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 studied 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. Acad. Sci. U. S.A . 1997, 94, 7667]. Fluorescence kinetics of the monomeric subunits that make up the LHC I-730 heterodimer are characterized at room temperature by three decay processes with lifetimes of 150−350 ps, 0.8−1.8 ns, and 2−3.5 ns. The 2−3.5 ns process represents an overall relaxation of light-harvesting complexes while the other decay processes possibly reflect kinetic heterogeneity due to different pigment−protein interactions. In LHC I-730 heterodimers, which are characterized by an assembly of more Chl b and a change in pigment−protein interactions, an additional 30−50 ps energy-transfer component was found. This component is absent in both Lhca1 and Lhca4 monomers. This energy-transfer component is due to intersubunit energy redistribution, from Lhca1 to Lhca4 in a heterodimer. The spectral overlap of fluorescence of Lhca1 and absorption of long wavelength spectral forms of Chl a in Lhca4 suggests the energy transfer is most possibly via the Förster inductive resonance mechanism.