Is the A_-1 Pigment in Photosystem I Part of P700? A (P700⁺-P700) FTIR Difference Spectroscopy Study of A_-1 Mutants

The involvement of the second pair of chlorophylls, termed A_-1A and A_-1B, in light-induced electron transfer in photosystem I (PSI) is currently debated. Asparagines at PsaA600 and PsaB582 are involved in coordinating the A_-1B and A_-1A pigments, respectively. Here we have mutated these asparagine residues to methionine in two single mutants and a double mutant in PSI from Synechocystis sp. PCC 6803, which we term NA600M, NB582M, and NA600M/NB582M mutants. (P700⁺-P700) FTIR difference spectra (DS) at 293 K were obtained for the wild-type and the three mutant PSI samples. The wild-type and mutant FTIR DS differ considerably. This difference indicates that the observed changes in the (P700⁺-P700) FTIR DS cannot be due to only the P_A and P_B pigments of P700. Comparison of the wild-type and mutant FTIR DS allows the assignment of different features to both A_-1 pigments in the FTIR DS for wild-type PSI and assesses how these features shift upon cation formation and upon mutation. While the exact role the A_-1 pigments play in the species we call P700 is unclear, we demonstrate that the vibrational modes of the A_-1A and A_-1B pigments are modified upon P700⁺ formation. Previously, we showed that the A_-1 pigments contribute to P700 in green algae. In this manuscript, we demonstrate that this is also the case in cyanobacterial PSI. The nature of the mutation-induced changes in algal and cyanobacterial PSI is similar and can be considered within the same framework, suggesting a universality in the nature of P700 in different photosynthetic organisms.