trapped holes and electrons. Electron-hole recombination follows approximately second-order kinetics, with rates of decay similar for sheets and scrolls. In addition, catalysts were characterized with UV/vis and fluorescence spectroscopy and transmission electron microscopy.

Research paper thumbnail of Ultrafast Carrier Dynamics in Water-Splitting Photocatalysts

Bulletin of the American Physical Society, Mar 12, 2008

Semiconductors that photocatalytically split water into hydrogen and oxygen using solar energy re... more Semiconductors that photocatalytically split water into hydrogen and oxygen using solar energy represent promising renewable energy technologies. Charge carrier dynamics on sub-nanosecond timescales dictate the overall efficiency of charge separation, and consequently, ...

Research paper thumbnail of A family of photoswitchable NMDA receptors

eLife, 2016

NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consi... more NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consist of several subtypes with distinct subunit compositions and functional properties. To enable spatiotemporally defined, rapid and reproducible manipulation of function of specific subtypes, we engineered a set of photoswitchable GluN subunits ('LiGluNs'). Photo-agonism of GluN2A or GluN2B elicits an excitatory drive to hippocampal neurons that can be shaped in time to mimic synaptic activation. Photo-agonism of GluN2A at single dendritic spines evokes spine-specific calcium elevation and expansion, the morphological correlate of LTP. Photo-antagonism of GluN2A alone, or in combination with photo-antagonism of GluN1a, reversibly blocks excitatory synaptic currents, prevents the induction of long-term potentiation and prevents spine expansion. In addition, photo-antagonism in vivo disrupts synaptic pruning of developing retino-tectal projections in larval zebrafish. By providing p...

Research paper thumbnail of An atomic force microscopy and molecular simulations study of the inhibition of barite growth by phosphonates

Surface Science, 2004

The effect of five phosphonic acids (hydroxyethylene diphosphonic acid, HEDP; nitro trimethyl pho... more The effect of five phosphonic acids (hydroxyethylene diphosphonic acid, HEDP; nitro trimethyl phosphonic acid, NTMP; methylene diphosphonic acid, MDP; amino methylene phosphonic acid, AMP; and sodium phosphonobutane tricarboxylic acid, PBTC) on the growth of the barite(OO 1) face has been investigated using atomic force microscopy (AFM). Experimental data have been obtained by in situ measurements of the velocities of barite monomolecular steps growing from solutions with different concentrations of each phosphonic acid. Adsorption isotherms, constructed by plotting individual monomolecular step rates versus inhibitor concentrations, indicate a Langrnuir adsorption mech anism in the range of concentrations from 0.5 to 10 Illliolll. Both affinity constants calculated from adsorption iso therms and measurements of growth rates of barite monomolecular steps as a function of inhibitor concentration allowed us to give the following ranking of inhibitor effectiveness: PBTC > NTMP > MDP > HEDP» AMP. Molecular simulations of the interaction of the phosphonic acids with barite(O 0 1) surfaces indicate that only kink sites along monomolecular steps can be considered as possible inhibition sites. This is in agreement with the AFM observations and measurements.

Research paper thumbnail of A single source femtosecond-millisecond broadband spectrometer

Review of Scientific Instruments, 2009

Time-resolved measurement of population dynamics extending over femtosecond to millisecond time s... more Time-resolved measurement of population dynamics extending over femtosecond to millisecond time scales typically requires a combination of transient absorption techniques involving different laser systems and detection schemes. The spectrometer design presented here facilitates transient absorption measurements over 12 decades with a single ultrafast laser system by picking pump and probe pulses independently from the laser oscillator pulse train. Unamplified pulses seed a photonic crystal fiber to a supercontinuum probe source for spectrally resolved measurements. The utility of the system is demonstrated by measuring triplet state dynamics following photoexcitation of vitamin B(6) in aqueous solution.

Research paper thumbnail of Photo-isomerization upshifts the pKa of the Photoactive Yellow Protein chromophore to contribute to photocycle propagation

Journal of Photochemistry and Photobiology A: Chemistry, 2013

ABSTRACT The influence of chromophore structure on the protonation constant of the Photoactive Ye... more ABSTRACT The influence of chromophore structure on the protonation constant of the Photoactive Yellow Protein chromophore is explored with isolated para-coumaric acid (pCA) and thiomethyl-para-coumaric acid model chromophores in solution. pH titration coupled with visible absorption spectra of the trans and photogenerated cis conformer of isolated pCA demonstrates that the isomerization of the chromophore increases the pKa of the phenolate group by 0.6 units (to 10.1 ± 0.22). Formation of the pCA thioester reduces the pKa of the phenolic group by 0.3 units (from 9.5 ± 0.15 to 9.2 ± 0.16). Unfortunately, a macroscopic cis-TMpCA population was not achieved via photoexcitation. Both trends were explained with electronic structure calculations including a Natural Bond Orbital analysis that resolves that the pKa upshift for the cis configuration is attributed to increased Columbic repulsion between the coumaryl tail and the phenolate moieties. This structurally induced pKa upshift after isomerization is argued to aid in the protonation of the chromophore within the PYP protein environment and the subsequent propagation of the photocycle response and in vivo photo-activity.

Research paper thumbnail of Extracting Information from Adaptive Control Experiments

Israel Journal of Chemistry, 2012

periments can reveal the pulse features responsible for control and also the underlying molecular... more periments can reveal the pulse features responsible for control and also the underlying molecular dynamics. In this article we review attempts to extract information from optical control experiments using adaptive learning algorithms to search the available parameter space, and we discuss how these kinds of experiments can be used to achieve and understand multiphoton optical control.

Research paper thumbnail of Phase control of the competition between electronic transitions in a solvated laser dye

Chemical Physics, 2008

Excited state population can be manipulated by resonant chirped laser pulses through pump–dump pr... more Excited state population can be manipulated by resonant chirped laser pulses through pump–dump processes. We investigate these processes in the laser dye LD690 as a function of wavelength by monitoring the saturated absorption of chirped ultrafast pulses. The resulting nonlinear absorption spectrum becomes increasingly complex as the pulse is tuned to shorter wavelengths. However, fluorescence measurements indicate that the excited

Research paper thumbnail of First demonstration of CdSe as a photocatalyst for hydrogen evolution from water under UV and visible light

Chemical Communications, 2008

CdSe nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na 2 S... more CdSe nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na 2 S/Na 2 SO 3 solution under irradiation with ultraviolet and visible light.

Research paper thumbnail of Influence of linear chirp on non-vertical transitions in a dye solution

Chirped laser pulses can manipulate vibrational coherences in dye solutions. We show that the eff... more Chirped laser pulses can manipulate vibrational coherences in dye solutions. We show that the effect of linear chirp also depends strongly on the power spectrum of the ultrafast pulse. We use a programmable phase mask to control the spectral phase of 25-fs visible pulses generated in a noncollinear optical parametric amplifier (NOPA). Following chirped pulse excitation of the oxazine laser dye LD690, coherent oscillations are observed in the time-resolved transient absorption. When a vertical transition is excited, negative chirp leads to strong ground state vibrational coherences as resonant stimulated Raman processes are enhanced by the frequency sweep of the laser pulse. When the chirped pulse excitation is higher frequency than the Franck-Condon transition, the optical response of the dye is significantly affected by excited state absorption. In this case, coherences are established in excited states but no chirp-enhancement of the ground state wavepacket is observed.

Research paper thumbnail of Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

The Journal of Physical Chemistry A, 2009

Time-resolved transient absorption spectroscopy was used to investigate the primary geminate reco... more Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.

Research paper thumbnail of Influence of Environment on the Electronic Structure of Cob(III)alamins:  Time-Resolved Absorption Studies of the S 1 State Spectrum and Dynamics

Journal of the American Chemical Society, 2007

Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate st... more Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate state populated following excitation of cob(III)alamin (Cbl(III)) compounds. This state is sensitive both to axial ligation and to solvent polarity. The excited-state lifetime as a function of temperature and solvent environment is used to separate the dynamic and electrostatic influence of the solvent. Two distinct types of excited states are identified, both assigned to pi3d configurations. The spectra of both types of excited states are characterized by a red absorption band (ca. 600 nm) assigned to Co 3d --&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; 3d or Co 3d --&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; corrin pi* transitions and by visible absorption bands similar to the corrin pi--&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;pi* transitions observed for ground state Cbl(III) compounds. The excited state observed following excitation of nonalkyl Cbl(III) compounds has an excited-state spectrum characteristic of Cbl(III) molecules with a weakened bond to the axial ligand (Type I). A similar excited-state spectrum is observed for adenosylcobalamin (AdoCbl) in water and ethylene glycol. The excited-state spectrum of methyl, ethyl, and n-propylcobalamin is characteristic of a Cbl(III) species with a sigma-donating alkyl anion ligand (Type II). This Type II excited-state spectrum is also observed for AdoCbl bound to glutamate mutase. The results are discussed in the context of theoretical calculations of Cbl(III) species reported in the literature and highlight the need for additional calculations exploring the influence of the alkyl ligand on the electronic structure of cobalamins.

Research paper thumbnail of Multiphoton control of the 1,3-cyclohexadiene ring-opening reaction in the presence of competing solvent reactions

The journal of physical chemistry. A, Jan 31, 2008

Although physical chemistry has often concentrated on the observation and understanding of chemic... more Although physical chemistry has often concentrated on the observation and understanding of chemical systems, the defining characteristic of chemistry remains the direction and control of chemical reactivity. Optical control of molecular dynamics, and thus of chemical reactivity provides a path to use photon energy as a smart reagent in a chemical system. In this paper, we discuss recent research in this field in the context of our studies of the multiphoton optical control of the photo-initiated ring-opening reaction of 1,3-cyclohexadiene (CHD) to form 1,3,5- cis-hexatriene (Z-HT). Closed-loop feedback and learning algorithms are able to identify pulses that increase the desired target state by as much as a factor of two. Mechanisms for control are discussed through the influence of the intensity dependence, the nonlinear power spectrum, and the projection of the pulses onto low orders of polynomial phase. Control measurements in neat solvents demonstrate that competing solvent frag...

Research paper thumbnail of Photoactivatable genetically encoded calcium indicators for targeted neuronal imaging

Nature methods, Jan 13, 2015

Circuit mapping requires knowledge of both structural and functional connectivity between cells. ... more Circuit mapping requires knowledge of both structural and functional connectivity between cells. Although optical tools have been made to assess either the morphology and projections of neurons or their activity and functional connections, few probes integrate this information. We have generated a family of photoactivatable genetically encoded Ca(2+) indicators that combines attributes of high-contrast photolabeling with high-sensitivity Ca(2+) detection in a single-color protein sensor. We demonstrated in cultured neurons and in fruit fly and zebrafish larvae how single cells could be selected out of dense populations for visualization of morphology and high signal-to-noise measurements of activity, synaptic transmission and connectivity. Our design strategy is transferrable to other sensors based on circularly permutated GFP (cpGFP).

Research paper thumbnail of Two-photon brightness of azobenzene photoswitches designed for glutamate receptor optogenetics

Proceedings of the National Academy of Sciences, 2015

Mammalian neurotransmitter-gated receptors can be conjugated to photoswitchable tethered ligands ... more Mammalian neurotransmitter-gated receptors can be conjugated to photoswitchable tethered ligands (PTLs) to enable photoactivation, or photoantagonism, while preserving normal function at neuronal synapses. &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;MAG&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; PTLs for ionotropic and metabotropic glutamate receptors (GluRs) are based on an azobenzene photoswitch that is optimally switched into the liganding state by blue or near-UV light, wavelengths that penetrate poorly into the brain. To facilitate deep-tissue photoactivation with near-infrared light, we measured the efficacy of two-photon (2P) excitation for two MAG molecules using nonlinear spectroscopy. Based on quantitative characterization, we find a recently designed second generation PTL, l-MAG0460, to have a favorable 2P absorbance peak at 850 nm, enabling efficient 2P activation of the GluK2 kainate receptor, LiGluR. We also achieve 2P photoactivation of a metabotropic receptor, LimGluR3, with a new mGluR-specific PTL, d-MAG0460. 2P photoswitching is efficiently achieved using digital holography to shape illumination over single somata of cultured neurons. Simultaneous Ca(2+)-imaging reports on 2P photoswitching in multiple cells with high temporal resolution. The combination of electrophysiology or Ca(2+) imaging with 2P activation by optical wavefront shaping should make second generation PTL-controlled receptors suitable for studies of intact neural circuits.

Research paper thumbnail of Calcium Niobate Semiconductor Nanosheets as Catalysts for Photochemical Hydrogen Evolution from Water

Journal of Physical Chemistry C, 2007

We have studied the photochemical water splitting activity of individually dispersed, semiconduct... more We have studied the photochemical water splitting activity of individually dispersed, semiconductor nanosheets derived from the Dion-Jacobsen phase HCa 2 Nb 3 O 10 by exfoliation with tetrabutylammonium hydroxide. The dimensions of the nanosheets are 0.001 × 1.2 × 1.2 µm 3 based on TEM measurements and crystallographic data, and their band gap is determined as 3.53 eV based on the absorption band edge at 350 nm. When an aqueous suspension of the nanosheets is irradiated with light from a Hg lamp, hydrogen is produced at rates of up to 2.28 µmol/h with a quantum efficiency (QE) of 0.22%. No oxygen is evolved. Derivatization of the nanosheets with Pt nanoparticles increases the hydrogen production rate to 78.37 µmol (QE ) 7.5%). Restacking of the nanosheets at low pH does not significantly affect the catalytic activity. Transient absorption measurements of the nanosheets reveal charge separation on a nanosecond time scale.

Research paper thumbnail of Ultrafast Carrier Dynamics in Exfoliated and Functionalized Calcium Niobate Nanosheets in Water and Methanol

Journal of Physical Chemistry C, 2008

We have characterized the initial charge carrier trapping and recombination processes in suspensi... more We have characterized the initial charge carrier trapping and recombination processes in suspensions of exfoliated calcium niobate nanosheets derived from the Dion Jacobsen phase KCa 2 Nb 3 O 10 . Niobate nanosheets have previously been found to catalyze hydrogen generation in photochemical water-splitting reactions. Photogenerated charge carriers relaxed with second-order kinetics on a sub-nanosecond time scale that depended on the nanosheet size. Methanol was used as a sacrificial electron donor to separate the electron and hole dynamics, and the photo-oxidation dynamics was found to directly compete with electron-hole recombination. Absorption spectra for trapped carriers were determined and used to separately analyze the trapped electron and hole dynamics in [H 1-x Ca 2 Nb 3 O 10 ] x-(x ) 0.15-0.20) sheets in water. Surface modification by 3-aminopropyl(trimethoxy)silane was found to quench trapped carrier absorptions, resulting in an observed optical

Research paper thumbnail of Rapid Photodynamics of Vitamin B 6 Coenzyme Pyridoxal 5‘-Phosphate and Its Schiff Bases in Solution

The Journal of Physical Chemistry B, 2008

The active form of vitamin B 6 , pyridoxal 5′-phosphate (PLP), is an important cofactor for numer... more The active form of vitamin B 6 , pyridoxal 5′-phosphate (PLP), is an important cofactor for numerous enzymes in amine and amino acid metabolism. Presented here is the first femtosecond transient absorption study of free PLP and two Schiff bases, PLP-valine and PLP-R-aminoisobutyric acid (AIB), in solution. Photoexcitation of free PLP leads to efficient triplet formation with an internal conversion rate that increases with increasing pH. The measured excited-state kinetics of the PLP-valine Schiff base exhibits a dramatic deuterium dependence as a result of excited-state proton transfer (ESPT) of the CR hydrogen in the amino acid substrate. This is consistent with formation of the key reaction carbanionic intermediate (quinonoid), which is resonance stabilized by the electron-deficient, conjugated π system of the Schiff base/pyridine ring. The transient absorption signals of the PLP-Schiff base with R-methylalanine (2-aminoisobutyric acid), which does not have a CR proton, lack an observable deuterium effect, verifying ESPT formation of the quinonoid intermediate. In contrast to previous studies, no dependence on the excitation wavelength of the femtosecond kinetics is observed with PLP or PLP-valine, which suggests that a rapid (<250 fs) tautomerization occurs between the enolimine (absorbing at 330 nm) and ketoenamine (absorbing at 410 nm) tautomers in solution.

Research paper thumbnail of Is the photoactive yellow protein a UV-B/blue light photoreceptor?

Photochemical & Photobiological Sciences, 2011

Research paper thumbnail of Synthesis and Characterization of Manganese-Doped Silicon Nanoparticles:  Bifunctional Paramagnetic-Optical Nanomaterial

Journal of the American Chemical Society, 2007

Mn doped Si nanoparticles have been synthesized via a low-temperature solution route and characte... more Mn doped Si nanoparticles have been synthesized via a low-temperature solution route and characterized by X-ray powder diffraction, transmission electron microscopy, optical and emission spectroscopy, and by electron paramagnetic resonance (EPR). The particle diameter was 4 nm, and the surface was capped by octyl groups. Mn doping at 5% resulted in a green emission with a slightly lower quantum yield than undoped Si nanoparticles prepared by the same method. Mn2+ doped into the nanoparticle is confirmed by EPR ...

Research paper thumbnail of K4Nb6O17-derived photocatalysts for hydrogen evolution from water: Nanoscrolls versus nanosheets

Journal of Solid State Chemistry, 2008

The layered hexaniobate K 4 Nb 6 O 17 is known as a photocatalyst for methanol dehydrogenation an... more The layered hexaniobate K 4 Nb 6 O 17 is known as a photocatalyst for methanol dehydrogenation and hydrogen evolution from water under ultraviolet (UV) light. Here we show that the activity is retained in propylammonium-(PA) or tetrabutylammonium-(TBA) stabilized H 2 K 2 Nb 6 O 17 nanosheets and TBAstabilized H 4 Nb 6 O 17 nanoscrolls that can be obtained by exfoliation of K 4 Nb 6 O 17 followed by cation exchange. The catalytic activity of the exfoliated systems is comparable to K 4 Nb 6 O 17 , with scrolls being most active in water, and PA sheets giving enhanced H 2 rates due to sacrificial electron donor action of PA. Femtosecond absorption spectra for TBA scrolls and PA sheets exhibit broad features between 450 and 700 nm due to trapped holes and electrons. Electron-hole recombination follows approximately second-order kinetics, with rates of decay similar for sheets and scrolls. In addition, catalysts were characterized with UV/vis and fluorescence spectroscopy and transmission electron microscopy.

Research paper thumbnail of Ultrafast Carrier Dynamics in Water-Splitting Photocatalysts

Bulletin of the American Physical Society, Mar 12, 2008

Semiconductors that photocatalytically split water into hydrogen and oxygen using solar energy re... more Semiconductors that photocatalytically split water into hydrogen and oxygen using solar energy represent promising renewable energy technologies. Charge carrier dynamics on sub-nanosecond timescales dictate the overall efficiency of charge separation, and consequently, ...