The Dynamics of Ultrafast Excited State Proton Transfer in Anionic Micelles † (original) (raw)
Related papers
Excited State Proton Transfer in Reverse Micelles
Journal of the American Chemical Society, 2002
The aqueous phase of water/AOT reversed micelles having varying diameters was probed by a single free diffusing proton that was released form a hydrophilic photoacid molecule (2-naphthol-6,8disulfonate). The fluorescence decay signals were reconstructed through the geminate recombination algorithm, accounting for the reversible nature of the proton-transfer reactions at the surface of the excited molecule and at the water/detergent interface. The radial diffusion of the proton inside the aqueous phase was calculated accounting for both the entropy of dilution and the total electrostatic energy of the ion pair, consisting of the pair-energy and self-energy of the ions. The analysis implied that micellar surface must be modeled with atomic resolution, assuming that the sulfono residue protrudes above the water/hydrocarbon interface by ∼2 Å. The analysis of the fluorescence decay curves implies that the molecule is located in a solvent with physical-chemical properties very similar to bulk water, except for the dielectric constant. For reversed micelles with rmax g 16 Å, the dielectric constant of the aqueous phase was ∼70 and for smaller micelles, where ∼60% of the water molecule is in contact with the van der Waals surface of the micelle, it is as low as 60. This reduction is a reflection of the increased fraction of water molecule that is in close interaction with the micelle surface.
Langmuir, 2006
The functionalized flavylium salt 6-hexyl-7-hydroxy-4-methyflavylium chloride (HHMF) was employed to probe some of the fundamental features of proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethylammonium chloride (CTAC) micelles. In contrast to most ordinary flavylium salts, HHMF is insoluble in water, but readily incorporates into SDS and CTAC micelles. In the ground state, the rate constant for deprotonation of the acid form (AH + ) of HHMF decreases 100-fold upon going from CTAC (k d ) 3.0 × 10 6 s -1 ) to SDS (k d ) 1.4 × 10 4 s -1 ), consistent with the presence of an activation barrier for proton transfer in the ground state and reflecting, respectively, stabilization or destabilization of the AH + cation by the micelle. Reprotonation of A is diffusion-controlled in both micelles (k p (SDS) ) (2.1 × 10 11 )[H + ] aq s -1 and k p (CTAC) ) (3.7 × 10 8 )[H + ] aq s -1 ), the difference reflecting the rate of proton entry into the micelles. In the excited singlet state, the rate constants for deprotonation of the AH + * form of HHMF are similar in the two micelles (2.4 × 10 10 s -1 ), consistent with activationless proton transfer. Reprotonation of the excited A* is dominated by fast geminate recombination of the photogenerated (A*-H + ) pair at the micelle surface (k rec (SDS) ) 6.1 × 10 9 s -1 and k rec (CTAC) ) 3.4 × 10 10 s -1 ) and the net efficiencies of geminate recombination are quite similar in SDS (0.89) and CTAC (0.86).
An Approach to a Model Free Analysis of Excited-State Proton Transfer Kinetics in a Reverse Micelle
The Journal of Physical Chemistry C, 2018
Time resolved area normalized emission spectra (TRANES) of a photo-acid dye HPTS, confined within the water-pools of reverse micelles (RMs), are analyzed for a direct and model free observation of excited state proton transfer (ESPT) kinetics. When area normalized emission spectra of HPTS at different times are overlapped in a single window, we find population of RO-* (PTS-*) form of HPTS increases at the cost of a concomitant decreasing of the population of ROH* (HPTS*) form as time progresses. Migration of excited state population from ROH* form to RO-* form causes the emergence of an isoemissive point in TRANES, which retains throughout the entire course of ESPT process. An estimation of ESPT timescale is obtained either from the population depleting rate of ROH* form or from the population increasing rate of RO-* form; both are practically same here. Emergence of an isoemissive point in TRANES is implying that there are only two kinetically and reversibly coupled emitting species (ROH* and RO-*) of HPTS are present within the water pools of RMs. Continuous spectral relaxations of HPTS due to excited state solvation dynamics apparently have no effect on the ESPT kinetics of HPTS within the RMs; otherwise spectral shifting, caused by the excited state solvation, would have destroyed the isoemissive point of TRANES.
Chemical Physics Letters, 2004
The effects of ionic and nonionic micelles on the excited state proton transfer processes of 2-hydroxy 1-naphthaldehyde (HNL) have been reported in this Letter. Deprotonation of HNL is considerably retarded in neutral and anionic micelles than that in cationic type as evinced from the increased neutral emission. Increased anion emission of HNL in cationic micelle is more due to the abundance of hydroxyl ions in the environment as well as less nonradiative deactivation. Anion emission is found to decrease in anionic micelle due to less formation of the conformer along with increase in nonradiative decay. The nonradiative processes from neutral to ionic form and ionic to ground state are less affected in neutral micelle as compared to HNL in ionic micelles.
The Journal of Physical Chemistry A, 2010
Three water-insoluble, micelle-anchored flavylium salts, 7-hydroxy-3-octyl-flavylium chloride, 4′-hexyl-7hydroxyflavylium chloride, and 6-hexyl-7-hydroxy-4-methyl-flavylium chloride, have been employed to probe excited-state prototropic reactions in micellar sodium dodecyl sulfate (SDS). In SDS micelles, the fluorescence decays of these three flavylium salts are tetraexponential functions in the pH range from 1.0 to 4.6 at temperatures from 293 to 318 K. The four components of the decays are assigned to four kinetically coupled excited species in the micelle: specifically, promptly deprotonable (AH + *) and nonpromptly deprotonable (AH h + *) orientations of the acid in the micelle, the base-proton geminate pair (A* · · · H + ), and the free conjugate base (A*). The initial prompt deprotonation to form the geminate pair occurs at essentially the same rate (k d ∼ 6-7 × 10 10 s -1 ) for all three photoacids. Recombination of the geminate pair is ∼3-fold faster than the rate of proton escape from the pair (k rec ∼ 3 × 10 10 s -1 and k diss ∼ 1 × 10 10 s -1 ), corresponding to an intrinsic recombination efficiency of the pair of ∼75%. Finally, the reprotonation of the short-lived free A* (200-350 ps, depending on the photoacid) has two components, only one of which depends on the proton concentration in the intermicellar aqueous phase. Ultrafast transfer of the proton to water and substantial compartmentalization of the photogenerated proton at the micelle surface on the picosecond time scale strongly suggest preferential transfer of the proton to preformed hydrogen-bonded water bridges between the photoacid and the anionic headgroups. This localizes the proton in the vicinity of the excited base much more efficiently than in bulk water, resulting in the predominance of geminate reprotonation at the micelle surface.
European Journal of Biochemistry, 1984
The dense packing of protogenic enzymes on the coupling membrane can furnish a route for a rapid proton flux which may avoid the adjacent bulk phase. In order to evaluate the role of proximity between reactants on the rate constant of proton transfer we generated a model system consisting of 2-naphthol and pH indicator (bromocresol green) both adsorbed on the same micelle of unchanged detergent. Excitation of the 2-naphthol by a short intensive laser pulse lowers its pK with subsequent synchronized proton ejection. The discharged protons are detected by their reaction with the indicator using a fast transient absorption technique.
Probing the micelle/water interface by rapid laser-induced proton pulse
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1981
The laser-induced pH jump (Gutman, M. and Huppert, D.J. (1979) Biochem. Biophys. Methods 1, 9-19) has a time resolution capable of measuring the diffusion-controlled rate constant of proton binding. In the present study we employed this technique for measuring the kinetics of protonation-deprotonation of surface groups of macromolecules. The heterogeneous surface of proteins excludes them from serving as a simple model, therefore we used micelles of a neutral detergent (Brij 58) as a high molecular weight structure. The charge was varied by the addition of a low concentration of sodium dodecyl sulfate and the surface group with which the protons react was an adsorbed pH indicator (bromocresol green or neutral red). The dissociation of a proton from adsorbed bromocresol green is slower than that from free indicator. This effect is attributed to the enhanced stabilization of the acid form of the indicator in the pallisade region of the miceUe. The pK shift of bromocresol green adsorbed on neutral micelles is thus quantitatively accounted for by the decreased rate of proton dissociation. Indicators such as neutral red, which are more lipid soluble in their alkaline form, do not exhibit such decelerated proton dissociation in their adsorbed state nor a pK shift on adsorption to neutral micelles. The protonation of an indicator is a diffusion-controlled reaction, whether it is free in solution or adsorbed on micelles. By varying the electric charge of the micelle this rate can be accelerated or decelerated depending on the total Abbreviations: SDS, sodium dodecyl sulfate; CTAB, cetyltximethylammonium bromide. 16 charge of the micelle. The micellar charge calculated from this method was corroborated by other measurements which rely only on equilibrium parameters. The high time resolution of the pH jump is exemplified by the ability to estimate the diffusion coefficient of protons through the hydrated shell of the micelle.
Excited-state proton transfer of 1-naphthol in micelles
The Journal of Physical …, 1998
The fast deprotonation of 1-naphthol, which occurs in 35 ps in aqueous solution, is studied in neutral (triton X 100, reduced, TX-100R), cationic (cetyl trimethylammonium bromide, CTAB), and anionic (sodium dodecyl sulfate, SDS) micelles. Drastically different effects on the ...
Kinetic Studies of Proton Transfer in the Microenvironment of a Binding Site
European Journal of Biochemistry, 1982
Excitation of 8-hydroxypyrene 1,3,6-trisulfonate to its first electronic singlet state convcrts the compound from weak base (pK" = 7.7) into a strong acid (pK* = 0.5). The dissociation of the proton in water or dilute salt solution is a very fast reaction, k l z = 1 x 10" s-'. In concentrated salt solutions the dissociation is slowed as an exponential function of the chemical activity of the water in the solution. This kinetic parameter has been used to gauge the properties of the microenvironment of the binding sites of bovine serum albumin at which this compound is bound. 40 ps which lasts z 0.5 ns, followed by a slower reaction with z = 3.3 ns. The first rapid phase represents proton dissociation taking place in the binding site. From the rate constant k = 3.3 x lo9 s-' we estimate that the ability of the water molecules in the site to hydrate the ejected proton is equivalent to a salt solution with water activity of 0.85.
2013
Department of Chemistry, West Bengal State University, Barasat, Kolkata-700 126, India <em>E-mail:</em> ranjan.das68@gmail.com Fax : 91-33-25241977 <em>Manuscript received online 19 July 2012, revised 21 August 2012, accepted 22 August 2012</em> The photo physics of 2-(2<em>' </em>-furyl)-3-hydroxychromone (FHC) was explored in three different non-ionic micelles of Triton X-100, Brij-58 and Tween-20. FHC exhibits a dual emission, attributable to the excited normal (<strong>N*</strong>) and tautomer (<strong>T*</strong>) forms resulting from an excited state intramolecular proton transfer (ESIPT) reaction (<strong>N*→T*</strong>). The ESIPT dynamics of FHC in the non-ionic micelles demonstrates a dependence on the hydrophile-lipophile balance (HLB) parameter of the surfactants by an increase in the kinetic constant of ESIPT reaction (<em><sup>k</sup></em>PT) with a decrease in HLB. A c...