Primary processes in isolated photosynthetic bacterial reaction centres from Chloroflexus aurantiacus studied by picosecond fluorescence spectroscopy

Abstract

The stationary and time-resolved fluorescence emission spectra of reaction centres (RCs) isolated from the thermophilic phototrophic bacterium Chloroflexus aurantiacus strain Ok-70-fl have been studied. Several lifetime components in the picosecond and nanosecond time range have been resolved at room temperature. A short-lived approx. 3 ps component is related to an energy-transfer process from the excited accessory BChl-αL to the special pair P. This is demonstrated by the existence of positive and negative amplitudes of this component, depending on the emission wavelength. A conformational relaxation of P∗ causing a bathochromic shift is excluded for this time constant due to the absence of this component at an excitation wavelength of 850 nm (P) and also by the temperature dependence of the stationary fluorescence. Two further short-lived components with positive amplitudes and lifetimes of about 7 ps and about 18 ps were also resolved. Their decay-associated spectra (DAS) and emission maxima at about 910–920 nm are similar, showing that their origin is fluorescence from P∗. Their relative amplitudes change strongly, depending on the excitation intensity. They represent the charge separation times of open RCs (7 ps from P∗HQA) and closed RCs (18 ps from P∗ HQA−). The time constant for open RCs is in full agreement with the value reported recently from transient absorption measurements (Becker et al. (1991) Biochim. Biophys. Acta 1057, 299–312). The presence of “heterogeneity” in the primary rates of charge separation in RC preparations proposed recently is discussed. It is suggested that the “heterogeneity” reported by other authors is possibly an open/closed RC heterogeneity.