Kevin S. Peters

Primary intermediates in the photochemical cycle of bacteriorhodopsin

Picosecond studies of the primary photochemical events in the light-adapted bacteriorhodopsin, bR570, indicate that the first metastable intermediate K610 is formed with a rise time of 11 ps. Difference spectra obtained at 50 ps after excitation show that K610 is the same species as that trapped in low temperature glasses. A precursor species (S) of the K610 intermediate has been observed which is red shifted with respect to K610 and is formed within the 6-ps time width of the excitation pulse. The formation of the precursor has no observable thermal dependence between 298 degrees and 1.8 degrees K. The formation of K610 has a very low thermal barrier and at very low temperatures, the rate of formation becomes practically temperature independent which is characteristic of a tunneling process. The rate of formation becomes practically temperature independent which is characteristic of a tunneling process. The rate of formation of K610 has a moderate deuterium isotope effect of kH/kD approximately 1.6 at 298 degrees K and 2.4 at 4 degrees K. The mechanism for formation of K610 is found to involve a rate-limiting proton transfer which occurs by tunneling at low temperatures.

Picosecond dynamics of primary electron-transfer processes in bacterial photosynthesis.

The primary electron transfer processes in Rhodopseudomonas sphaeroides R-26 were studied as a function of temperature by means of picosecond spectroscopy. The first chemical step of the bacterial photosynthesis involves an electron transfer from the excited state of a bacteriochlorophyll a dimer, (BChl)2, to a bacteriopheophytin (BPh) to form the radical ion pair (BChl)2+. BPh-.. The upper limit for the formation time of this ion-pair was found to be 10 ps, at temperatures in the range 300-4.2 degree K. Similarly, the second chemical step, involving electron transfer from BPh-. to an ubiquinone-iron complex (QFe), was found to have a lifetime of approximately 150 ps, also independent of temperature in the same range. We interpret the absence of temperature dependence as indicating that process 2 proceeds via a tunneling mechanism. Utilizing our results in conjunction with electron tunneling theories, we calculate the distance between BPh-. and Q(Fe) to be 9--13 A. Our results also imply a closer proximity between (BChl)2 and BPh.

Application of time-resolved photoacoustic calorimetry to CrL bond enthalpies in Cr(CO)5L

Using time-resolved photoacoustic calorimetry, the CrL bond enthalpies in Cr(CO)5L for a variety of ligands have been measured. Also, the enthalpy and entropy of activation for the displacement of heptane in Cr(CO)5(heptane) by pyridine and 2-picoline have been determined.

A photoacoustic calorimetry study of horse carboxymyoglobin on the 10-nanosecond time scale

The development of a photoacoustic calorimeter with a time resolution of 10 ns is presented, and the dynamics of the enthalpy and volume changes found in the photodissociation of CO from horse carboxymyoglobin are examined. With this enhanced time resolution a new transient species, the lifetime of which is 29 ns at 20 degrees C, is observed in the ligand dissociation process.

A photoacoustic calorimetric study of horse myoglobin

The dynamics of the enthalpy and volume changes for the photodissociation of CO from horse myoglobin has been studied by time-resolved photoacoustic calorimetry which measures the dynamics of enthalpy and volume changes on the nanosecond time scale. The role of the Lys 45 salt bridge in the ligand dissociation is discussed.

Strength of the metal-ligand bond in LCr(CO)5 measured by photoacoustic calorimetry

Abstract The Cr-L bond strengths for Cr(CO)5L in heptane have been determined using photoacoustic calorimetry. For L = CO, PBu3, CH3CN and THF, ΔHBDE = 27.0, 21.6, 18.2, 12.4 kcal mol respectively. Comparison of the Cr-CO bond dissociation energies in the gas phase and in heptane solution suggests a 10 kcal mol stabilization of Cr(CO)5 by coordinated heptane.

The sub-picosecond dynamics of diphenylmethylchloride ion pairs and radical pairs

Abstract The dynamics for the photoinduced homolysis and heterolysis of diphenylmethylchloride in acetonitrile and cyclohexane are examined on the 100 fs time scale. In acetonitrile, the radical pair and the ion pair are formed in 345 and 833 fs, respectively. The radical pair is then observed to decay into the ion pair on the 15 ps time scale, a kinetic process which is found to be time-dependent. In cyclohexane, the radical pair and ion pair are formed within 230 fs, and the ion pair is observed to decay into the radical pair on the 5 ps time scale.

Picosecond Kinetic Study of the Photoinduced Homolysis of Benzhydryl Acetates: The Nature of the Conversion of Radical Pairs into Ion Pairs

The conversion of benzhydryl acetate geminate radical pairs to contact ion pairs following photoinduced homolysis in solution is studied using picosecond pump-probe spectroscopy. The dynamics for the decay of the geminate radical pairs into contact ion pairs is modeled within a Marcus-like theory for nonadiabatic electron transfer. A second decay channel for the geminate radical pairs is diffusional separation to free radicals. The kinetics of this latter process reveals an energy of interaction between the two radicals in the geminate pair.

Thin-layer microcalorimetric studies of oxygen and carbon monoxide binding to hemoglobin and myoglobin

A thin-layer gas-solution microcalorimeter has been developed to study the binding reactions of gaseous ligands with ligand binding macromolecules. We have measured the enthalpy of binding oxygen and carbon monoxide to horse myoglobin, human hemoglobin A0 and sperm whale myoglobin in phosphate buffer at pH 7.6, with the enzyme reducing system of Hayashi. Reactions of human hemoglobin were also done under various buffer conditions in order to elucidate the Bohr effect. These binding reactions were found not to exhibit a detectable enthalpy change over the temperature range of 10 degrees C to 25 degrees C. The enzyme reducing system was shown to react with oxygen in a manner that releases a substantial amount of heat. This problem was corrected by using a minimum amount and by placing the buffer and enzyme system in the reference cell effectively cancelling the oxygen enzyme reaction heat as well as the heat of gas dissolution. It was also demonstrated that glucose-6-phosphate, one of the reducing system components, in 50 mM concentrations can influence the heat of binding oxygen and carbon monoxide to hemoglobin. This effect was shown to be absent in the myoglobins and also with hemoglobin at glucose-6-phosphate concentrations less than 5 mM.

Time-Resolved Photoacoustic Calorimetry of Carbenes and Biradicals

To develop an understanding of the reactivity of biradicals and carbenes, it is essential that the energetics of the reactants, intermediates, and products be established. The most successful approach to date for determining the energetics of intermediates, such as radicals, biradicals, and carbenes, has been to estimate their energies through a combination of Benson group additivities(1) and bond enthalpy data.(2) Regrettably, this data base is not sufficiently extensive to allow for the calculation of the energetics for most intermediates of interest. For example, little is known regarding the enthalpic contributions of substituents at a radical center. Also, there is no satisfactory method for estimating the energetics of carbenes and biradicals. Finally, it is not clear how reliable even the best calculation may be for reactive intermediates in the condensed phase as solvent interactions may make significant contributions to the overall energetics. Clearly, it would be advantageous to have experimental methods that could directly probe the energies of transient species.