Fast photoinduced reactions in the condensed phase are nonexponential.

Time-resolved measurements of photoinduced reactions reveal that many ultrafast reactions in the femto- to picosecond time scale are nonexponential. In this article we provide several examples of reactions that exhibit a nonexponential rate. We explain the wide range of the nonexponential reaction by the lack of time separation between τ(s), the characteristic fast equilibration time of the population in the reactant potential well, and the longer time τ(e), the characteristic time to cross the energy barrier between the reactant and the product.

How fast can a proton-transfer reaction be beyond the solvent-control limit?

In this article, we review the field of photoacids. The rate of excited-state proton transfer (ESPT) to solvent spans a wide range of time scales, from tens of nanoseconds for the weakest photoacids to short time scales of about 100 fs for the strongest photoacids synthesized so far. We divide the photoacid strength into four regimes.

Insight into the structure and the mechanism of the slow proton transfer in the GFP double mutant T203V/S205A.

Mutations near the fluorescing chromophore of the green fluorescent protein (GFP) have direct effects on the absorption and emission spectra. Some mutants have significant band shifts and most of the mutants exhibit a loss of fluorescence intensity. In this study we continue our investigation of the factors controlling the excited state proton transfer (PT) process of GFP, in particular to study the effects of modifications to the key side chain Ser205 in wt-GFP, proposed to participate in the proton wire.

Acid effect on photobase properties of curcumin.

Femtosecond UV-vis pump-probe spectroscopy was employed to study the acid effect on curcumin in the excited state. Curcumin in solutions of weak acids was found to be a photobase forming a protonated curcumin within a few tens of picoseconds from the time of excitation. The excited-state protonation reaction is also observed in the steady-state emission spectrum as a new red emission band with a maximum at 620 nm in the presence of weak acids.

Excited-state proton transfer from quinone-cyanine 9 to protic polar-solvent mixtures.

Steady-state and time-resolved emission techniques were used to study the excited-state proton-transfer (ESPT) process of quinone cyanine 9 (QCy9) in solvent mixtures. We found that the ESPT rate from QCy9 in water/methanol mixtures is independent of the mixture composition and the rate constant is k(PT) ∼ 10(13) s(-1). In ethanol/trifluoroethanol (TFE) mixtures the ESPT rate strongly depends on the solvent-mixture composition.

Effect of acid on the ultraviolet-visible absorption and emission properties of curcumin.

Steady-state and time-resolved emission techniques were employed to study the acid-base effects on the UV-vis spectrum of curcumin in several organic solvents. The fluorescence-decay rate of curcumin increases with increasing acid concentration in all of the solvents studied. In methanol and ethanol solutions containing about 1 M HCl, the short-wavelength fluorescence (λ < 560 nm) decreases by more than an order of magnitude.

Temperature dependence of the excited-state proton-transfer reaction of quinone-cyanine-7.

Steady-state and time-resolved fluorescence techniques were used to study the temperature dependence of the photoprotolytic process of quinone-cyanine-7 (QCy7), a very strong photoacid, in H2O and D2O ice, over a wide temperature range, 85-270 K. We found that the excited-state proton-transfer (ESPT) rate to the solvent decreases as the temperature is lowered with a very low activation energy of 10.5 ± 1 kJ/mol.

Temperature and viscosity dependence of the nonradiative decay rates of auramine-O and thioflavin-T in glass-forming solvents.

Both auramine-O (AuO) and thioflavin-T (ThT) behave as fluorescent molecular rotors, meaning that their (non)radiative properties are markedly affected by the intramolecular rotation of the molecule. In this article, steady-state and time-resolved fluorescence of AuO and ThT were measured in three alcohols, 1-propanol, 1-butanol, and 1-pentanol, over a wide range of temperatures (86-260 K). These solvents are glass-forming liquids, and their viscosity and dielectric relaxation time increase by more than 10 orders of magnitude as the temperature is lowered from room temperature to ~100 K.

Excited-state proton transfer of firefly dehydroluciferin.

Steady-state and time-resolved emission techniques were used to study the protolytic processes in the excited state of dehydroluciferin, a nonbioluminescent product of the firefly enzyme luciferase. We found that the ESPT rate coefficient is only 1.1 × 10(10) s(-1), whereas those of d-luciferin and oxyluciferin are 3.

Comparative study of the photoprotolytic reactions of D-luciferin and oxyluciferin.

Optical steady-state and time-resolved spectroscopic methods were used to study the photoprotolytic reaction of oxyluciferin, the active bioluminescence chromophore of the firefly's luciferase-catalyzed reaction. We found that like D-luciferin, the substrate of the firefly bioluminescence reaction, oxyluciferin is a photoacid with pK(a)* value of ∼0.5, whereas the excited-state proton transfer (ESPT) rate coefficient is 2.

Ultrafast proton transfer of three novel quinone cyanine photoacids.

Steady-state and time-resolved emission techniques were used to study the photoprotolytic properties of three recently synthesized strong quinone cyanine photoacids (QCy7 and its sulfonated derivatives). The rate coefficient of the excited-state proton transfer (ESPT), k(PT), of the three dyes is roughly 1.5 × 10(12) s(-1), a high value that is comparable to the solvation dynamics rate of large polar organic molecules in H(2)O and D(2)O.

The effect of a mild base on curcumin in methanol and ethanol.

Steady-state and time-resolved emission techniques were employed to study the effect of acetate, a mild base, on the luminescence of curcumin in methanol and ethanol. We found that the steady-state emission intensity as well as the average fluorescence decay time are reduced by a factor of 5 when the acetate concentration is raised to about 1.8 M.

Ultrafast excited-state intermolecular proton transfer of cyanine fluorochrome dyes.

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D(2)O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is ~1.5 × 10(12) s(-1).

Structure and excited-state proton transfer in the GFP S205A mutant.

To further explore excited state proton transfer (ESPT) pathways within green fluorescent protein (GFP), mutagenesis, X-ray crystallography, and time-resolved and steady-state optical spectroscopy were employed to create and study the GFP mutant S205A. In wild type GFP (wt-GFP), the proton transfer pathway includes the hydroxyl group of the chromophore, a water molecule, Ser205, and Glu222. We found that the ESPT rate constant of S205A is smaller by a factor of 20 than that of wt-GFP and larger by a factor of 2 in comparison to the ESPT rate of S205V mutant which we previously characterized.

Temperature dependence of the fluorescence properties of curcumin.

Steady-state and time-resolved techniques were employed to study the nonradiative process of curcumin dissolved in ethanol and 1-propanol in a wide range of temperatures. We found that the nonradiative rate constants at temperatures between 175-250 K qualitatively follow the same trend as the dielectric relaxation times of both neat solvents. We attribute the nonradiative process to solvent-controlled proton transfer.

Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases.

We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.

Study of thioflavin-T immobilized in porous silicon and the effect of different organic vapors on the fluorescence lifetime.

Steady-state and time-resolved emission techniques have been employed to study the fluorescence properties of thioflavin-T (ThT) adsorbed on oxidized porous silicon (PSi) surfaces, with an average pore size of ∼10 nm. We found that the average fluorescence decay time of ThT, when it is adsorbed on the PSi surface, is rather long, τ(av) = 1.3 ns.

Pressure effect on the nonradiative process of thioflavin-T.

Time-resolved emission techniques were employed to study the nonradiative process of thioflavin-T (ThT) in 1-propanol, 1-butanol, and 1-pentanol as a function of the hydrostatic pressure. Elevated hydrostatic pressure increases the alcohol viscosity, which in turn strongly influences the nonradiative rate of ThT. A diamond-anvil cell was used to increase the pressure up to 2.

Temperature dependence of the fluorescence properties of thioflavin-T in propanol, a glass-forming liquid.

Steady-state and time-resolved emission techniques were employed to study the nonradiative process of Thioflavin-T (ThT) in 1-propanol as a function of temperature. We found that the nonradiative rate, k(nr), decreased by about 3 orders of magnitude when the temperature was lowered to 88 K. We found remarkably good correspondence between the temperature dependence of k(nr) of ThT and the dielectric relaxation times of the 1-propanol solvent.

Excited-state intermolecular proton transfer of firefly luciferin IV. Temperature and pH dependence.

Time-resolved emission as well as steady-state UV-vis techniques were employed to study the photoprotolytic processes that d-luciferin, the natural substrate of the firefly luciferase, undergoes in both acidic aqueous solutions and ice. The emission spectrum of D-luciferin in a 20 mM HCl aqueous solution or higher has an additional emission band at 590 nm red-shifted with respect to the strongest emission band positioned at 530 nm of the deprotonated NRO(-*) form in a pH-neutral aqueous solution. We attribute this emission band to the zwitterion form designated as (+)HNRO(-).

Excited state proton transfer in the red fluorescent protein mKeima.

mKeima is an unusual monomeric red fluorescent protein (lambda(em)(max) approximately 620 nm) that is maximally excited in the blue (lambda(ex)(max) approximately 440 nm). The large Stokes shift suggests that the chromophore is normally protonated. A 1.

Structure and mechanism of the photoactivatable green fluorescent protein.

Crystal structures of the photoactivatable green fluorescent protein T203H variant (PA-GFP) have been solved in the native and photoactivated states, which under 488 nm illumination are dark and brightly fluorescent, respectively. We demonstrate that photoactivation of PA-GFP is the result of a UV-induced decarboxylation of the Glu222 side chain that shifts the chromophore equilibrium to the anionic form. Coupled with the T203H mutation, which stabilizes the native PA-GFP neutral chromophore, Glu222 decarboxylation yields a 100-fold contrast enhancement relative to wild-type GFP (WT).

An alternative excited-state proton transfer pathway in green fluorescent protein variant S205V.

Wild-type green fluorescent protein (wt-GFP) has a prominent absorbance band centered at approximately 395 nm, attributed to the neutral chromophore form. The green emission arising upon excitation of this band results from excited-state proton transfer (ESPT) from the chromophore hydroxyl, through a hydrogen-bond network proposed to consist of a water molecule and Ser205, to Glu222. Although evidence for Glu222 as a terminal proton acceptor has already been obtained, no evidence for the participation of Ser205 in the proton transfer process exists.

Proton antenna effect of the gamma-cyclodextrin outer surface, measured by excited state proton transfer.

The reversible proton dissociation and geminate recombination of the common photoacid, 8-hydroxypyrene-1,3,6-trisulfonate (pyranine), either in dilute aqueous solution or when forming a complex with gamma-cyclodextrin (gamma-CD), has been studied by time-resolved fluorescence spectroscopy and supplemented by molecular modeling and dynamics simulations. We find that the dissociation rate of the proton from the excited molecule was decreased to about approximately 50% of its value in water, while the rate of recombination was doubled. These observations were evaluated by molecular modeling of the reactants at atomic resolution.

Testing the three step excited state proton transfer model by the effect of an excess proton.

In a previous work, we proposed an extended model for intermolecular excited-state proton transfer to the solvent. The model invoked an intermediate species, the contact ion-pair RO(-)..