ENERGY REDISTRIBUTION IN HETERODIMERIC LIGHT-HARVESTING COMPLEX LHCI-730 OF PHOTOSYSTEM-I

Time-resolved fluorescence of the LHC I-730 complex and its monomeric subunits of the light-harvesting complex (LHC) of photosystem I was st udied in complexes reconstituted from Lhca1 and Lhca4 apoproteins and HPLC purified chlorophyll a, chlorophyll b, and carotenoids [Schmid, V . H. R.; Cammarata, K. V.; Bruns, B. U.; Schmidt, G. W. Proc. Natl. Ac ad. Sci. U.S.A. 1997, 94, 7667]. Fluorescence kinetics of the monomeri c subunits that make up the LKC I-730 heterodimer are characterized at room temperature by three decay processes with lifetimes of 150-350 p s, 0.8-1.8 ns, and 2-3.5 ns. The 2-3.5 ns process represents an overal l relaxation of light-harvesting complexes while the other decay proce sses possibly reflect kinetic heterogeneity due to different pigment-p rotein interactions. In LHC I-730 heterodimers, which are characterize d by an assembly of more Chl b and a change in pigment-protein interac tions, an additional 30-50 ps energy-transfer component was found. Thi s component is absent in both Lhca1 and Lhca4 monomers. This energy-tr ansfer component is due to intersubunit energy redistribution, from Lh ca1 to Lhca4 in a heterodimer. The spectral overlap of fluorescence of Lhcal and absorption of long wavelength spectral forms of Chi a in Lh ca4 suggests the energy transfer is most possibly via the Forster indu ctive resonance mechanism.