Excited-state Proton Transfer Kinetics Research Articles

Could Chemical Reaction at the Molecular Level Show Distinction between Two Liquid-Water States? Study of the Excited-State Water-Catalyzed Proton Transfer Reaction Provides a Clue.

The two liquid-water states, which lead to some anomalies when temperature crosses over 50 ± 10 °C at the atmospheric pressure, have been continuously catching popular attention. In this study, using the excited-state proton transfer (ESPT) catalyzed by water molecules as a prototypical reaction, we demonstrate that the kinetics of ESPT indeed is influenced by the two liquid-water states. In the water-catalyzed ESPT of 3-cyano-7-azaindole (3CAI), a repetitive and comprehensive temperature-dependent study of ESPT in H2O from 0 to 90 °C shows anomalous behavior. The plot of the logarithm of ESPT rate constant as a function of inverse of absolute temperature deviates from a straight line. The convex-Arrhenius behavior manifests the activation free energy for water-assisted ESPT being dependent on temperature and hence the liquid water structure. To simplify the discussion, the plot is well fitted by using two straight lines that are crossed over in the vicinity of 40 °C. The free energy difference between water-solvated 3CAI and the 1:1 H2O:3CAI complex is deduced to be 2.29 ± 0.04 and 1.96 ± 0.04 kcal·mol-1 in the regions of 0-40 and 40-90 °C water, respectively, which also results in different frequency factors, i.e., the proton transfer/tunneling rates of (5.83 ± 0.36) × 1010 and (3.48 ± 0.27) × 1010 s-1, respectively. In a qualitative manner, the results are then rationalized by the different types of H-bonding configuration as proposed for two liquid-water phases, rendering experimental evidence to support the different water phases in ambient temperatures at 1 bar.

An Approach to a Model Free Analysis of Excited-State Proton Transfer Kinetics in a Reverse Micelle

Time resolved area normalized emission spectra (TRANES) of a photoacid 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 time scale is obtained either from the population depleting rate of ROH* form or from the population increasing rate of RO–* form; both are practically the same here. Emergence of an isoemissive point in TRANES implies 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.

The change in excited-state proton transfer kinetics of 1-naphthol in micelles upon the binding of polymers: The influence of hyaluronan hydration

The fast deprotonation of 1-naphthol was studied in aqueous solution and in polymer–cetyltrimethylammonium bromide (CTAB) mixtures using picosecond fluorescence spectroscopy to study the influence of hyaluronan hydration in polymer–surfactant interactions. The aqueous micelle solution showed the expected change of proton transfer rate around the reported critical micelle concentration (∼1mM). The proton transfer rate dependence on CTAB concentration in the hyaluronan–CTAB and polystyrenesulfonate–CTAB systems differed significantly from that in the aqueous micelle solution. The dynamic study of excited state proton transfer (ESPT) revealed the significant influence of hyaluronan hydration in CTAB micelles. When hyaluronan–CTAB aggregates were formed at the CTAB concentration of 0.5mM, a tenfold decrease in the rate of deprotonation was observed when compared to polystyrenesulfonate–CTAB aggregates due to hyaluronan hydration. In 2mM CTAB–hyaluronan aggregates, the rate of deprotonation was found to be almost two times faster than in the 2mM CTAB or polystyrenesulfonate–CTAB system. Furthermore, the study of excited-state proton transfer of 1-naphthol confirmed that hyaluronan hydration layer penetrates into the micelle and changes the emission characteristics of 1-naphthol.

Unveiling the Eigen-Weller ion pair from the excited state proton transfer kinetics of 3-chloro-4-methyl-7-hydroxycoumarin.

The prototropic reactions of the first excited singlet state of 3-chloro-4-methylumbelliferone (3Cl4MU), in dioxane:water mixtures (Dx:H2O), were revisited using ps-time-resolved fluorescence techniques. The data response to the dielectric constant of the mixtures revealed the presence of an additional fourth kinetic species, kinetically coupled to the neutral (N*), the tautomeric (T*), and anionic (A(-)*) forms of 3Cl4MU, which is assigned to the elusive geminate (A(-)*···H(+)) ion pair. From the data analysis, all rate constants of the prototropic and diffusion processes involved were separately evaluated. The results showed that, whenever the geminate ionic pair is not kinetically detected, the evaluated values for deprotonation and protonation rate constants can substantially deviate from the real ones, depending on the efficiencies of pair recombination and dissociation. Finally, the results provide convincing kinetic evidence for the Eigen-Weller mechanism (intermediacy of the geminate ionic pair) in a quasi-aqueous medium, which to our knowledge had not yet been given.

Improved analysis of excited state proton transfer kinetics by the combination of standard and convolution methods

We wish to report a powerful methodology for analysis and interpretation of complex fluorescence decays and other kinetic data, based on the addition of convolution analysis to standard (individual and global) analysis. The method was applied to cases involving excited state proton transfer of flavylium salts in water, in water-1,4-dioxane mixtures and in micellar solutions of sodium dodecyl sulfate (SDS). The results clearly support the proposal of the intermediacy of a proton-base geminate pair in environments other than water.

Photoinduced proton transfer of 6-hydroxynaphthalene-1-sulfonic acid in n-propanol/water mixtures

Kinetics of excited-state proton transfer from 6-hydroxynaphthalene-1-sulfonic acid to water in n-propanol/water mixtures has been studied at room temperature. The fluorescence decay is single exponential in n-propanol and becomes bi-exponential in n-propanol/water mixtures. The proton transfer occurs in the binary mixtures above the critical water concentration which is 4M. A water dimer has been found to be an effective proton acceptor. The presence of reverse reaction indicates the need of the stable solvation to excited conjugate base aftermath of the proton transfer.

Can Luminescent Ru(II) Polypyridyl Dyes Measure pH Directly?

Two molecularly engineered Ru(II) complexes for direct pH optosensing in environmental or physiological media based on luminescence lifetime measurements-namely, Na(2)[Ru(bpds)(2)(F(15)ap)] and Na(2)[Ru(pbbs)(2)(pyim)] (where bpds = 2,2'-bipyridine-4,4'-disulfonate, F(15)ap = 5-perfluorooctanamide-1,10-phenanthroline, pbbs = 1,10-phenanthroline-4,7-(diyl)bis(benzenesulfonate), and pyim = 2-(2'-pyridyl)imidazole)-have been prepared. The suitability of these two luminophores as general-purpose pH indicators has been assessed to determine the general features of Ru(II) dyes required for such application. Their photochemical properties were investigated at different pH values in various buffer solutions using absorption spectroscopy, as well as steady-state and time-resolved luminescence. Both dyes display a parallel absorption and emission behavior as a function of pH (2-10), namely, higher luminescence in acidic solutions together with a 8-10 nm bathochromic shift in their (blue) absorption and 6-39 nm bathochromic shift in their (red) luminescence maxima in basic media, respectively. Similar ground-state acidity values (pK(a)) of 6.5 +/- 0.2 for the amide group of the F(15)ap complex and 6.9 +/- 0.2 for the imidazole NH moiety of the pyim complex have been measured. However, dramatic differences in their luminescence lifetimes as a function of pH were found. The HA and A(-) forms of *[Ru(bpds)(2)(F(15)ap)](2-) conveniently display lifetimes of 372 and 263 ns, respectively, regardless of the solution acidity and buffer nature. Their relative contributions to the overall decay (0%-100%) are dependent on the solution pH indicating excited-state proton exchange rates well below the decay rates of the acidic and basic forms. However, *[Ru(pbbs)(2)(pyim)](2-) deactivation kinetics show a pH-independent component of 80 ns at high pH and an acidity-sensitive one that varies from 610 ns (at pH 2) to 170 ns (at pH 10). Both components are also dependent on the buffer nature and concentration, also indicating the lack of an acid-base equilibrium in its excited-state but an irreversible proton transfer by the buffer species. Density functional theory calculations have demonstrated the difficult accessibility of the base to the acidic perfluoroamide proton of the F(15)ap complex, severely slowing the excited-state proton transfer kinetics of the luminescent dye. Therefore, we conclude that the design of Ru(II) polypyridyl lifetime-based pH indicators not affected by the buffer nature and concentration requires the absence of proton exchange during the radiative deactivation of both the acidic and basic species, which then would remain in their ground-state relative ratio. This feature may be achieved by (a) mild excited-state acidities and (b) structural features that shield the exchangeable proton from the buffer access.

Balance between Ultrafast Parallel Reactions in the Green Fluorescent Protein Has a Structural Origin

The fluorescence photocycle of the green fluorescent protein is functionally dependent on the specific structural protein environment. A direct relationship between equilibrium protein side-chain conformation of glutamate 222 and reactivity is established, particularly the rate of ultrafast proton transfer reactions in the fluorescence photocycle. We show that parallel transformations in the photocycle have a structural origin, and we report on the vibrational properties of responsive amino acids on an ultrafast timescale. Blue excitation of GFP drives two parallel, excited-state deuteron transfer reactions with 10 ps and 75 ps time constants to the buried carboxylic acid side chain of glutamate 222 via a hydrogen-bonding network. Assignment of 1456 cm −1 and 1441 cm −1 modes to ν sym and assignment of 1564 cm −1 and 1570 cm −1 features to ν asym of E222 in the 10 ps and 75 ps components, respectively, was possible from the analysis of the transient absorption data of an E222D mutant and was consistent with photoselection measurements. In contrast to the wild-type, measurements of E222D can be described with only one difference spectrum, with the ν sym mode at 1435 cm −1 and the ν asym mode at 1567 cm −1, also correlating a large Δ ν asym-sym with slow excited-state proton transfer kinetics. Density Functional Theory calculations and published model compound and theoretical studies relate differences in Δ ν asym-sym to the strength and number of hydrogen-bonding interactions that are detected via equilibrium geometry and COO − stretching frequency differences of the carboxylate. The correlation of photocycle kinetics with side-chain conformation of the acceptor suggests that proton transfer from S205 to E222 controls the rate of the overall excited-state proton transfer process, which is consistent with recent theoretical predictions. Photoselection measurements show agreement for localized C O vibrations of chromophore, Q69, and E222 with Density Functional Theory and ab initio calculations placed in the x-ray geometry and provide their vibrational response in the intermediates in the photocycle.

Excited-State Proton-Transfer Dynamics of 7-Hydroxyquinoline in Room Temperature Ionic Liquids

Excited-state proton transfer (ESPT) reaction of 7-hydroxyquinoline (7-HQ) mediated by methanol molecules has been studied in two room temperature ionic liquids (RTILs) using steady-state and time-resolved fluorescence measurements. While no ESPT is observable in neat RTILs, characteristic tautomer fluorescence of 7-HQ could be observed in the presence of small quantity of methanol (0.5-4.1 M). The observation of a rise time (350 ps-1.4 ns) associated with the tautomer fluorescence suggests that proton transfer in 7-HQ is indeed an excited-state phenomenon that requires considerable solvent reorganization prior to the relay of proton from the hydroxyl group to the distant ring nitrogen atom through suitably organized dimeric chain of methanol molecules. The rise time of the tautomer fluorescence, which has been found to decrease with increasing methanol concentration, is attributed to the change of viscosity of the medium upon methanol addition. While the influence of viscosity on the ESPT kinetics is evident from the data, lack of any definite correlation between the bulk viscosity and the rise time has been interpreted in terms of the microheterogeneous nature of the media that does not allow assessment of the microviscosity around 7-HQ from the bulk viscosity.

Encapsulation of 6-Hydroxyquinoline in Heptakis(2,6-di-O-methyl)-β-cyclodextrin

Cyclodextrins have attracted long-standing interests in the field of artificial catalysts. Their cone-shaped cages consist of α,D-glucose subunits and possess nonpolar cavities with polar rims containing primary hydroxyl and secondary methoxy groups. Cyclodextrins are soluble in water and their hydrophobic interiors are capable of sequestering a variety of polar and nonpolar compounds, controlling the chemistry of reactive molecules. Three different types of water are generally assumed to exist in aqueous cyclodextrin solutions: water inside the cavity, water near the rim, and bulk water. A number of studies have reported the effects of micelles and cyclodextrins on proton transfer to shed light on the influence of lipophilic environment. Proton transfers of hydroxyquinolines having two prototropic groups of enol and imine in a molecule have been extensively explored. The excited-state proton transfer of 7-hydroxyquinoline (7HQ) in the molecular cage of βcyclodextrin or heptakis(2,6-di-O-methyl)-β-cyclodextrin (CD) has also been studied. The anionic intermediate during the excited-state proton transfer of 7HQ forms slower but decays faster in β-cyclodextrin cages than in water. However, the fluorescence spectral overlaps of normal molecule, enol-deprotonated anion, and imine-protonated and enol-deprotonated tautomer at S1, as well as the low solubility in water and the low association constant of βcyclodextrin with 7HQ, make the excited-state proton transfer kinetics of 7HQ very complex to require a sophisticated analysis. This has led us to investigate the encapsulation of 6-hydroxyquinoline (6HQ) in the molecular cage of CD in this work. CD is a derivative of β-cyclodextrin with a greatly improved solubility in water, encapsulating a predominant fraction of 6HQ to form host-guest complexes. The observed H chemical shifts of CD in water decrease

Experimental evidence for a kinetic transition in reversible reactions.

We provide a first experimental verification of a theoretical prediction of a kinetic transition in a reversible binding reaction, AB right harpoon over left harpoon A+B, driven by the difference in effective lifetimes of the bound and the unbound states. We consider the kinetics of excited-state proton transfer to solvent from a photoacid whose conjugate anionic base possesses an extremely short unbound anion lifetime. Its solvent variation relative to the overall dissociation rate coefficient induces a transition in the kinetics, from power law to exponential.

Kinetics of Excited-State Proton Transfer of Doubly Protonated 2-Aminoacridine

2-Aminoacridine has been used to extend the quantitation of excited-state proton-transfer kinetics in concentrated acid media to ≃13 M sulfuric acid. The reasons for the large discrepancy between the excited-state dissociation constant determined from kinetic measurements and that calculated from a thermodynamic cycle are discussed.

Dual asymptotic behavior in geminate diffusion-influenced reaction

The kinetics of excited-state proton transfer to solvent from the strong photoacid 5-cyano-2-naphthol exhibits two different power-law asymptotic tails for acid and base, arising from competition between reversible and irreversible geminate reprotonation. We show that the data are in quantitative agreement with a recently developed theory for this diffusion-influenced reaction.

Excited state proton transfer kinetics of 4-hydroxydiphenyl ether

Fluorimetric titrations of 4-hydroxydiphenyl ether and its anion give stretched sigmoid curves with two inflection points. This reveals that the rates of excited state proton exchange are comparable to the rates of deactivation of the conjugate pair. These curves are analyzed using the lifetimes of the species. The excited state equilibrium constants determined from rate constants and by other methods are compared. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 861–867, 1997

Time-resolved spectroscopy of wild-type and mutant Green Fluorescent Proteins reveals excited state deprotonation consistent with fluorophore-protein interactions

Recently steady-state and picosecond time-resolved absorption and fluorescence spectroscopy on the Green Fluorescent Protein (GFP) have been interpreted by a mechanism where the key process is an excited state deprotonation of the chromophore (M. Chattoraij, B.A. King, G.U. Bublitz and S.G. Boxer, Proc. Natl. Acad. Sci. USA, 93 (1996) 8362–8367). Such a conclusion was borne out by the mirror image of the picosecond decay of the protonated species RH ∗ in the blue and the concomitant picosecond rise of the green fluorescence of the deprotonated fluorophore R −∗ as well as the significant slowing of both kinetic features upon deuteration. We report similar experiments confirming this mechanism. The results of ultrafast spectroscopy on wild-type GFP together with two important mutants combined with the recent crystal structures are shown to shed more light on the interplay between absorption and emission phenomena in GFP. Beyond some differences with previous results pertaining, for instance, to the assignment of vibronic progressions in absorption spectra and the temperature dependence of excited state deprotonation, several new features have been identified. These concern the deprotonated ground state R − in equilibrium as well as the excited state RH ∗. In particular, we have studied the distributed fluorescence kinetics in the time and frequency domain, excited state absorption features observed in femtosecond time-resolution, and the dependence of excited state proton transfer kinetics on the aggregational state of the protein.

Excited-state proton-transfer kinetics: a theoretical model

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTExcited-state proton-transfer kinetics: a theoretical modelLuis G. Arnaut and Sebastiao J. FormosinhoCite this: J. Phys. Chem. 1988, 92, 3, 685–691Publication Date (Print):February 1, 1988Publication History Published online1 May 2002Published inissue 1 February 1988https://doi.org/10.1021/j100314a022RIGHTS & PERMISSIONSArticle Views164Altmetric-Citations25LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (795 KB) Get e-Alerts

Excited-state proton transfer kinetics of carbazole

Rate constants for the excited-state proton transfer reaction of carbazole in aqueous alkaline solution have been determined using picosecond single photon counting. Fluorescence decay measurements show that the back reaction is slow compared to the fluorescence decay time and therefore equilibrium is not attained in the excited state. The validity of a pK value for the lowest excited state determined from steady-state fluorescence measurements assuming equilibrium is discussed. It is concluded that the thermodynamic pK * value for carbazole is 10.98.

Kinetics of excited-state proton transfer of doubly protonated 3-aminoacridine

3-aminoacridine (1) has beens been used to extend the quantitation of excited-state proton-transfer kinetic in concentrated acid media to ≈ 13 M sulfuric acid. The hydration requirement of the reaction and the reasons for its not attaining equilibrium within the lifetime of the excited state are discussed.

Hydrogen/deuterium isotope effects on the excited-state proton transfer kinetics of 3-hydroxyflavone

The static and transient fluorescence spectroscopy of 3-hydroxyflavone, 3HF, has been investigated as a function of deuteration of the 3-hydroxy group and the solvent. Small H/D isotope effects are observed for at least two of the simple rate constants for the excited-state dynamics. The isotope effects are not significantly temperature dependent. The results suggest that the proton transfer process is not a simple barrier crossing reaction.