The secondary electron acceptor in Photosystem I (PS I) is called A1. The A1 binding site in the PS I particles is occupied by a phylloquinone (PhQ) molecule (2-methyl, 3-phytyl, 1, 4-naphthoquinone). In menB mutants, a plastoquinone-9 (PQ9) molecule occupies the A1 binding site instead of PhQ. Using menB mutant PS I particles, it has been shown that it is possible to replace PQ9 in the A1 site with PhQ, by incubating the isolated particles in a large molar excess of PhQ.
To probe the molecular properties of PhQ and its environment in both the neutral and reduced state in PS I, we have been using time-resolved step-scan FTIR difference spectroscopy (TRSS FTIR DS). We have produced time-resolved A1-/A1 FTIR DS using menB mutant PS I particles in which PhQ has been reintroduced into the A1 binding site. Also we have obtained time-resolved A1-/A1 FTIR difference spectra for menB PS I particles that are globally 13C labeled where 12C labeled PhQ was incorporated into the A1 binding site. By introducing 12C labeled PhQ into 13C labeled PS I, we are able to identify the bands in A1- /A1 FTIR DS that are due to the carbonyl (C=O) modes of neutral and reduced PhQ.