Ehud Pines

Real-Time Observation of Carbonic Acid Formation in Aqueous Solution

A Glimpse of Wet Carbonic Acid Both carbon dioxide and bicarbonate play extraordinarily widespread roles in biochemical and geochemical reactions. It is surprising therefore that carbonic acid, the intermediate in the water-coupled interconversion of these two compounds, has never been directly characterized in aqueous solution. Adamczyk et al. (p. 1690, published online 12 November) have succeeded in glimpsing the elusive acid by inducing an aqueous photoacid (a compound rendered transiently more acidic upon light absorption) to react with dissolved bicarbonate. Using infrared spectroscopy, they show that the carbonic acid product persists for nanoseconds. Analysis of its formation kinetics affords a direct pKa value of 3.5, substantially lower than the effective value derived from observations of the net bicarbonate/carbon dioxide equilibrium. The use of a photoacid enables the long-sought characterization of the conjugate acid of bicarbonate. Despite the widespread importance of aqueous bicarbonate chemistry, its conjugate acid, carbonic acid, has remained uncharacterized in solution. Here we report the generation of deuterated carbonic acid in deuterium oxide solution by ultrafast protonation of bicarbonate and its persistence for nanoseconds. We follow the reaction dynamics upon photoexcitation of a photoacid by monitoring infrared-active marker modes with femtosecond time resolution. By fitting a kinetic model to the experimental data, we directly obtain the on-contact proton-transfer rate to bicarbonate, previously inaccessible with the use of indirect methods. A Marcus free-energy correlation supports an associated pKa (Ka is the acid dissociation constant) of 3.45 ± 0.15, which is substantially lower than the value of 6.35 that is commonly assumed on the basis of the overall carbon dioxide–to–bicarbonate equilibrium. This result should spur further exploration of acid-base reactivity in carbon dioxide–rich aqueous environments such as those anticipated under sequestration schemes.

Direct measurement of intrinsic proton transfer rates in diffusion-controlled reactions

Abstract Acid–base reactions are usually treated as diffusion-controlled. In this study we directly measure the intrinsic proton transfer rates at the reaction contact of several naphthols photoacids–carboxylic base pairs. Deviations from the diffusion-controlled limit result from the finite intrinsic proton transfer rates at contact and these rates correlate well with the total free-energy change following the reactions. The correction was made over 8 Δp K a units and yielded k r 0 =3×10 11 s −1 and G a 0 =2.9 kcal/mol for the activationless proton transfer rate and the intrinsic free-energy barrier, respectively. A similar behavior is found when the proton dissociation rates of all previously studied naphthol-like photoacids are included in the correlation. The combined correlation, over 11 Δp K a units, yielded k r 0 =2×10 11 s −1 and G a 0 =2.5 kcal/mol.

Photophysical characterization of trans-4,4′-disubstituted stilbenes

Abstract Absorption and emission spectra of twenty trans -4,4′-disubstituted stilbenes have been measured in four solvents: cyclohexane (CH), chlorobenzene (CB), 2-butanone (methylethyl ketone, MEK) and dimethylsulfoxide (DMSO) at room temperature. Fluorescence quantum yields ( Φ f ) and fluorescence lifetimes ( τ f ) have been measured for these stilbenes. The lifetimes and quantum yields of fluorescence were found to be dependent on the donor-acceptor properties of the substituents and correlate with the Hammett ν-constants. In addition, we experimentally observed the appearance of a second emitting state which is close energetically to the lowest exited singlet state 1 t ∗ in cases of strong donor-acceptor substituents in polar solvents.

Coexistence of Domains with Distinct Order and Polarity in Fluid Bacterial Membranes

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan‐labeled Escherichia coli and temperature dependencies of both laurdans GP and fluorescence anisotropy of 1,3‐diphenyl‐1,3,5‐hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid‐crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37°C reflects membrane heterogeneity. Time‐resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdans emission. In the proteo‐lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple‐Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid‐crystalline bacterial membrane: a lipid‐enriched and a proteo‐lipid domain. In bacteria with chloramphenicol (Cam)–inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature‐dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam‐induced domain dissipation.

Self quenching of 1-naphthol. Connection between time-resolved and steady-state measurements

Abstract The photochemistry of 1-naphthol following its electronic excitation in aqueous solution was investigated by a single-photon counting apparatus. It was found that following the proton dissociation the excited naphtholate anion decays non-exponentially with a t− sol1 2 dependence over time. This we attribute to a self-quenching reaction of the naphtholate anion by its geminate proton. The quenching rate on contact is 3.5 × 1010 s−1. We also found that some of the protons interact adiabatically with a geminate recombination rate in D2O fo 8 × 109 s−1. The time-dependent escape probability of the naphtholate iopn-par was sam;ed by carrying out the experiment in presence of a proton scavenger. We show that the leading term in the ultimate scavenging probability is given by the time integral of the homogeneous scavenging rate of the isolated pair at relative long times following its creation.

Solvent dependence of pyranine fluorescence and UV-visible absorption spectra

Fluorescence and UV–visible absorption spectra of HPTS (8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt, pyranine) were measured in a variety of solvents. Fluorescence maxima (in kcal mol-1) can be correlated with the Kosower Z parameter (r = 0.901), the Dimroth–Reichardt ET(30) parameter (r = 0.900) and the Winstein Y parameter (r = 0.916) using one-parameter fits. Good correlations (r = 0.98) were obtained for HPTS fluorescence in ethanol–water mixtures using the Y, YOTs and Z parameters. Fluorescence maxima of HPTS in aqueous sulphuric acid solutions gave an excellent correlation with YOTs (r = 0.991). Multi-parameter correlations, indicating the significance of specific solvent interactions, were also studied. In addition, fluorescence maxima correlate well with maximum/minimum ratios obtained from UV–visible absorption experiments. Results can be applied in the use of HPTS as a molecular probe of solvent environments and for extension of the YOTs scale in acidic solutions. HPTS is a unique molecular probe, not only because of its photoacidic properties and its widespread use as a pH-sensitive biosensor, but also because of its relative stability in acidic environments.

Ultrafast Excited-State Proton-Transfer Reaction of 1-Naphthol-3,6-Disulfonate and Several 5-Substituted 1-Naphthol Derivatives

The 1-naphthol molecule has been the subject of intense research activity for the past 60 years due to its complex behavior as a photoacid upon optical excitation. We have utilized femtosecond mid-infrared spectroscopy and time-resolved fluorescence spectroscopy to investigate the excited-state proton-transfer reaction of 1-naphthol-3,6-disulfonate (1N-3,6diS) and several 5-substituted 1-naphthol derivatives. The proton dissociation rate constant of 1N-3,6-diS was found to be about 3 times faster and the pKa* about 2 pKa units more acidic than the values previously reported in the literature. A Marcus (free-energy) plot of excited-state proton dissociation rate constants vs the excited-state equilibrium constant of the photoacids, Ka*, was constructed using the C-5 series of 1-naphthol derivatives. The newly measured values for the ESPT rate constant and pKa* of 1N-3,6diS was found to fit well with the Marcus correlation. We discuss our findings in the context of the photoacidity phenomenon in general, and the photoacidity of 1-naphthol and its derivatives in particular.

Apparent Stoichiometry of Water in Proton Hydration and Proton Dehydration Reactions in CH3CN/H2O Solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH(+) + n(H(2)O) = X + (H(2)O)(n)H(+) was studied for strong mineral acids (negatively charged X, X = ClO(4)¯, Cl¯, Br¯, I¯, CF(3)SO(3)¯) and for strong cationic acids (uncharged X, X = R*NH(2), H(2)O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H(+)(H(2)O)(2), remain almost unchanged at large n values up to at least 10 M of water.

Direct observation of power-law behavior in the asymptotic relaxation to equilibrium of a reversible bimolecular reaction

We report the first direct observation of power-law relaxation to equilibrium of the diffusion influenced reversible reaction, AB⇆A+B (B≫A). Our experimental findings confirm the predicted t−3/2 decay law relaxation of AB population a long time after the photoinitiation of the reaction. This t−3/2 relaxation of the excess-over-equilibrium population is similar to that found in diffusion influenced geminate recombination reactions.

The Hydrated Excess Proton in the Zundel Cation H5O2+: The Role of Ultrafast Solvent Fluctuations

The nature of the excess proton in liquid water has remained elusive after decades of extensive research. In view of ultrafast structural fluctuations of bulk water scrambling the structural motifs of excess protons in water, we selectively probe prototypical protonated water solvates in acetonitrile on the femtosecond time scale. Focusing on the Zundel cation H5 O2 (+) prepared in room-temperature acetonitrile, we unravel the distinct character of its vibrational absorption continuum and separate it from OH stretching and bending excitations in transient pump-probe spectra. The infrared absorption continuum originates from a strong ultrafast frequency modulation of the H(+) transfer vibration and its combination and overtones. Vibrational lifetimes of H5 O2 (+) are found to be in the sub-100 fs range, much shorter than those of unprotonated water. Theoretical results support a picture of proton hydration where fluctuating electrical interactions with the solvent and stochastic thermal excitations of low-frequency modes continuously modify the proton binding site while affecting its motions.