A Stepanov relation analysis of steady-state absorption and fluorescence spectra in the isolated D1/D2/cytochrome b-559 complex

The relation between room-temperature absorption and fluorescence spectra of the freshly prepared D1/D2/cytb-559 complex was analysed according to the Stepanov equation (Stepanov, B.I. (1957) Sov. Phys. Dokl. 2, 81-84). It is shown that there is a close correspondence between the emission spectrum calculated from the absorption spectrum and the measured emission spectrum in the wavelength range 665-695 nm. This correspondence was not found for samples which had either been aged at room temperature or subjected to a brief illumination with high intensity light. The data are interpreted to indicate that in freshly prepared D1/D2/cytb-559: (i) excited states are thermally equilibrated between all chromophores including P680, (ii) the fluorescence yield of all chromophores is similar, (iii) the freshly prepared complex does not contain a significant amount of uncoupled pigment. This latter conclusion is supported by straight line Stern-Volmer fluorescence quenching kinetics using the quinone quencher of chlorophyll fluorescence dibromothymoquinone.