Electron transfer following charge separation in the photosystem I (PSI) reaction center of Synechococcus elongatus is studied using theoretical methods. The difference in rate between two almost symmetrical A- and B-branches is caused by a difference in a single residue (Trp B673 versus Gly A693), close to the F-X iron-sulfur cluster. Partly due to its polar environment, Trp B673 acts as an electron acceptor in its pi-system. The rate increases on the B-side due to shortened distances for electron transfer. (C) 2003 Elsevier Science B.V. All rights reserved.