Evidence for hydrogen bond formation to the PsaB chlorophyll of P700 in photosystem I mutants of Synechocystis sp. PCC 6803.

P700, the primary electron donor of photosystem I, is an asymmetric dimer made of one molecule of chlorophyll a' (P A ) and one of chlorophyll a (P B ) that are bound to the homologous PsaA and PsaB polypeptides. While the carbonyl groups of P A are involved in hydrogen-bonding interactions with several surrounding amino acid side chains and a water molecule, P B does not engage hydrogen bonds with the protein. Notably, the residue Thr A739 is donating a strong hydrogen bond to the 9-keto C=O group of P A and the homologous residue Tyr B718 is free from interaction with P B . Light-induced FTIR difference spectroscopy of the photooxidation of P700 has been combined with a site-directed mutagenesis attempt to introduce hydrogen bonds to the carbonyl groups of P B in Synechocystis sp. PCC 6803. The FTIR study of the Y(B718)T mutant provides evidence that the 9-keto C=O group of P B and P B + engages a relatively strong hydrogen-bonding interaction with the surroundings in a significant fraction (40 ′ 10%) of the reaction centers. Additional mutations on the two PsaB residues homologous to those involved in the main interactions between the PsaA polypeptide and the 10a-carbomethoxy groups of P A affect only marginally the vibrational frequency of the 10a-ester C=O group of P B . The FTIR data on single, double, and triple mutants at these PsaB sites indicate a plasticity of the interactions of the carbonyl groups of P B with the surrounding protein. However, these mutations do not perturb the hydrogen-bonding interactions assumed by the 9-keto and 10a-ester C=O groups of P A and P A + with the protein and have only a limited effect on the relative charge distribution between P A + and P B + .