Spectroscopic characterization of a 5 Chl a photosystem II reaction center complex

It is now well established that the isolated photosystem II (PS II) reaction center complex in its most stable form binds 6 chlorophyll a, 2 beta-carotene and 2 pheophytin a molecules. By using immobilised metal affinity chromatography, however, it is possible to isolate PS II reaction center particles binding 5 Chl a molecules [Vacha et al. (1995) Proc. Natl. Acad. Sci. USA 92, 2929-2933], In this report we present a number of steady-state spectroscopic characteristics at very low temperature(s) of the 5 Chl preparation (RC-5) and compare those with data obtained for 6 Chl preparations (RC-6). The results confirm the loss of a chlorophyll molecule absorbing at 670 nm in RC-5, and in addition reveal that the shoulder near 684 nm is more pronounced in this preparation than in any other PS II RC preparation. The RC-5 preparation is therefore ideally suited to obtain more information on the nature of the low-energy absorption. Based on the fluorescence and triplet-minus-singlet absorbance-difference data presented in this paper, we propose that all absorption around 680 and 684 nm arises from the weakly excitonically coupled 'core' of the RC-5 complex, and that the remaining peripheral Chl molecule absorbs at 670 nm. Furthermore, from the temperature dependence of the spectroscopic data we conclude that the 684 nm absorption in isolated PS II reaction center complexes contains about equal contributions from the primary electron donor and from the red-absorbing 'trap' states.