Fourier transform infrared difference spectroscopy shows no evidence for an enolization of chlorophyll a upon cation formation either in vitro or during P700 photooxidation.

Molecular changes associated with the photooxidation of the primary electron donor P700 in photosystem I from cyanobacteria have been investigated with Fourier transform infrared (FTIR) difference spectroscopy. Highly resolved signals are observed in the carbonyl stretching frequency region of the light-induced FTIR spectra. In order to assign and to interpret these signals, the FTIR spectra of isolated chlorophyll a and pyrochlorophyll a (lacking the 10a-ester carbonyl) in both their neutral and cation states were investigated. Comparison of the redox-induced FTIR difference spectra of these two model compounds demonstrates that upon chlorophyll a cation formation in tetrahydrofuran the 7c-ester carbonyl is essentially unperturbed while the 10a-ester carbonyl is upshifted from 1738 to 1751 cm-1. For the 9-keto group, the shift is from 1693 to 1718 cm-1 in chlorophyll a and from 1686 to 1712 cm-1 in pyrochlorophyll a. The 1718-cm-1 band in the difference spectrum of chlorophyll a is thus unambiguously assigned to the 9-keto carbonyl of the cation. Comparison of the light-induced FTIR difference spectrum associated with the photooxidation of P700 in vivo with the difference FTIR spectrum of chlorophyll a cation formation leads to the assignment of the frequencies of the 9-keto carbonyl group(s) at 1700 cm-1 in P700 and at 1717 cm-1 in P700+.(ABSTRACT TRUNCATED AT 250 WORDS)