Identification of the special pair chlorophyll of photosystem II in Acaryochloris marina by means of FTIR spectroscopy

Abstract

The cyanobacterium Acaryochloris marina has Chld as a major pigment and contains a small amount of Chla. Whether the special pair chlorophyll (P) of PSII in A. marina is formed by Chld or Chla remains controversial. In this study, we have identified chlorophyll species of P in A. marina using FTIR spectroscopy. Light-induced FTIR difference spectrum upon P oxidation was measured for purified PSII core complexes of A. marina and compared with the spectra of P680 in Synechocystis 6803 and spinach, which consists of Chla. The chlorin ring vibrations of A. marina showed clear differences from those of Synechocystis and spinach, indicating that P of A. marina is formed by Chld. The feature of keto C=O bands of P⁺ was similar to that of P680⁺, suggesting that the charge distribution on P⁺ is also similar. It was thus proposed that P of A. marina is a homo-dimer of Chld molecules.