FTIR spectroscopy of synechocystis 6803 mutants affected on the hydrogen bonds to the carbonyl groups of the PsaA chlorophyll of P700 supports an extensive delocalization of the charge in P700+.

Abstract

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)). 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 in hydrogen bonding with the protein. Light-induced FTIR difference spectroscopy of the photooxidation of P700 has been combined with site-directed mutagenesis in Synechocystis sp. PCC 6803 to investigate the influence of these hydrogen bonds on the structure of P700 and P700(+). When the residue Thr A739, which donates a hydrogen bond to the 9-keto C=O group of P(A), is changed to Phe, a differential signal at 1653(+)/1638(-) cm(-1) in the P700(+)/P700 FTIR difference spectrum upshifts by approximately 30-40 cm(-1), as expected for the rupture of the hydrogen bond or, at least, a strong decrease of its strength. The same upshift is also observed in the FTIR spectrum of a triple mutant in which the residues involved in the three main hydrogen bonds to the 9-keto and 10a-carbomethoxy groups of P(A) have been changed to the symmetry-related side chains present around P(B). In addition, the spectrum of the triple mutant shows a decrease of a differential signal around 1735 cm(-1) and the appearance of a new signal around 1760 cm(-1). This is consistent with the perturbation of a bound 10a-ester C=O group that becomes free in the triple mutant. All of these observations support the assignment scheme proposed previously for the carbonyls of P700 and P700(+) [Breton, J., Nabedryk, E., and Leibl, W. (1999) Biochemistry 38, 11585-11592] and therefore reinforce our previous conclusions that the positive charge in P700(+) is largely delocalized over P(A) and P(B).