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Picosecond studies of rhodopsin in low-temperature glasses have been carried out in order to observe directly the risetime of prelumirhodopsin, the first intermediate in the visual pathway. Only at 20 K or below can the risetime of this intermediate be resolved and even at 4 K it is astoundingly rapid, about 36 psec. An examination of the Arrhenius dependence on temperature of the rate of formation of prelumirhodopsin shows a strong deviation from linearity at low temperatures, i.e., non-Arrhenius behavior. This marked nonlinear behavior is characteristic of a quantum mechanical tunneling event such as the translocation of hydrogen. An excellent candidate for the tunnelling process is the hydrogen of the protonated Schiff base formed between opsin and its retinal chromophore. Deuterium-exchanged rhodopsin, in which the Schiff base hydrogen is replaced by a deuterium, also shows a marked non-Arrhenius temperature dependence at low temperatures, consistent with tunneling. The rate of formation of prelumirhodopsin in deuterium-exchanged samples is much slower and a deuterium isotope effect kH/kD t 7 is observed. The data support a model in which the formation of prelumirhodopsin involves translocation of a roton toward the Schiff base nitrogen of the retinal chromopfiore.The primary process in visual excitation is initiated by a photochemical event, the absorption of a photon by the photoreceptor, rhodopsin, resulting in the formation of a new species, prelumirhodopsin. The characterization of this new species has been carried out by photostationary studies in low-temperature glasses (1, 2) and by picosecond kinetic studies near room temperatures (3, 4). Prelumirhodopsin is formed within 6 X 10-12 sec (6 psec) following excitation of rhodopsin and has an absorption maximum at 543 nm which is bathochromically shifted compared to that of rhodopsin. This event has been classically described as the isomerization of the 11-cis-retinal chromophore of rhodopsin to the all-trans-retinal form (5, 6). That full isomerization of a bulky chromophore could occur within this time scale has been questioned (3, 7), and speculation still exists as to the nature of this photochemical event.The time resolution for most ultrafast kinetic studies is about 6 psec, which is slower than the actual risetime of prelumirhodopsin at room temperature. To overcome this restriction, we have excited rhodopsin in low-temperature glasses and monitored the formation rate of prelumirhodopsin. The kinetics are sufficiently slow at 20 K or below to allow us to measure directly the formation of prelumirhodopsin. Our picosecond data suggest that the initial photochemical step in the visual process is not cis-trans isomerization, but rather proton translocation. Plausible models are presented and discussed. METHODOLOGY Rod outer segments were isolated from frozen bovine retinas (G. Hormel Co.) and were solubilized in 0.3 M Ammonyx LO The costs of publication of this article were defrayed in part by the payment of page charges from funds made avai...

Picosecond absorption spectroscopy has been employed in the study of the dynamics of proton transfer within substituted benzophenones/N,N-dimethylaniline contact radical ion pairs. The reactions were investigated in the solvents cyclohexane, benzene, and dimethylformamide. The correlation of the reaction rates with the change in free energy reveals that the reaction pathway corresponds to a nonadiabatic process, that is the reaction proceeds by proton tunneling. In nonpolar solvents, an “inverted region” is observed in the proton-transfer process.

Photoacoustic calorimetry was used to quantify the antiaromaticity of 1,3-cyclobutadiene (CBD) by measuring the heat release accompanying its formation via photofragmentation of a polycyclic precursor. In combination with quantum yield measurements and thermochemical calculations, this measurement provides an enthalpy of formation for CBD of 114 +/- 11 (2final sigma) kilocalories per mole (kcal/mol). The extraordinary reactivity of this prototypical antiaromatic hydrocarbon had previously made its heat of formation inaccessible except by theoretical calculations. Relative to a hypothetical strainless, conjugated diene reference, CBD is destabilized by a total of 87 kcal/mol, 32 kcal/mol of which can be attributed to ring strain and 55 kcal/mol to antiaromaticity (compared with 21 kcal/mol for the aromatic stabilization of benzene). Relative to a reference with isolated double bonds, CBD's antiaromaticity is 48 kcal/mol (compared with 32 kcal/mol for the aromaticity of benzene).

The dynamics of proton transfer within a variety of substituted benzophenones/N,N-dimethylaniline contact radical ion pairs are examined in a wide range of solvent polarities. The correlation of the rate constants with the thermodynamic driving force reveals both a normal and inverted region for proton transfer in solvents with an E T 30 value of less than 43.1; in solvents with E T 30 greater than 43.8, only the normal region is observed. Also, the kinetic deuterium isotope effect is examined. The solvent and isotope dependence for the transfer process is examined within the context of the Lee-Hynes model for nonadiabatic proton transfer. The theoretical analysis of the experimental data suggests that the reaction path for proton transfer involves tunneling. Conventional transition state theory with the inclusion of tunneling in the region of the transition state cannot account for the observed kinetic behavior.

Time-resolved photoacoustic calorimetry is a new experimental technique that measures the dynamics of enthalpy changes on the time scale of nanoseconds to microseconds for reactions initiated by absorption of light. When the reaction is carried out in water, it is also possible to obtain the dynamics of the corresponding volume changes. This method has been applied to a variety of biochemical, organic, and organometallic reactions.

For the past 60 years, the framework for understanding the kinetic behavior of proton transfer has been transition state theory. Found throughout textbooks, this theory, along with the Bell tunneling correction, serves as the standard model for the analysis of proton/hydrogen atom/hydride transfer. In comparison, a different theoretical model has recently emerged, one which proposes that the transition state occurs within the solvent coordinate, not the proton transfer coordinate, and proton transfer proceeds either adiabatically or nonadiabatically toward product formation. This Account discusses the central tenets of the new theoretical model of proton transfer, contrasts these with the standard transition state model, and presents a discrepancy that has arisen between our experimental studies on a nonadiabatic system and the current understanding of proton transfer. Transition state theory posits that in the proton transfer coordinate, the proton must surmount an electronic barrier prior to the formation of the product. This process is thermally activated, and the energy of activation is associated with the degree of bond making and bond breaking in the transition state. In the new model, the reaction path involves the initial fluctuation of the solvent, serving to bring the reactant state and the product state into resonance, at which time the proton is transferred either adiabatically or nonadiabatically to form the product. If this theory is correct, then all of the deductions derived from the standard model regarding the nature of the proton transfer process are called into question. For weakly hydrogen-bonded complexes, two sets of experiments are presented supporting the proposal that proton transfer occurs as a nonadiabatic process. In these studies, the correlation of rate constants to driving force reveals both a normal region and an inverted region for proton transfer. Yet, the experimentally observed kinetic behavior does not align with the recent theoretical formulation for nonadiabatic proton transfer, underscoring the gap in the collective understanding of proton transfer phenomena.

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