Abstract
A discrepancy is explored regarding the spin exchange integral estimated from magnetic field effects on recombination of the bacteriochlorophyll dimer cation (Bchl)(2)+ and the bacteriopheophytin anion Bph(-) and the measured electron transfer rate between Bph and electronically excited (Bchl)(2). In one explanation considered here there are two sites for the electron or for the hole, with a hopping between sites.