The recombination after flash photolysis of carbon monoxide (CO) to protoheme (PH) in glycerol: water is studied over ten decades in time (1 ps to 10 ms). The rebinding consists of an initial nonexponential geminate phase followed by a slower exponential bimolecular phase. The entire time course of this reaction between 260 and 300 K can be explained in a unified way using a simple, analytically tractable diffusion model involving just three parameters: the relative diffusion constant, the contact radius, and the intrinsic rate of reaction at contact.

Photoinduced electron transfer in ion pairs where the anion is a tetrasubstituted borate gives a boranyl radical. This species is reactive. When the boranyl radical contains an alkyl group, the carbon-boron bond cleaves in ca. 250 fs. The lifetime of tetraarylboranyl radicals is greater than 40 ps. The short lifetime of boranyl radicals prohibits measurement of the oxidation potentials of borates by electrochemical methods. These quantities were estimated by application of Marcus electron-transfer theory, which relates the energetics to the kinetics of the reaction. We find that the use of the empirical Rehm-Weller equation for this purpose is not appropriate and leads to errors in estimation of both the reorganization energy of the reaction and the oxidation potential of the borates. The oxidation potentials were determined in acetonitrile solution by this approach for two series of borates, (tolyl)n(Ph)knB-, where the tolyl group might be ortho or para substituted and n = 0-4. The data reveal that A, the reorganization energy, is 1 .O eV. The oxidation potentials of the borates vary systematically in the p-tolyl series from 0.8 1 to 1.03 V vs SCE with decreasing number of tolyl groups on the borate. The o-tolyl series shows higher than expected oxidation potentials for the tetra-and tritolyl cases, which may be due to greater steric hindrance to the acceptor for these two borates.

Ultrafast spectroscopy is used to investigate the temperature dependence of a bimolecular chemical reaction occurring at reaction centers embedded in a glycerol:water glass. The reaction centers consist of carbon monoxide bound to protoheme (PH–CO), or to myoglobin at pH=3 (Mb3–CO), a protein containing PH–CO with a broken proximal histidine–Fe bond. These systems have in common a small energetic barrier for rebinding of the photodissociated ligand. In the glass, the ligand is caged, so that only geminate rebinding is possible. Rebinding is not exponential in time. For t≳20 ps, the survival fraction of deligated heme N(t)∝t−n(n≥0). Below 100 K, rebinding is dominated by an inhomogeneous distribution of activation enthalpy P(ΔH‡) and n is temperature dependent. Inhomogeneous means that every site has a unique barrier. Above 150 K, n becomes independent of temperature. In this high temperature limit, the distribution of preexponential factors, attributed to a distribution of activation entropy P(ΔS‡), dominates rebinding. A picosecond two-pulse experiment demonstrates that the entropy distribution is also inhomogeneous. This work is the first study of heme–ligand rebinding in both low and high temperature limits, which allows a direct investigation of the nature of the activation entropy distribution in a glass. Because ligand rebinding in Mb3–CO and PH–CO is similar, despite the existence of a protein in Mb3–CO which provides a larger free volume for the ligand than does PH–CO, it is concluded that the low energetic barrier encourages immediate ligand rebinding and that the ligand does not diffuse far from the rebinding site at low temperature.

No abstract

No abstract