A Fourier transform infrared absorption difference spectrum associated with the reduction of A1 in photosystem I: are both phylloquinones involved in electron transfer?

Abstract

Photoaccumulated Fourier transform infrared difference spectra associated with P700(+) and P700(+)A(1)(-) formation have been obtained using purified photosystem I particles from Synechocystis sp. PCC 6803. From these spectra, a difference spectrum associated with phylloquinone reduction (A(1)(-) - A(1)) has been calculated. Infrared absorption changes associated with both the loss of the ground state and formation of the anion radical are observed in the difference spectrum. Fourier transform infrared difference spectra obtained in various spectral regions indicate that two, structurally distinct phylloquinones are photoaccumulated. This could indicate that phylloquinones on both the PsaA and PsaB branches are involved in electron transfer, and that electron transfer is bi-directional in photosystem I. It could also indicate an intrinsic structural heterogeneity in the A(1) binding site of the active branch. Several FTIR difference features taken together indicate that a glutamic acid residue (at position 699 or 702 on PsaA and/or 679 or 682 on PsaB) is perturbed upon A(1) anion formation. It is suggested that the protonation state of the perturbed glutamic acid residue is influenced by hydrogen bonding to a nearby tyrosine residue at position 696/676 on PsaA/PsaB.