The primary electron donor of photosystem I (PS1), called P 700 , is a heterodimer of chlorophyll (Chl) a and a′. The crystal structure of photosystem I reveals that the chlorophyll a′ (P A ) could be hydrogenbonded to the protein via a threonine residue, while the chlorophyll a (P B ) does not have such a hydrogen bond. To investigate the influence of this hydrogen bond on P 700 , PsaA-Thr739 was converted to alanine to remove the H-bond to the 13 1 -keto group of the chlorophyll a′ in Chlamydomonas reinhardtii. The PsaA-T739A mutant was capable of assembling active PS1. Furthermore the mutant PS1 contained approximately one chlorophyll a′ molecule per reaction center, indicating that P 700 was still a Chl a/a′ heterodimer in the mutant. However, the mutation induced several band shifts in the visible P 700 + -P 700 absorbance difference spectrum. Redox titration of P 700 revealed a 60 mV decrease in the P 700 /P 700 + midpoint potential of the mutant, consistent with loss of a H-bond. Fourier transform infrared (FTIR) spectroscopy indicates that the ground state of P 700 is somewhat modified by mutation of ThrA739 to alanine. Comparison of FTIR difference band shifts upon P 700 + formation in WT and mutant PS1 suggests that the mutation modifies the charge distribution over the pigments in the P 700 + state, with ∼14-18% of the positive charge on P B in WT being relocated onto P A in the mutant. 1 H-electron-nuclear double resonance (ENDOR) analysis of the P 700 + cation radical was also consistent with a slight redistribution of spin from the P B chlorophyll to P A , as well as some redistribution of spin within the P B chlorophyll. High-field electron paramagnetic resonance (EPR) spectroscopy at 330-GHz was used to resolve the g-tensor of P 700 + , but no significant differences from wild-type were observed, except for a slight decrease of anisotropy. The mutation did, however, provoke changes in the zero-field splitting parameters of the triplet state of P 700 ( 3 P 700 ), as determined by EPR. Interestingly, the mutation-induced change in asymmetry of P 700 did not cause an observable change in the directionality of electron transfer within PS1.In higher plants and green algae, the photosynthetic reaction occurs within two large pigment-protein complexes located in the thylakoid membranes of the chloroplast: photosystems I and II (PS1 and PS2). 1 These two systems exemplify the type 1 ("iron-sulfur") and type 2 ("quinone") reaction centers, respectively, based on their terminal electron acceptors. In oxygenic phototrophs, PS2 and PS1 act in tandem to oxidize water and reduce NADP + , but there are several anoxygenic photosynthetic bacteria that use only one type of reaction center protein.All known reaction centers share a similar structural motif with a core of reaction centers composed of two similar or identical integral membrane subunits, to which the various redox cofactors are bound. Both the protein structural motifs and the cofactor arrangements are characterized by a pseudo-C 2 symmetry ...