Roman Shchepin - Academia.edu (original) (raw)

Papers by Roman Shchepin

Bioorganic & Medicinal Chemistry, 2008

A series of synthetic molecules combining a geranyl backbone with a heterocyclic or oxime head gr... more A series of synthetic molecules combining a geranyl backbone with a heterocyclic or oxime head group are quorum-sensing molecules that block the yeast to mycelium transition in the dimorphic fungus Candida albicans. A number of the analogs have an IC 50 ≤ 10 μM, a level of potency essentially identical to the natural quorum sensing signal, the sesquiterpene farnesol. Two of the most potent analogs, neither toxic toward healthy mice, display remarkably different effects when co-administered with C. albicans. While neither offers protection from candidiasis, one analog mimics farnesol in acting as a virulence factor, whereas the other has no effect. The results offer the first example of highly potent synthetic fungal quorum-sensing molecules, and provide the first evidence for the ability to decouple quorum sensing and virulence.

Journal of the American Chemical Society, 2012

We demonstrate that potassium 1-(13)C-phosphoenolpyruvate becomes hyperpolarized potassium 1-(13)... more We demonstrate that potassium 1-(13)C-phosphoenolpyruvate becomes hyperpolarized potassium 1-(13)C-phospholactate with (13)C T(1) = 36 s after molecular hydrogenation by PASADENA (Parahydrogen and Synthesis Allows Dramatically Enhanced Nuclear Alignment). This proof-of-principle study was conducted with a fully protonated molecular precursor. (13)C was polarized to a level of 1%, corresponding to nearly 4000-fold sensitivity enhancement at 3 T. The relevant homo- and heteronuclear spin-spin couplings are reported.

15N nuclear spins of imidazole-15N2 were hyperpolarized using NMR signal amplification by reversi... more 15N nuclear spins of imidazole-15N2 were hyperpolarized using NMR signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). A 15N NMR signal enhancement of ∼2000-fold at 9.4 T is reported using parahydrogen gas (∼50% para-) and ∼0.1 M imidazole-15N2 in methanol:aqueous buffer (∼1:1). Proton binding to a 15N site of imidazole occurs at physiological pH (pKa ∼ 7.0), and the binding event changes the 15N isotropic chemical shift by ∼30 ppm. These properties are ideal for in vivo pH sensing. Additionally, imidazoles have low toxicity and are readily incorporated into a wide range of biomolecules. 15N-Imidazole SABRE-SHEATH hyperpolarization potentially enables pH sensing on scales ranging from peptide and protein molecules to living organisms.

The journal of physical chemistry letters, Jan 21, 2015

We report NMR Signal Amplification by Reversible Exchange (SABRE) hyperpolarization of the rare i... more We report NMR Signal Amplification by Reversible Exchange (SABRE) hyperpolarization of the rare isotopes in "neat" liquids, each composed only of an otherwise pure target compound with isotopic natural abundance (n.a.) and millimolar concentrations of dissolved catalyst. Pyridine (Py) or Py derivatives are studied at 0.4% isotopic natural abundance (15)N, deuterated, (15)N enriched, and in various combinations using the SABRE-SHEATH variant (microTesla magnetic fields to permit direct (15)N polarization from parahydrogen via reversible binding and exchange with an Ir catalyst). We find that the dilute n.a. (15)N spin bath in Py still channels spin order from parahydrogen to dilute (15)N spins, without polarization losses due to the presence of (14)N or (2)H. We demonstrate P 15N ≈ 1% (a gain of 2900 fold relative to thermal polarization at 9.4 T) at high substrate concentrations. This fundamental finding has a significant practical benefit for screening potentially hyperpo...

The journal of physical chemistry. C, Nanomaterials and interfaces, Jan 23, 2015

NMR signal amplification by reversible exchange (SABRE) is a NMR hyperpolarization technique that... more NMR signal amplification by reversible exchange (SABRE) is a NMR hyperpolarization technique that enables nuclear spin polarization enhancement of molecules via concurrent chemical exchange of a target substrate and parahydrogen (the source of spin order) on an iridium catalyst. Recently, we demonstrated that conducting SABRE in microtesla fields provided by a magnetic shield enables up to 10% (15)N-polarization (Theis, T.; et al. J. Am. Chem. Soc. 2015, 137, 1404). Hyperpolarization on (15)N (and heteronuclei in general) may be advantageous because of the long-lived nature of the hyperpolarization on (15)N relative to the short-lived hyperpolarization of protons conventionally hyperpolarized by SABRE, in addition to wider chemical shift dispersion and absence of background signal. Here we show that these unprecedented polarization levels enable (15)N magnetic resonance imaging. We also present a theoretical model for the hyperpolarization transfer to heteronuclei, and detail key pa...

Chemistry & biology, 2003

The dimorphic fungus Candida albicans produces extracellular farnesol (3,7,11-trimethyl-2,6,10-do... more The dimorphic fungus Candida albicans produces extracellular farnesol (3,7,11-trimethyl-2,6,10-dodecatriene-1-ol) which acts as a quorum-sensing molecule (QSM) to suppress filamentation. Of four possible geometric isomers of farnesol, only the E,E isomer possesses QSM activity. We tested 40 natural and synthetic analogs of farnesol for their activity in an N-acetylglucosamine-induced differentiation assay for germ tube formation (GTF). Modified structural features include the head group, chain length, presence or absence of the three double bonds, substitution of a backbone carbon by S, O, N, and Se heteroatoms, presence or absence of a 3-methyl branch, and the bulkiness of the hydrophobic tail. Of the 40 compounds, 22 showed QSM activity by their ability to reduce GTF by 50%. However, even the most active of the analogs tested had only 7.3% of the activity of E,E-farnesol. Structure-activity relationships were examined in terms of the likely presence in C. albicans of a farnesol bi...

Analytical Chemistry, 2014

We demonstrate the feasibility of microscale molecular imaging using hyperpolarized proton and ca... more We demonstrate the feasibility of microscale molecular imaging using hyperpolarized proton and carbon-13 MRI contrast media and low-field (47.5 mT) preclinical scale (38 mm i.d.) 2D magnetic resonance imaging (MRI). Hyperpolarized proton images with 94 × 94 μm(2) spatial resolution and hyperpolarized carbon-13 images with 250 × 250 μm(2) in-plane spatial resolution were recorded in 4-8 s (largely limited by the electronics response), surpassing the in-plane spatial resolution (i.e., pixel size) achievable with micro-positron emission tomography (PET). These hyperpolarized proton and (13)C images were recorded using large imaging matrices of up to 256 × 256 pixels and relatively large fields of view of up to 6.4 × 6.4 cm(2). (13)C images were recorded using hyperpolarized 1-(13)C-succinate-d2 (30 mM in water, %P(13C) = 25.8 ± 5.1% (when produced) and %P(13C) = 14.2 ± 0.7% (when imaged), T1 = 74 ± 3 s), and proton images were recorded using (1)H hyperpolarized pyridine (100 mM in methanol-d4, %P(H) = 0.1 ± 0.02% (when imaged), T1 = 11 ± 0.1 s). Both contrast agents were hyperpolarized using parahydrogen (>90% para-fraction) in an automated 5.75 mT parahydrogen induced polarization (PHIP) hyperpolarizer. A magnetized path was demonstrated for successful transportation of a (13)C hyperpolarized contrast agent (1-(13)C-succinate-d2, sensitive to fast depolarization when at the Earth's magnetic field) from the PHIP polarizer to the 47.5 mT low-field MRI. While future polarizing and low-field MRI hardware and imaging sequence developments can further improve the low-field detection sensitivity, the current results demonstrate that microscale molecular imaging in vivo is already feasible at low (<50 mT) fields and potentially at low (~1 mM) metabolite concentrations.

Analytical Chemistry, 2014

Parahydrogen-induced polarization (PHIP) was used to demonstrate the concept that highly polarize... more Parahydrogen-induced polarization (PHIP) was used to demonstrate the concept that highly polarized, catalyst-free fluids can be obtained in a catalysis-free regime using a chemical reaction with molecular addition of parahydrogen to a water-soluble Rh(I) complex carrying a payload of compound with unsaturated (C═C) bonds. Hydrogenation of norbornadiene leads to formation of norbornene, which is eliminated from the Rh(I) complex and, therefore, leaves the aqueous phase and becomes a gaseous hyperpolarized molecule. The Rh(I) metal complex resides in the original liquid phase, while the product of hydrogen addition is found exclusively in the gaseous phase based on the affinity. Hyperpolarized norbornene (1)H NMR signals observed in situ were enhanced by a factor of approximately 10,000 at a static field of 47.5 mT. High-resolution (1)H NMR at a field of 9.4 T was used for ex situ detection of hyperpolarized norbornene in the gaseous phase, where a signal enhancement factor of approximately 160 was observed. This concept of stoichiometric as opposed to purely catalytic use of PHIP-available complexes with an unsaturated payload precursor molecule can be extended to other contrast agents for both homogeneous and heterogeneous PHIP. The Rh(I) complex was employed in aqueous medium suitable for production of hyperpolarized contrast agents for biomedical use. Detection of PHIP hyperpolarized gas by low-field NMR is demonstrated here for the first time.

The Journal of Physical Chemistry B, 2014

Activation of a catalyst [IrCl(COD)(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylide... more Activation of a catalyst [IrCl(COD)(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; COD = cyclooctadiene)] for signal amplification by reversible exchange (SABRE) was monitored by in situ hyperpolarized proton NMR at 9.4 T. During the catalyst-activation process, the COD moiety undergoes hydrogenation that leads to its complete removal from the Ir complex. A transient hydride intermediate of the catalyst is observed via its hyperpolarized signatures, which could not be detected using conventional nonhyperpolarized solution NMR. SABRE enhancement of the pyridine substrate can be fully rendered only after removal of the COD moiety; failure to properly activate the catalyst in the presence of sufficient substrate can lead to irreversible deactivation consistent with oligomerization of the catalyst molecules. Following catalyst activation, results from selective RF-saturation studies support the hypothesis that substrate polarization at high field arises from nuclear cross-relaxation with hyperpolarized (1)H spins of the hydride/orthohydrogen spin bath. Importantly, the chemical changes that accompanied the catalyst's full activation were also found to endow the catalyst with water solubility, here used to demonstrate SABRE hyperpolarization of nicotinamide in water without the need for any organic cosolvent--paving the way to various biomedical applications of SABRE hyperpolarization methods.

Journal of the American Chemical Society, 2015

Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrog... more Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrogen-15 in pyridine and nicotinamide (vitamin B(3) amide) by conducting "signal amplification by reversible exchange" (SABRE) at microtesla fields within a magnetic shield. Following transfer of the sample from the magnetic shield chamber to a conventional NMR spectrometer, the (15)N NMR signals for these molecules are enhanced by ∼30,000- and ∼20,000-fold at 9.4 T, corresponding to ∼10% and ∼7% nuclear spin polarization, respectively. This method, dubbed "SABRE in shield enables alignment transfer to heteronuclei" or "SABRE-SHEATH", promises to be a simple, cost-effective way to hyperpolarize heteronuclei. It may be particularly useful for in vivo applications because of longer hyperpolarization lifetimes, lack of background signal, and facile chemical-shift discrimination of different species.

Journal of labelled compounds & radiopharmaceuticals, 2014

Previously unreported (15) N labeled Azidothymidine (AZT) was prepared as an equimolar mixture of... more Previously unreported (15) N labeled Azidothymidine (AZT) was prepared as an equimolar mixture of two isotopomers: 1-(15) N-AZT and 3-(15) N-AZT. Polarization decay of (15) N NMR signal was studied in high (9.4 T) and low (~50 mT) magnetic fields. (15) N T1 values were 45 ± 5 s (1-(15) N-AZT) and 37 ± 2 s (3-(15) N-AZT) at 9.4 T, and 140 ± 16 s (3-(15) N-AZT) at 50 mT. (15) N-AZT can be potentially (15) N hyperpolarized by several methods. These sufficiently long (15) N-AZT T1 values potentially enable hyperpolarized in vivo imaging of (15) N-AZT, because of the known favorable efficient (i.e., of the time scale shorter than the longest reported here (15) N T1 ) kinetics of uptake of injected AZT. Therefore, 3-(15) N-AZT can be potentially used for HIV molecular imaging using hyperpolarized magnetic resonance imaging.

Journal of labelled compounds & radiopharmaceuticals, Jan 30, 2014

Here, we present a new approach for the delivery of a metabolic contrast agent for in vivo molecu... more Here, we present a new approach for the delivery of a metabolic contrast agent for in vivo molecular imaging. The use of a phosphate-protecting group that facilitates parahydrogen-induced polarization of 1-(13)C-phospholactate potentially enables the in vivo administration of a hydrogenated hyperpolarized adduct. When injected, nonhyperpolarized 1-(13)C-phospholactate is retained in the vasculature during its metabolic conversion to 1-(13)C-lactate by blood phosphatases as demonstrated here using a mucin 1 mouse model of breast cancer and ex vivo high-resolution (13)C NMR. This multisecond process is a suitable mechanism for the delivery of relatively short-lived (13)C and potentially (15)N hyperpolarized contrast agents using -OH phosphorylated small molecules, which is demonstrated here for the first time as an example of 1-(13)C-phospholactate. Through this approach, DL-1-(13)C-lactate is taken up by tissues and organs including the liver, kidneys, brain, heart, and tumors accord...

The Journal of Physical Chemistry B, 2013

Spin order obtained in the strong coupling regime of protons from parahydrogen-induced hyperpolar... more Spin order obtained in the strong coupling regime of protons from parahydrogen-induced hyperpolarization (PHIP) is initially captured as an ensemble of singlet states. For biomedical applications of PHIP, locking this spin order on long-lived heteronuclear storage nuclei increases spectral dispersion, reduces background interference from water protons, and eliminates the need to synchronize subsequent detection pulse sequences to accrued singlet-state evolution. A variety of traditional sequences such as INEPT or HMQC are available to interconvert heteronuclear single quantum coherences at high field, but new approaches are required for converting singlet states into heteronuclear single quantum coherences at low field in the strong coupling regime of protons. Described here is a consolidated pulse sequence that achieves this transformation of singlet-state spin order into heteronuclear magnetization across a wide range of scalar couplings in AA'X spin systems. Analytic solutions to the spin evolution are presented, and performance was validated experimentally in the parahydrogen addition product, 2-hydroxyethyl 1-(13)C-propionate-d(3). Hyperpolarized carbon-13 signals were enhanced by a factor of several million relative to Boltzmann polarization in a static magnetic field of 47.5 mT (~13% polarization). We anticipate that this pulse sequence will provide efficient conversion of parahydrogen spin order over a broad range of emerging PHIP agents that feature AA'X spin systems.

Organic Letters, 2010

B(C 6 F 5 ) 3 promotes regio-and stereoselective cyclizations of unsaturated alkoxysilanes to gen... more B(C 6 F 5 ) 3 promotes regio-and stereoselective cyclizations of unsaturated alkoxysilanes to generate oxasilinanes and oxasilepanes. The same products are available directly from alkenols via tandem silylation and hydrosilylation. Intramolecular hydrosilylation of alkenes is an important transformation in organic synthesis.1 Initially investigated for unsaturated silanes,2 the methodology is now often applied to unsaturated alkoxy-and aminosilanes,3 where stereospecific oxidative cleavage of the newly formed C-Si bond enables stereodefined synthesis of diols and aminoalcohols. 4 , 5 The majority of examples involve metal-catalyzed 5-endo or 5-exo ring closures, although six-membered cyclizations have been reported.1 , 3 , 6 We now report regio-and stereoselective formation of oxasilinanes and oxasilepanes via formation and cyclization of unsaturated alkoxysilanes in the presence of a nonmetal catalyst.

Journal of the American Chemical Society, 2014

1 H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridin... more 1 H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridine-d 5 at 9.4 T, a field that is orders of magnitude higher than what is typically utilized to achieve the conventional low-field SABRE effect. In addition to emissive peaks for the hydrogen spins at the ortho positions of the pyridine substrate (both free and bound to the metal center), absorptive signals are observed from hyperpolarized orthohydrogen and Ir-complex dihydride. Real-time kinetics studies show that the polarization buildup rates for these three species are in close agreement with their respective 1 H T 1 relaxation rates at 9.4 T. The results suggest that the mechanism of the substrate polarization involves cross-relaxation with hyperpolarized species in a manner similar to the spin-polarization induced nuclear Overhauser effect. Experiments utilizing pyridine-d 5 as the substrate exhibited larger enhancements as well as partial H/D exchange for the hydrogen atom in the ortho position of pyridine and concomitant formation of HD molecules. While the mechanism of polarization enhancement does not explicitly require chemical exchange of hydrogen atoms of parahydrogen and the substrate, the partial chemical modification of the substrate via hydrogen exchange means that SABRE under these conditions cannot rigorously be referred to as a non-hydrogenative parahydrogen induced polarization process. N MR hyperpolarization techniques increase nuclear spin polarization by several orders of magnitude, 1−3 which leads to the corresponding increase in NMR signal enabling an array of applications including studies of catalytic processes 4 and biomedical use of hyperpolarized substrates as MRI contrast agents. 5−8 There are several hyperpolarization methods, including dynamic nuclear polarization (DNP), 9 spin-exchange optical pumping, 10 parahydrogen-induced polarization (PHIP) 11 using parahydrogen and synthesis allow dramatically enhanced nuclear alignment (PASADENA), 12 and others. One of the newest methods is signal amplification by reversible exchange (SABRE), 13,14 with the experiments conducted by shaking the solutions of an Ir catalyst (i.e., Crabtree's catalyst 15 or Nheterocyclic carbene complex 16 ) with parahydrogen and a polarizable substrate at a relatively low magnetic field of a few mT, followed by physical transfer of the sample to the high-field NMR spectrometer. Alternatively, the in situ detection of SABRE effects at low magnetic fields has been demonstrated. 17 However, the latter approach does not provide sufficient chemical shift resolution, and therefore interpretation of the low-field NMR studies of SABRE often relies on the previous reports of ex situ high-field detection. Furthermore, high-field SABRE is commonly thought to be unobservable, because of the expectation that canonical SABRE 13 would be quenched by the fact that the J-coupling mediated flip-flops would no longer be energy conserving. The control experiments in the early SABRE studies seemingly confirm these expectations. In addition, a common misconception is that SABRE is not a chemical process since the polarized substrate appears to be chemically identical to its thermally polarized counterpart. Here, we show that generation of SABRE in high magnetic fields is possible and that the substrate is clearly involved in a chemical (hydrogen exchange) process while coordinated to a metal center. To the best of our knowledge, this is the first time the SABRE effects are generated and detected in situ in a high magnetic field.

Journal of the American Chemical Society, 2010

Free radical co-oxidation of polyunsaturated lipids with tyrosine or phenolic analogues of tyrosi... more Free radical co-oxidation of polyunsaturated lipids with tyrosine or phenolic analogues of tyrosine gave rise to lipid peroxide-tyrosine (phenol) adducts in both aqueous micellar and organic solutions. The novel adducts were isolated and characterized by 1D and 2D NMR spectroscopy as well as by mass spectrometry (MS). The spectral data suggest that the polyunsaturated lipid peroxyl radicals give stable peroxide coupling products exclusively at the para position of the tyrosyl (phenoxy) radicals. These adducts have characteristic 13 C chemical shifts at 185 ppm due to the cross-conjugated carbonyl of the phenolderived cyclohexadienone. The primary peroxide adducts subsequently undergo intramolecular Diels-Alder (IMDA) cyclization, affording a number of diastereomeric tricyclic adducts that have characteristic carbonyl 13 C chemical shifts at ∼198 ppm. All of the NMR HMBC and HSQC correlations support the structure assignments of the primary and Diels-Alder adducts, as does MS collision-induced dissociation data. Kinetic rate constants and activation parameters for the IMDA reaction were determined, and the primary adducts were reduced with cuprous ion to give a phenol-derived 4-hydroxycyclohexa-2,5-dienone. No products from adduction of peroxyls at the phenolic ortho position were found in either the primary or cuprous reduction product mixtures. These studies provide a framework for understanding the nature of lipid-protein adducts formed by peroxyl-tyrosyl radical-radical termination processes. Coupling of lipid peroxyl radicals with tyrosyl radicals leads to cyclohexenone and cyclohexadienone adducts, which are of interest in and of themselves since, as electrophiles, they are likely targets for protein nucleophiles. One consequence of lipid peroxyl reactions with tyrosyls may therefore be protein-protein cross-links via interprotein Michael adducts.

Bioorganic & Medicinal Chemistry, 2008

A series of synthetic molecules combining a geranyl backbone with a heterocyclic or oxime head gr... more A series of synthetic molecules combining a geranyl backbone with a heterocyclic or oxime head group are quorum-sensing molecules that block the yeast to mycelium transition in the dimorphic fungus Candida albicans. A number of the analogs have an IC 50 ≤ 10 μM, a level of potency essentially identical to the natural quorum sensing signal, the sesquiterpene farnesol. Two of the most potent analogs, neither toxic toward healthy mice, display remarkably different effects when co-administered with C. albicans. While neither offers protection from candidiasis, one analog mimics farnesol in acting as a virulence factor, whereas the other has no effect. The results offer the first example of highly potent synthetic fungal quorum-sensing molecules, and provide the first evidence for the ability to decouple quorum sensing and virulence.

Journal of the American Chemical Society, 2012

We demonstrate that potassium 1-(13)C-phosphoenolpyruvate becomes hyperpolarized potassium 1-(13)... more We demonstrate that potassium 1-(13)C-phosphoenolpyruvate becomes hyperpolarized potassium 1-(13)C-phospholactate with (13)C T(1) = 36 s after molecular hydrogenation by PASADENA (Parahydrogen and Synthesis Allows Dramatically Enhanced Nuclear Alignment). This proof-of-principle study was conducted with a fully protonated molecular precursor. (13)C was polarized to a level of 1%, corresponding to nearly 4000-fold sensitivity enhancement at 3 T. The relevant homo- and heteronuclear spin-spin couplings are reported.

15N nuclear spins of imidazole-15N2 were hyperpolarized using NMR signal amplification by reversi... more 15N nuclear spins of imidazole-15N2 were hyperpolarized using NMR signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). A 15N NMR signal enhancement of ∼2000-fold at 9.4 T is reported using parahydrogen gas (∼50% para-) and ∼0.1 M imidazole-15N2 in methanol:aqueous buffer (∼1:1). Proton binding to a 15N site of imidazole occurs at physiological pH (pKa ∼ 7.0), and the binding event changes the 15N isotropic chemical shift by ∼30 ppm. These properties are ideal for in vivo pH sensing. Additionally, imidazoles have low toxicity and are readily incorporated into a wide range of biomolecules. 15N-Imidazole SABRE-SHEATH hyperpolarization potentially enables pH sensing on scales ranging from peptide and protein molecules to living organisms.

The journal of physical chemistry letters, Jan 21, 2015

We report NMR Signal Amplification by Reversible Exchange (SABRE) hyperpolarization of the rare i... more We report NMR Signal Amplification by Reversible Exchange (SABRE) hyperpolarization of the rare isotopes in "neat" liquids, each composed only of an otherwise pure target compound with isotopic natural abundance (n.a.) and millimolar concentrations of dissolved catalyst. Pyridine (Py) or Py derivatives are studied at 0.4% isotopic natural abundance (15)N, deuterated, (15)N enriched, and in various combinations using the SABRE-SHEATH variant (microTesla magnetic fields to permit direct (15)N polarization from parahydrogen via reversible binding and exchange with an Ir catalyst). We find that the dilute n.a. (15)N spin bath in Py still channels spin order from parahydrogen to dilute (15)N spins, without polarization losses due to the presence of (14)N or (2)H. We demonstrate P 15N ≈ 1% (a gain of 2900 fold relative to thermal polarization at 9.4 T) at high substrate concentrations. This fundamental finding has a significant practical benefit for screening potentially hyperpo...

The journal of physical chemistry. C, Nanomaterials and interfaces, Jan 23, 2015

NMR signal amplification by reversible exchange (SABRE) is a NMR hyperpolarization technique that... more NMR signal amplification by reversible exchange (SABRE) is a NMR hyperpolarization technique that enables nuclear spin polarization enhancement of molecules via concurrent chemical exchange of a target substrate and parahydrogen (the source of spin order) on an iridium catalyst. Recently, we demonstrated that conducting SABRE in microtesla fields provided by a magnetic shield enables up to 10% (15)N-polarization (Theis, T.; et al. J. Am. Chem. Soc. 2015, 137, 1404). Hyperpolarization on (15)N (and heteronuclei in general) may be advantageous because of the long-lived nature of the hyperpolarization on (15)N relative to the short-lived hyperpolarization of protons conventionally hyperpolarized by SABRE, in addition to wider chemical shift dispersion and absence of background signal. Here we show that these unprecedented polarization levels enable (15)N magnetic resonance imaging. We also present a theoretical model for the hyperpolarization transfer to heteronuclei, and detail key pa...

Chemistry & biology, 2003

The dimorphic fungus Candida albicans produces extracellular farnesol (3,7,11-trimethyl-2,6,10-do... more The dimorphic fungus Candida albicans produces extracellular farnesol (3,7,11-trimethyl-2,6,10-dodecatriene-1-ol) which acts as a quorum-sensing molecule (QSM) to suppress filamentation. Of four possible geometric isomers of farnesol, only the E,E isomer possesses QSM activity. We tested 40 natural and synthetic analogs of farnesol for their activity in an N-acetylglucosamine-induced differentiation assay for germ tube formation (GTF). Modified structural features include the head group, chain length, presence or absence of the three double bonds, substitution of a backbone carbon by S, O, N, and Se heteroatoms, presence or absence of a 3-methyl branch, and the bulkiness of the hydrophobic tail. Of the 40 compounds, 22 showed QSM activity by their ability to reduce GTF by 50%. However, even the most active of the analogs tested had only 7.3% of the activity of E,E-farnesol. Structure-activity relationships were examined in terms of the likely presence in C. albicans of a farnesol bi...

Analytical Chemistry, 2014

We demonstrate the feasibility of microscale molecular imaging using hyperpolarized proton and ca... more We demonstrate the feasibility of microscale molecular imaging using hyperpolarized proton and carbon-13 MRI contrast media and low-field (47.5 mT) preclinical scale (38 mm i.d.) 2D magnetic resonance imaging (MRI). Hyperpolarized proton images with 94 × 94 μm(2) spatial resolution and hyperpolarized carbon-13 images with 250 × 250 μm(2) in-plane spatial resolution were recorded in 4-8 s (largely limited by the electronics response), surpassing the in-plane spatial resolution (i.e., pixel size) achievable with micro-positron emission tomography (PET). These hyperpolarized proton and (13)C images were recorded using large imaging matrices of up to 256 × 256 pixels and relatively large fields of view of up to 6.4 × 6.4 cm(2). (13)C images were recorded using hyperpolarized 1-(13)C-succinate-d2 (30 mM in water, %P(13C) = 25.8 ± 5.1% (when produced) and %P(13C) = 14.2 ± 0.7% (when imaged), T1 = 74 ± 3 s), and proton images were recorded using (1)H hyperpolarized pyridine (100 mM in methanol-d4, %P(H) = 0.1 ± 0.02% (when imaged), T1 = 11 ± 0.1 s). Both contrast agents were hyperpolarized using parahydrogen (>90% para-fraction) in an automated 5.75 mT parahydrogen induced polarization (PHIP) hyperpolarizer. A magnetized path was demonstrated for successful transportation of a (13)C hyperpolarized contrast agent (1-(13)C-succinate-d2, sensitive to fast depolarization when at the Earth's magnetic field) from the PHIP polarizer to the 47.5 mT low-field MRI. While future polarizing and low-field MRI hardware and imaging sequence developments can further improve the low-field detection sensitivity, the current results demonstrate that microscale molecular imaging in vivo is already feasible at low (<50 mT) fields and potentially at low (~1 mM) metabolite concentrations.

Analytical Chemistry, 2014

Parahydrogen-induced polarization (PHIP) was used to demonstrate the concept that highly polarize... more Parahydrogen-induced polarization (PHIP) was used to demonstrate the concept that highly polarized, catalyst-free fluids can be obtained in a catalysis-free regime using a chemical reaction with molecular addition of parahydrogen to a water-soluble Rh(I) complex carrying a payload of compound with unsaturated (C═C) bonds. Hydrogenation of norbornadiene leads to formation of norbornene, which is eliminated from the Rh(I) complex and, therefore, leaves the aqueous phase and becomes a gaseous hyperpolarized molecule. The Rh(I) metal complex resides in the original liquid phase, while the product of hydrogen addition is found exclusively in the gaseous phase based on the affinity. Hyperpolarized norbornene (1)H NMR signals observed in situ were enhanced by a factor of approximately 10,000 at a static field of 47.5 mT. High-resolution (1)H NMR at a field of 9.4 T was used for ex situ detection of hyperpolarized norbornene in the gaseous phase, where a signal enhancement factor of approximately 160 was observed. This concept of stoichiometric as opposed to purely catalytic use of PHIP-available complexes with an unsaturated payload precursor molecule can be extended to other contrast agents for both homogeneous and heterogeneous PHIP. The Rh(I) complex was employed in aqueous medium suitable for production of hyperpolarized contrast agents for biomedical use. Detection of PHIP hyperpolarized gas by low-field NMR is demonstrated here for the first time.

The Journal of Physical Chemistry B, 2014

Activation of a catalyst [IrCl(COD)(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylide... more Activation of a catalyst [IrCl(COD)(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; COD = cyclooctadiene)] for signal amplification by reversible exchange (SABRE) was monitored by in situ hyperpolarized proton NMR at 9.4 T. During the catalyst-activation process, the COD moiety undergoes hydrogenation that leads to its complete removal from the Ir complex. A transient hydride intermediate of the catalyst is observed via its hyperpolarized signatures, which could not be detected using conventional nonhyperpolarized solution NMR. SABRE enhancement of the pyridine substrate can be fully rendered only after removal of the COD moiety; failure to properly activate the catalyst in the presence of sufficient substrate can lead to irreversible deactivation consistent with oligomerization of the catalyst molecules. Following catalyst activation, results from selective RF-saturation studies support the hypothesis that substrate polarization at high field arises from nuclear cross-relaxation with hyperpolarized (1)H spins of the hydride/orthohydrogen spin bath. Importantly, the chemical changes that accompanied the catalyst's full activation were also found to endow the catalyst with water solubility, here used to demonstrate SABRE hyperpolarization of nicotinamide in water without the need for any organic cosolvent--paving the way to various biomedical applications of SABRE hyperpolarization methods.

Journal of the American Chemical Society, 2015

Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrog... more Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrogen-15 in pyridine and nicotinamide (vitamin B(3) amide) by conducting "signal amplification by reversible exchange" (SABRE) at microtesla fields within a magnetic shield. Following transfer of the sample from the magnetic shield chamber to a conventional NMR spectrometer, the (15)N NMR signals for these molecules are enhanced by ∼30,000- and ∼20,000-fold at 9.4 T, corresponding to ∼10% and ∼7% nuclear spin polarization, respectively. This method, dubbed "SABRE in shield enables alignment transfer to heteronuclei" or "SABRE-SHEATH", promises to be a simple, cost-effective way to hyperpolarize heteronuclei. It may be particularly useful for in vivo applications because of longer hyperpolarization lifetimes, lack of background signal, and facile chemical-shift discrimination of different species.

Journal of labelled compounds & radiopharmaceuticals, 2014

Previously unreported (15) N labeled Azidothymidine (AZT) was prepared as an equimolar mixture of... more Previously unreported (15) N labeled Azidothymidine (AZT) was prepared as an equimolar mixture of two isotopomers: 1-(15) N-AZT and 3-(15) N-AZT. Polarization decay of (15) N NMR signal was studied in high (9.4 T) and low (~50 mT) magnetic fields. (15) N T1 values were 45 ± 5 s (1-(15) N-AZT) and 37 ± 2 s (3-(15) N-AZT) at 9.4 T, and 140 ± 16 s (3-(15) N-AZT) at 50 mT. (15) N-AZT can be potentially (15) N hyperpolarized by several methods. These sufficiently long (15) N-AZT T1 values potentially enable hyperpolarized in vivo imaging of (15) N-AZT, because of the known favorable efficient (i.e., of the time scale shorter than the longest reported here (15) N T1 ) kinetics of uptake of injected AZT. Therefore, 3-(15) N-AZT can be potentially used for HIV molecular imaging using hyperpolarized magnetic resonance imaging.

Journal of labelled compounds & radiopharmaceuticals, Jan 30, 2014

Here, we present a new approach for the delivery of a metabolic contrast agent for in vivo molecu... more Here, we present a new approach for the delivery of a metabolic contrast agent for in vivo molecular imaging. The use of a phosphate-protecting group that facilitates parahydrogen-induced polarization of 1-(13)C-phospholactate potentially enables the in vivo administration of a hydrogenated hyperpolarized adduct. When injected, nonhyperpolarized 1-(13)C-phospholactate is retained in the vasculature during its metabolic conversion to 1-(13)C-lactate by blood phosphatases as demonstrated here using a mucin 1 mouse model of breast cancer and ex vivo high-resolution (13)C NMR. This multisecond process is a suitable mechanism for the delivery of relatively short-lived (13)C and potentially (15)N hyperpolarized contrast agents using -OH phosphorylated small molecules, which is demonstrated here for the first time as an example of 1-(13)C-phospholactate. Through this approach, DL-1-(13)C-lactate is taken up by tissues and organs including the liver, kidneys, brain, heart, and tumors accord...

The Journal of Physical Chemistry B, 2013

Spin order obtained in the strong coupling regime of protons from parahydrogen-induced hyperpolar... more Spin order obtained in the strong coupling regime of protons from parahydrogen-induced hyperpolarization (PHIP) is initially captured as an ensemble of singlet states. For biomedical applications of PHIP, locking this spin order on long-lived heteronuclear storage nuclei increases spectral dispersion, reduces background interference from water protons, and eliminates the need to synchronize subsequent detection pulse sequences to accrued singlet-state evolution. A variety of traditional sequences such as INEPT or HMQC are available to interconvert heteronuclear single quantum coherences at high field, but new approaches are required for converting singlet states into heteronuclear single quantum coherences at low field in the strong coupling regime of protons. Described here is a consolidated pulse sequence that achieves this transformation of singlet-state spin order into heteronuclear magnetization across a wide range of scalar couplings in AA'X spin systems. Analytic solutions to the spin evolution are presented, and performance was validated experimentally in the parahydrogen addition product, 2-hydroxyethyl 1-(13)C-propionate-d(3). Hyperpolarized carbon-13 signals were enhanced by a factor of several million relative to Boltzmann polarization in a static magnetic field of 47.5 mT (~13% polarization). We anticipate that this pulse sequence will provide efficient conversion of parahydrogen spin order over a broad range of emerging PHIP agents that feature AA'X spin systems.

Organic Letters, 2010

B(C 6 F 5 ) 3 promotes regio-and stereoselective cyclizations of unsaturated alkoxysilanes to gen... more B(C 6 F 5 ) 3 promotes regio-and stereoselective cyclizations of unsaturated alkoxysilanes to generate oxasilinanes and oxasilepanes. The same products are available directly from alkenols via tandem silylation and hydrosilylation. Intramolecular hydrosilylation of alkenes is an important transformation in organic synthesis.1 Initially investigated for unsaturated silanes,2 the methodology is now often applied to unsaturated alkoxy-and aminosilanes,3 where stereospecific oxidative cleavage of the newly formed C-Si bond enables stereodefined synthesis of diols and aminoalcohols. 4 , 5 The majority of examples involve metal-catalyzed 5-endo or 5-exo ring closures, although six-membered cyclizations have been reported.1 , 3 , 6 We now report regio-and stereoselective formation of oxasilinanes and oxasilepanes via formation and cyclization of unsaturated alkoxysilanes in the presence of a nonmetal catalyst.

Journal of the American Chemical Society, 2014

1 H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridin... more 1 H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridine-d 5 at 9.4 T, a field that is orders of magnitude higher than what is typically utilized to achieve the conventional low-field SABRE effect. In addition to emissive peaks for the hydrogen spins at the ortho positions of the pyridine substrate (both free and bound to the metal center), absorptive signals are observed from hyperpolarized orthohydrogen and Ir-complex dihydride. Real-time kinetics studies show that the polarization buildup rates for these three species are in close agreement with their respective 1 H T 1 relaxation rates at 9.4 T. The results suggest that the mechanism of the substrate polarization involves cross-relaxation with hyperpolarized species in a manner similar to the spin-polarization induced nuclear Overhauser effect. Experiments utilizing pyridine-d 5 as the substrate exhibited larger enhancements as well as partial H/D exchange for the hydrogen atom in the ortho position of pyridine and concomitant formation of HD molecules. While the mechanism of polarization enhancement does not explicitly require chemical exchange of hydrogen atoms of parahydrogen and the substrate, the partial chemical modification of the substrate via hydrogen exchange means that SABRE under these conditions cannot rigorously be referred to as a non-hydrogenative parahydrogen induced polarization process. N MR hyperpolarization techniques increase nuclear spin polarization by several orders of magnitude, 1−3 which leads to the corresponding increase in NMR signal enabling an array of applications including studies of catalytic processes 4 and biomedical use of hyperpolarized substrates as MRI contrast agents. 5−8 There are several hyperpolarization methods, including dynamic nuclear polarization (DNP), 9 spin-exchange optical pumping, 10 parahydrogen-induced polarization (PHIP) 11 using parahydrogen and synthesis allow dramatically enhanced nuclear alignment (PASADENA), 12 and others. One of the newest methods is signal amplification by reversible exchange (SABRE), 13,14 with the experiments conducted by shaking the solutions of an Ir catalyst (i.e., Crabtree's catalyst 15 or Nheterocyclic carbene complex 16 ) with parahydrogen and a polarizable substrate at a relatively low magnetic field of a few mT, followed by physical transfer of the sample to the high-field NMR spectrometer. Alternatively, the in situ detection of SABRE effects at low magnetic fields has been demonstrated. 17 However, the latter approach does not provide sufficient chemical shift resolution, and therefore interpretation of the low-field NMR studies of SABRE often relies on the previous reports of ex situ high-field detection. Furthermore, high-field SABRE is commonly thought to be unobservable, because of the expectation that canonical SABRE 13 would be quenched by the fact that the J-coupling mediated flip-flops would no longer be energy conserving. The control experiments in the early SABRE studies seemingly confirm these expectations. In addition, a common misconception is that SABRE is not a chemical process since the polarized substrate appears to be chemically identical to its thermally polarized counterpart. Here, we show that generation of SABRE in high magnetic fields is possible and that the substrate is clearly involved in a chemical (hydrogen exchange) process while coordinated to a metal center. To the best of our knowledge, this is the first time the SABRE effects are generated and detected in situ in a high magnetic field.

Journal of the American Chemical Society, 2010

Free radical co-oxidation of polyunsaturated lipids with tyrosine or phenolic analogues of tyrosi... more Free radical co-oxidation of polyunsaturated lipids with tyrosine or phenolic analogues of tyrosine gave rise to lipid peroxide-tyrosine (phenol) adducts in both aqueous micellar and organic solutions. The novel adducts were isolated and characterized by 1D and 2D NMR spectroscopy as well as by mass spectrometry (MS). The spectral data suggest that the polyunsaturated lipid peroxyl radicals give stable peroxide coupling products exclusively at the para position of the tyrosyl (phenoxy) radicals. These adducts have characteristic 13 C chemical shifts at 185 ppm due to the cross-conjugated carbonyl of the phenolderived cyclohexadienone. The primary peroxide adducts subsequently undergo intramolecular Diels-Alder (IMDA) cyclization, affording a number of diastereomeric tricyclic adducts that have characteristic carbonyl 13 C chemical shifts at ∼198 ppm. All of the NMR HMBC and HSQC correlations support the structure assignments of the primary and Diels-Alder adducts, as does MS collision-induced dissociation data. Kinetic rate constants and activation parameters for the IMDA reaction were determined, and the primary adducts were reduced with cuprous ion to give a phenol-derived 4-hydroxycyclohexa-2,5-dienone. No products from adduction of peroxyls at the phenolic ortho position were found in either the primary or cuprous reduction product mixtures. These studies provide a framework for understanding the nature of lipid-protein adducts formed by peroxyl-tyrosyl radical-radical termination processes. Coupling of lipid peroxyl radicals with tyrosyl radicals leads to cyclohexenone and cyclohexadienone adducts, which are of interest in and of themselves since, as electrophiles, they are likely targets for protein nucleophiles. One consequence of lipid peroxyl reactions with tyrosyls may therefore be protein-protein cross-links via interprotein Michael adducts.