Ephraim Buhks

Charge transfer in polypyrrole

Experimental electrical conductivity and optical absorption data are presented for fully oxidized polypyrrole. A charge transfer theory is described which gives an excellent account of the temperature dependence, carrier density dependence, and magnitude of electrical conductivity. The reorganization energy calculated from the temperature dependence of conductivity, 0.23 eV, is in good agreement with the value obtained from the near IR dielectric loss spectrum, ∼0.25 eV. Self‐consistent hopping distances of about 5–10 A are obtained from the reorganization energy, conductivity preexponential factor, and carrier density. Hopping between localized sites associated with the counteranion is indicated.

Effects of medium modes on electron transfer in a biological system

Abstract In this paper we report the results of model calculations on the temperature dependence of the electron transfer (ET) rate from cytochrome C to the reaction center in the photosynthetic bacterium chromatium. The effects of both polar medium phonons and of molecular vibrations of the electron donor and the acceptor center were incorporated within the framework of a nonadiabatic multiphonon description of the ET process. The nuclear parameters required to account for the temperature dependence are in accord with the available physical and chemical information regarding this system, while the large electronic coupling indicates the possible role of intermediate states in the ET process.

Activationless electron transfer processes in biological systems

Electron transfer (ET) processes between ions in solutions [ 11, as well as in biological systems [2,3], can adequately be described within a unified theoretical framework in terms of non-adiabatic multiphonon non-radiative processes [2-41. A central prediction of the theory [2-41 is that at sufficiently low temperatures the ET rate is constant and independent of temperature manifesting the effects of nuclear tunnelling, while at high temperatures the rate exhibits the conventional Arrehnius-type activated temperature dependence. This pattern has been observed for the DeVault-Chance ET process between cytochrome and the reaction centre in chromatium [ 51. Experimental studies of the primary charge separation steps in bacterial photosynthesis [6,7] have established that several of the elementary ET processes are temperature independent over a wide temperature range [8]. The ET process between bacteriopheophtin (BPh) and ubiquionone10 (Q) [ 6-81:

The dynamics of carbon monoxide binding to hemoglobin and to cytochrome

In this paper we apply the theory of nonadiabatic multiphonon group transfer processes to describe the low temperature recombination reaction between CO and hemoglobin (CO–Hb) and between CO and carboxylmethyl cytochrome c (CO–cmcytc). The CO–Hb recombination is accompanied by an electronic transition from an S=2 high spin ‘‘free’’ state to an S=0 low open ‘‘bound’’ state, with the iron heme moving from a domed‐type structure into a planar structure, while the characteristic low frequency of relevant vibration mode(s) involving the heme iron coupled to the protein (〈ω〉∼50 cm−1). The CO‐cmcytc recombination characterized by an electronic transition from an S=1 spin free state to an S=0 bound state, with the iron heme ring staying planar and configurational frequencies prevail for a high‐frequency (ωc∼400 cm−1) in‐plane iron‐porphyrin vibrational mode. The distinct electronic and nuclear parameters for these two recombination processes account for the large difference in the onset of the temperature depende...

Low temperature recombination of CO with haemoglobina

Abstract In this paper we apply the theory of non-adiabatic multiphonon group transfer processes to describe the low-temperature recombination reaction between CO and haemoglobin (COHb). The COHb recombination is accompanied by an electronic transition from an S = 2 high spin “free” state to an S = 0 low spin “bound” state, with the iron heme moving from a domed-type structure into a planar structure, while the characteristic low frequency of the relevant vibrational mode(s) involving the heme iron coupled to the protein is ≅ 50 cm−1. We were able to provide a quantitative account of the temperature dependence of the recombiantion rate over the temperature range 2–40 K, which exhibits a continuous transition from nuclear tunnelling to an activated process. These low temperature kinetic data allow for the exploration of microscopic mechanisms of biological reactions.