Dan Huppert

Geminate recombination in excited‐state proton‐transfer reactions: Numerical solution of the Debye–Smoluchowski equation with backreaction and comparison with experimental results

The well‐known phenomenon of proton dissociation from excited‐state hydroxy‐arenes is analyzed by the Debye–Smoluchowski equation which is solved numerically with boundary conditions which account for the reversibility of the reaction. The numerical solution is then compared with the measured dissociation profiles which were obtained by picosecond time‐resolved fluorescence spectroscopy. The intrinsic rate constants thus determined are used to predict steady‐state rates, yields, and pK values, in agreement with experiment.

Geminate recombination in proton‐transfer reactions. II. Comparison of diffusional and kinetic schemes

The diffusional and kinetic approaches are compared for geminate dissociation–recombination reactions. When steady‐state rate coefficients to and from a distance defined as a ‘‘complex cage’’ are evaluated from the diffusion equation, one obtains encouraging agreement between the transient analytic solution of the rate equations and the exact numerical solution for diffusion with backreaction over a finite time regime. However, the rate equations cannot accurately describe the decay of the dissociating molecule for very long times, since as we prove below, the asymptotic decay according to the diffusional scheme is t−3/2, while for the rate equations it is exponential. New experiments, over an extended time regime confirm these conclusions.

Solvent motion controls the rate of intramolecular electron transfer in solution

Abstract The fluorescence decay times (τ- F ) for conversion (by intramolecular electron transfer) of the S 1,np state into the S 1,ct state of 6-(4-methylphenyl) amino-2-naphthalenesulfon-N,N-dimethylamide (TNSDMA) correlate well with the constant-charge dielectric relaxation times [τ 1 v = (e 00 /e s ) τ 1 ] in linear alcohols. Solvent motion thus controls certain intramolecular electron transfers.

Water consumption during the early stages of the sol-gel tetramethylorthosilicate polymerization as probed by excited state proton transfer☆

Abstract Proton transfer from the excited state of 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt to surrounding water molecules is used as a sensitive fluorimetric method for following directly and in detail the kinetics of water consumption during the early stages of teh tetramethylorthosilicate sol-gel polymerization process. Changes in water/silane ratio and in pH were found to affect markedly the kinetic behavior of water consumption.

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). Additionally, the structures provide insights into the spectroscopic differences between WT and PA-GFP steady-state fluorescence maxima and excited-state proton transfer dynamics.

Picosecond proton ejection: an ultrafast pH jump

Abstract The rates of proton ejection from 2-naphthol-3,6-disulfonate (pK* = 0.5 ± 0.1) and 8-hydroxy 1,3,6-pyrene trisulfonate (pK* = 0.4 ± 0.1) have been found to be 3.1 × 1010 s−1 and 3.2 × 1010 s−1, respectively. This is in keeping with the scaling of the ejection rate inversely with the excited state pK*.

Picosecond proton transfer studies in water-alcohols solutions

Abstract Picosecond proton ejection rates from 8-hydroxy-1,3,6-pyrene trisulfonate and 2-naphthol-6-sulfonate in water-alcohol solutions have been found to decrease rapidly with increasing alcohol concentrations. It was found that the rate of proton transfer decreases more rapidly in water propanol solutions than in water ethanol solutions. This phenomenon can be interpreted in terms of water breaking structure by the organic solvent.

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 ∼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. To examine the role of Ser205 in the proton transfer, we mutated Ser205 to valine. However, the derived GFP variant S205V, upon excitation at 400 nm, still produces green fluorescence. Time‐resolved emission spectroscopy suggests that ESPT contributes to the green fluorescence, and that the proton transfer takes place ∼30 times more slowly than in wt‐GFP. The crystal structure of S205V reveals rearrangement of Glu222 and Thr203, forming a new hydrogen‐bonding network. We propose this network to be an alternative ESPT pathway with distinctive features that explain the significantly slowed rate of proton transfer. In support of this proposal, the double mutant S205V/T203V is shown to be a novel blue fluorescent protein containing a tyrosine‐based chromophore, yet is incapable of ESPT. The results have implications for the detailed mechanism of ESPT and the photocycle of wt‐GFP, in particular for the structures of spectroscopically identified intermediates in the cycle.

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 1.63 A resolution structure of mKeima reveals the chromophore to be imbedded in a novel hydrogen bond network, different than in GFP, which could support proton transfer from the chromophore hydroxyl, via Ser142, to Asp157. At low temperatures the emission contains a green component (lambda(em)(max) approximately 535 nm), enhanced by deuterium substitution, presumably resulting from reduced proton transfer efficiency. Ultrafast pump/probe studies reveal a rising component in the 610 nm emission with a lifetime of approximately 4 ps, characterizing the rate of proton transfer. Mutation of Asp157 to neutral Asn changes the chromophore resting charge state to anionic (lambda(ex)(max) approximately 565 nm, lambda(em)(max) approximately 620 nm). Thus, excited state proton transfer (ESPT) explains the large Stokes shift. This work unambiguously characterizes green emission from the protonated acylimine chromophore of red fluorescent proteins.

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. Regime I includes the weak photoacids 0 < pKa* < 3. These photoacids can transfer a proton only to water or directly to a mild-base molecule in solution. The ESPT rate to other protic solvents, like methanol or ethanol, is too small in comparison with the radiative rate. The second regime includes stronger photoacids whose pKa*s range from -4 to 0. They are capable of transferring a proton to other protic solvents and not only to water. The third regime includes even stronger photoacids. Their pKa* is ∼ -6, and the ESPT rate constant, kPT, is limited by the orientational time of the solvent which is characterized by the average solvation correlation function ⟨S(t)⟩. The fourth regime sets a new time limit for the ESPT rate of the strongest photoacids synthesized so far. The kPT value of such photoacids is 10(13) s(-1), and τPT = 100 fs. We attribute this new time limit (beyond the solvent control) to intermolecular vibration between the two heavy atoms of the proton donor and the proton acceptor, which assist the ESPT by lowering the height and width of the potential barrier, thus enhancing the ESPT rate.