Photo-Oxidation of P740, the Primary Electron Donor in Photosystem I from Acaryochloris marina

Abstract

Fourier transform infrared spectroscopy (FTIR) difference spectroscopy in combination with deuterium exchange experiments has been used to study the photo-oxidation of P740, the primary electron donor in photosystem I from Acaryochloris marina. Comparison of (P740 +-P740) and (P700 +-P700) FTIR difference spectra show that P700 and P740 share many structural similarities. However, there are several distinct differences also: 1), The (P740 +-P740) FTIR difference spectrum is significantly altered upon proton exchange, considerably more so than the (P700 +-P700) FTIR difference spectrum. The P740 binding pocket is therefore more accessible than the P700 binding pocket. 2), Broad, “dimer” absorption bands are observed for both P700 + and P740 +. These bands differ significantly in substructure, however, suggesting differences in the electronic organization of P700 + and P740 +. 3), Bands are observed at 2727(−) and 2715(−) cm −1 in the (P740 +-P740) FTIR difference spectrum, but are absent in the (P700 +-P700) FTIR difference spectrum. These bands are due to formyl CH modes of chlorophyll d. Therefore, P740 consists of two chlorophyll d molecules. Deuterium-induced modification of the (P740 +-P740) FTIR difference spectrum indicates that only the highest frequency 13 3 ester carbonyl mode of P740 downshifts, indicating that this ester mode is weakly H-bonded. In contrast, the highest frequency ester carbonyl mode of P700 is free from H-bonding. Deuterium-induced changes in (P740 +-P740) FTIR difference spectrum could also indicate that one of the chlorophyll d 3 1 carbonyls of P740 is hydrogen bonded.