A Reaction-Induced FT-IR Study of Cyanobacterial Photosystem I

In oxygenic photosynthesis, photosystem I (PSI) conducts light-driven electron transfer from plastocyanin to ferredoxin. The reactions are initiated when the primary chlorophyll donor, P700, is photooxidized. P700 is a chlorophyll dimer ligated by the core subunits psaA and psaB. A difference Fourier transform infrared spectrum, associated with P700 +-minus-P700, can be acquired using PSI from the cyanobacterium Synechocystis sp. PCC 6803. This spectrum reflects contributions from oxidation-sensitive modes of chlorophyll, as well as from oxidation-induced structural changes in amino acid residues and the peptide backbone. Oxidation-induced structural changes may play a role in the facilitation and control of electron-transfer reactions involving the primary donor. In this paper, we report that photooxidation of P700 in cyanobacterial PSI perturbs a cysteine residue. At 264 and 80 K, a downshift of a SH stretching vibration from 2560 to 2551 cm-1 is observed. Such a downshift is consistent with an increase in hydrogen bonding, with a change in C−S−H conformation, or with an electric field effect. Deuterium exchange experiments were also performed. While the perturbed cysteine is in a protein region that is resistant to exchange, other 2H-sensitive vibrational chl and amino acid bands were observed. From the 2H exchange experiments, we conclude that photooxidation of P700 perturbs internal or bound water molecules in PSI and that the P700 +-minus-P700 spectrum is 2H exchange-sensitive. The results are consistent with structural complexity in the PSI primary donor, as previously suggested [Kim, S., and Barry, B. A. (2000) J. Am. Chem. Soc. 122, 4980−4981]. Possible explanations, including a partial enolization of P700 +, are discussed.