No evidence from FTIR difference spectroscopy that aspartate-342 of the D1 polypeptide ligates a Mn ion that undergoes oxidation during the S0 to S1, S1 to S2, or S2 to S3 transitions in photosystem II

In the recent X-ray crystallographic structural models of photosystem II, Asp342 of the D1 polypeptide is assigned as a ligand of the oxygen-evolving Mn-4 cluster. To determine if D1-Asp342 ligates a Mn ion that undergoes oxidation during one or more of the S-0 -> S-1, S-1 -> S-2, and S-2 -> S-3 transitions, the FTIR difference spectra of the individual S state transitions in D1-D342N mutant PSII particles from the cyanobacterium Synechocystis sp. PCC 6803 were compared with those in wild-type PSII particles. Remarkably, the data show that the mid-frequency (1800-1200 cm(-1)) FTIR difference spectra of wild-type and D1-D342N PSII particles are essentially identical. Importantly, the mutation alters none of the carboxylate vibrational modes that are present in the wild-type spectra. The absence of significant mutation-induced spectral alterations in D1-D342N PSII particles shows that the oxidation of the Mn-4 cluster does not alter the frequencies of the carboxylate stretching modes of D1-Asp342 during the S-0 -> S-1, S-1 -> S-2, or S-2 -> S-3 transitions. One explanation of these data is that D1-Asp342 ligates a Mn ion that does not increase its charge or oxidation state during any of these S state transitions. However, because the same conclusion was reached previously for D1-Asp170, and because the recent X-ray crystallographic structural models assign D1-Asp170 and D1-Asp342 as ligating different Mn ions, this explanation requires that (1) the extra positive charge that develops on the Mn-4 cluster during the S-1 -> S-2 transition be localized on the Mn ion that is ligated by the alpha-COO- group of D1-Ala344 and (2) any increase in positive charge that develops on the Mn-4 cluster during the S-0 -> S-1 and S-2 -> S-3 transitions be localized on the one Mn ion that is not ligated by D1-Asp170, D1-Asp342, or D1-Ala344. In separate experiments that were conducted with L-[1-C-13]alanine, we found no evidence that D1-Asp342 ligates the same Mn ion that is ligated by the alpha-COO- group of D1-Ala344.