Fluctuational tunnel effects and vibrational dispersion in primary electron transfer processes of bacterial photosynthesis

Abstract

We have investigated forward electron transfer (ET) from reduced bacteriopheophytin (bph −) to the primary quinone (Q A), and the recombination ET from Q A − to the oxidized special pair (bch) 2 + in the photosynthetic reaction centre of Rh. sphaeroides. The same reactant, Q A, is here engaged in two ET reactions of widely different distances. The processes can also be followed over broad temperature ranges. Focus is on two elements not commonly considered in ET theory. One is continuous environmental dielectric dispersion which is both formally and physically important. The other one is environmental fluctuational modulation of the tunnel factor which is important when the donor and acceptor are separated by environmental matter. The analysis shows, first that a broad dielectric dispersion is in line with the ET data up to ≈180°K. Fluctuational modulation of the tunnel factor is, secondly, unimportant in shorter-range forward ET from bph − to Q A (13 Å centre-to-centre distance) but could be important in the much longer-range recombination ET from Q A − to (bch) 2 + (28 Å).