Spectroscopy of individual light-harvesting 2 complexes of Rhodopseudomonas acidophila: Diagonal disorder, intercomplex heterogeneity, spectral diffusion, and energy transfer in the B800 band

This paper reports a detailed spectroscopic study of the B800 absorption ba nd of individual light-harvesting 2 (LH2) complexes of the photosynthetic p urple bacterium Rhodopseudomonas acidophila at 1.2 K. By applying single-mo lecule detection techniques to this system, details and properties can be r evealed that remain obscured in conventional ensemble experiments. For inst ance, from fluorescence-excitation spectra of the individual complexes a mo re direct measure of the diagonal disorder could be obtained. Further spect ral diffusion phenomena and homogeneous linewidths of individual bacterioch lorophyll a (BChl a) molecules are observed, revealing valuable information on excited-state dynamics. This work demonstrates that it is possible to o btain detailed spectral information on individual pigment-protein complexes , providing direct insight into their electronic structure and into the mec hanisms underlying the highly efficient energy transfer processes in these systems.