Coupling of protein modes to electron transfer in bacterial photosynthesis

Mordechai Bixon, Joshua Jortner*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


This paper explores the contribution of the protein medium phonon modes to the Franck-Condon nuclear overlap factors, which determine the rates of electron-transfer (ET) reactions in the reaction centers of photosynthetic bacteria. From the analysis of the temperature dependence and some free energy relationships for the quinone reduction reaction and for the back recombination between the quinone and the primary donor, it is concluded that the average vibrational frequency involved in these two activationless ET processes is ℏω = 100 cm-1, whereupon the dominant contribution to the electron-phonon coupling originates from the exterior protein modes while the contribution of intramolecular vibrations of the prosthetic groups is minor. It is proposed that the unique temperature dependence of the Chance-DeVault cytochrome oxidation reaction in Chromatium is not due to a transition from low-temperature nuclear tunneling to a high-temperature activated ET, but rather originates from two parallel ET processes from two distinct low-potential cytochromes to the dimer cation. These involve a slow activationless process, which dominates at low temperatures (T ≤ 120 K) and an activated process, which is practically exclusive at high temperatures. This conjecture provides plausible nuclear and electronic coupling terms for the two cytochrome oxidation reactions, which are in full accord with the quantitative features of other ET processes in the reaction center.

Original languageEnglish
Pages (from-to)3795-3800
Number of pages6
JournalJournal of Physical Chemistry
Issue number16
StatePublished - 1986


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