On the basis of ab initio molecular orbital calculations of individual cofactor molecules, we have obtained estimations of intermolecular electron transfer and spin-exchange integrals between cofactor molecules in photosystem I. These calculations are based on the spatial arrangement of the cofactor molecules in photosystem I of Thermosynechococcus elongatus. The results of these calculations are used in discussion of the influence of the protein environment upon interactions between cofactor molecules in the early stages of electron transfer and directionality of photo-induced electron transfer in photosystem I. (C) 2004 Elsevier B.V. All rights reserved.