ULTRAFAST CHLOROPHYLL-B CHLOROPHYLL-A EXCITATION-ENERGY TRANSFER IN THE ISOLATED LIGHT-HARVESTING COMPLEX, LHC-II, OF GREEN PLANTS - IMPLICATIONS FOR THE ORGANIZATION OF CHLOROPHYLLS

The excitation energy transfer between chlorophyll b (Chl b) and chlor ophyll a (Chl a) in the isolated trimeric chlorophyll-a/b-binding prot ein complex of spinach photosystem 2 (LHC II) has been studied by femt osecond spectroscopy. In the main absorption band of Chl b the ground state recovery consists of two components of 0.5 ps and 2.0 ps, respec tively. Also in the Chl a absorption band, at 665 nm, the ground state recovery is essentially bi-exponential. In this case is, however, the fastest relaxation lifetime is a 2.0 ps component followed by a slowe r component with a lifetime in the order of 10-20 ps. In the Chl b abs orption band a more or less constant anisotropy of r = 0.2 was observe d during the 3 ps the system was monitored. In the Chl a absorption ba nd there was, however, a relaxation of the anisiotropy from r = 0.3 to a quasi steady state level of r = 0.18 in about 1 ps. Since the 0.5 p s component is only seen upon selective excitation of Chl b we assign this component to the energy transfer between Chl b and Chl a. The oth er components most likely represents redistribution processes of energ y among spectrally different forms of Chl a. The energy transfer proce ss between Chl b and Chl a can well be explained by the Forster mechan ism which also gives a calculated distance of 13 Angstrom between inte racting chromophores. The organisation of chlorophylls in LHC II is di scussed in view of the recent crystal structure data (1991) Nature 350 , 130].