Fluorescence detected magnetic resonance (FDMR) spectroscopy of chlorophyll-proteins from barley

Abstract

Fluorescence detected magnetic resonance (FDMR) spectra and fluorescence emission spectra at 4.2 K of chlorophyll-proteins isolated and purified from barley thylakoids are presented. The FDMR spectra show the occurrence of chlorophyll a triplet states in all five chlorophyll-proteins studied, namely Chla -P1, Chla -P2, Chla -P3, Chla/b -P1 and Chla/b -P2. The presence of more than one chlorophyll triplet each associated with a chlorophyll emitting at a specific wavelength gives rise to a characteristic wavelength dependence of the FDMR spectrum of chlorophyll-proteins. The zero field splitting parameters measured, combined with the observed fluorescence emission wavelengths suggest that three types of interactions of the Mg atom of chlorophyll a occur in these proteins: a type similar to that in the parallel dimer (Chl a·H2O)2, seen at 721 nm for Chla -P1 leading to a positive FDMR signal; a type like that in Chl a· 2 pyridine also giving a positive FDMR signal, seen in Chla -P2 and Chla -P3; and a third type similar to that in Chl a· 2H2O leading to a negative FDMR signal, seen for Chla -P1 at 679 nm, and for Chla/b -P1 and Chla/b -P2. The FDMR spectrum in the antenna of photosystem I (Chla -P1) can probably be ascribed to that of a trap formed by a pair of interacting chlorophyll a molecules, indicating that the organisation of chlorophyll in the antenna may not in principle be very different from that in the photosystem I reaction centre, and that it contains approximately plane-parallel chlorophyll a pairs. Chla -P2 and Chla -P3 do not show a long wavelength (>700 nm) emission, suggesting a much weaker interaction between chlorophyll molecules in these proteins compared to that in Chla -P1. For Chla/b -P1 and Chla/b -P2 the absence of a long wavelength emission and the observation of zero field splitting (ZFS) parameters similar to that of monomeric Chl a·2H2O both indicate the absence of strong interactions between chlorophyll a molecules in these proteins also, and it is suggested that chlorophyll a and chlorophyll b molecules occur in interacting pairs. © 1981 Carlsberg Laboratory.