Role of intramolecular vibrations in long-range electron transfer between pheophytin and ubiquinone in bacterial photosynthetic reaction centers

¹ Centro Italiano Ricerche Aereospaziali
² University of Salerno

^* To whom correspondence should be addressed. E-mail: apeluso{at}unisa.it.

Submitted on February 3, 2005
Revised on March 21, 2005
Accepted on 4 May 2005

	Abstract

The dynamics of the elementary electron transfer step between pheophytin and primary ubiquinone in bacterial photosynthetic reaction centers is investigated by using a discrete state approach, including only the intramolecular normal modes of vibration of the two redox partners. The whole set of normal coordinates of the acceptor and donor groups have been employed in the computations of the Hamiltonian matrix, in order to reliably account both for shifts and mixing of the normal coordinates, and for changes in vibrational frequencies upon ET. It is shown that intramolecular modes provide not only a discrete set of states strongerly coupled to the initial state but also a quasi-continuum of weakly coupled states, which account for the spreading of the wave packet after ET. The computed transition probabilities are sufficiently high for asserting that electron transfer from bacteriopheophytin to the primary quinone can occur via tunneling solely promoted by intramolecular modes; the transition times, computed for different values of the electronic energy difference and coupling term, are of the same order of magnitude (10² ps) of the observed one.

Key Words: electron transfer, pheophytin, photosynthesis, quantum dynamics, ubiquinone