MECHANISM OF DELTA-PH-DEPENDENT DISSIPATION OF ABSORBED EXCITATION-ENERGY BY PHOTOSYNTHETIC MEMBRANES .1. SPECTROSCOPIC ANALYSIS OF ISOLATED LIGHT-HARVESTING COMPLEXES

It has been proposed that the increase in thermal dissipation of excitation energy by thylakoid membranes at high light intensity is dependent upon the formation of an aggregated state of the light-harvesting complexes of Photosystem II (LHCII) (Horton et al. (1991) FEBS Lett. 292, 1-4). Therefore, a study of the spectroscopic changes occurring upon LHCII aggregation in vitro has been undertaken. Aggregation of LHCII, brought about by the removal of detergent, is associated with quenching of chlorophyll fluorescence and the appearance of the major emission band at 700 nm at 77 K (F700). Aggregation is associated with absorption bands with maxima at approx. 510 nm, 660 nm and 690 nm; each of these bands preferentially sensitises F700. The temperature-dependence of the emissions at 680 nm and 700 nm is very different; whereas the yield of F680 saturated at 240 K, that of F700 was not saturated at 77 K. The results are discussed in terms of the unusual and unique properties of aggregated LHCII, providing the basis for energy dissipation in vivo.