A Fourier transform infrared absorption difference spectrum associated with the reduction of A(1) in photosystem I: Are both phylloquinones involved in electron transfer?

Photoaccumulated Fourier transform infrared difference spectra associated w ith 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 observ ed in the difference spectrum. Fourier transform infrared difference spectr a obtained in various spectral regions indicate that two, structurally dist inct phylloquinones are photoaccumulated. This could indicate that phylloqu inones on both the PsaA and PsaB branches are involved in electron transfer , and that electron transfer is bi-directional in photosystem I. It could a lso indicate an intrinsic structural heterogeneity in the A(1) binding site of the active branch. Several FTIR difference features taken together indi cate that a glutamic acid residue (at position 699 or 702 on PsaA and/or 67 9 or 682 on PsaB) is perturbed upon A(1) anion formation. It is suggested t hat the protonation state of the perturbed glutamic acid residue is influen ced by hydrogen bonding to a nearby tyrosine residue at position 696/676 on PsaA/PsaB.