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 chlorophylla triplet states in all five chlorophyll-proteins studied, namely Chl a -P1, Chl a -P2, Chl a -P3, Chl a/b -P1 and Chl a/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 chlorophylla occur in these proteins: a type similar to that in the parallel dimer (Chla·H2O)2, seen at 721 nm for Chl a -P1 leading to a positive FDMR signal; a type like that in Chla· 2 pyridine also giving a positive FDMR signal, seen in Chl a -P2 and Chl a -P3; and a third type similar to that in Chla· 2H2O leading to a negative FDMR signal, seen for Chl a -P1 at 679 nm, and for Chl a/b -P1 and Chl a/b -P2. The FDMR spectrum in the antenna of photosystem I (Chl a -P1) can probably be ascribed to that of a trap formed by a pair of interacting chlorophylla 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 chlorophylla pairs. Chl a -P2 and Chl a -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 Chl a -P1. For Chl a/b -P1 and Chl a/b -P2 the absence of a long wavelength emission and the observation of zero field splitting (ZFS) parameters similar to that of monomeric Chla·2H2O both indicate the absence of strong interactions between chlorophylla molecules in these proteins also, and it is suggested that chlorophylla and chlorophyllb molecules occur in interacting pairs.