Energy transfer and aggregate size effects in the inhomogeneously broadened core light-harvesting complex of Rhodobacter sphaeroides

Abstract

The 4 K fluorescence emission spectrum of the core light-harvesting complex of the photosynthetic purple bacterium Rhodobacter sphaeroides is shown to depend strongly on the wavelength of excitation. It is concluded that the dominant factor determining the absorption spectrum is inhomogeneous broadening. The site-selected emission spectra are interpreted in terms of energy transfer in a cluster of weakly coupled pigments. Each individual cluster is supposed to be a random sample from the total inhomogeneously broadened pigment pool. For small clusters, with efficient energy transfer leading to fast thermalization, the effects of site-selection depend on the cluster-size only. It is shown that site-selective excitation can yield information about the number of coupled pigments in such a cluster. For the purified LH-1 of Rb. sphaeroides the cluster size was estimated to be 16 pigments.