Photosynthesis is the process by which plants and cyanobacteria convert light to chemical energy. Photosystem I and photosystem II are the two photosynthetic reaction centers that perform light-driven electron transport across thylakoid membranes. Using photon energy, photosystem I oxidizes plastocyanin, transfers the electron through a series of cofactors, and finally reduces ferredoxin. Photosystem I, consists of 12 or more proteins, 96 chlorophyll alpha molecules, 22 beta-carotenes, two phylloquinones, two lipid molecules, and three iron-sulfur clusters. PsaA and PsaB, the largest subunits, surround the electron transfer chain cofactors with a two-fold symmetry. These cofactors are: P700, the primary electron donor chlorophylls, another pair of molecules of Chlalpha (A0), two phylloquinone molecules (A1) and the iron-sulfur cluster Fx. A0 and A1 are symmetrically arranged in pairs, thus forming two branches. The directionality of electrons down one or both branches has been questioned. P700, a dimer of chlorophylls may play an important role in the determination of the electron directionality since P700 itself exhibits asymmetry, both in structure and in interaction with the protein. This asymmetry may influence the electron spin distribution over the chlorophylls.;P700 is a heterodimer of two chemically different chlorophyll alpha molecules. The chlorophyll molecule coordinated by PsaA is the C13 2 epimer of chlorophyll alpha. PsaB coordinates the other half of P700. There are five hydrogen bonds between PsaA and Chlalpha' (some through a water molecule) that are absent on the other half of the dimer. To understand the importance of protein environment to the biophysical properties of P700, the hydrogen bonding residues in PsaA and the homologous positions in PsaB were altered using site-directed mutagenesis of the psaA and psaB genes of Synechocystis sp. PCC 6803. Fourier transform infrared (FTIR) spectroscopy was used to study the protein-cofactor binding interaction in P700. The electron spin density distribution over P700+ was determined by electron nuclear double resonance spectroscopy (ENDOR). Our results show that changes of the hydrogen bonding residues in PsaA and the respective amino acids in PsaB, has not only altered the bonding pattern between P700 and the protein but has affected its electronic structure as well.