Resonance Raman spectroscopic study of the chromophores in isolated PSII reaction centres: neutral and cationic species

Abstract

Our aim is to measure the resonance Raman (RR) spectrum of P680, in order to determine the pigment-protein interactions which induce its extremely high oxidising potential. Radical cation states are photoaccumulated when isolated PSII complexes (RC2) are illuminated at low temperature in the presence of added electron acceptor. However, in addition to P680+, which is relatively unstable even at 20K, other more stable secondary electron donors are oxidised i.e. chlorophyll (Chl) and b -carotene (Car). By varying the wavelength for excitation of the RR spectra we are able to distinguish many of the pigments in their neutral and cationic states. For example RC2 bind Car507 and Car489 (as defined by their red-most absorption maxima). By specifically exciting either neutral Car we find distinct differences in the frequency maxima of their RR signals in isolated RC2 from spinach. We ascribe our recently published FTRR spectrum of Car+ in RC2 to the cation radical of Car507, as it is the main species absorbing at 1064 nm. Here, we will present RR spectra of various other neutral and cationic states of chromophores that can be detected in RC2.