Low temperature single molecule spectroscopy of light-harvesting complexes

Abstract

Single molecule spectroscopy can provide detailed information about the relationship between structure, spectroscopy, interaction and energy transfer of photosynthetic pigment-protein complexes. The spectral position of the absorption lines gives information about the structural inhomogeneities around the different chromophores inside the complex, and about the nature of excitation transfer. The polarization of the individual transitions gives insight into the structure. Bulk excitation spectra display only broad lines as a result of inhomogeneities in the properties of individual complexes. Therefore, the information above is most directly accessible by spectroscopy on single complexes. We have studied the fluorescence-detected absorption spectra of individual light harvesting (LH) 1 complexes of the photosynthetic purple bacteria Rhodospirillum rubrum at 1.4 K, with and without the reaction center enclosed in the LH1 ring. It is generally believed that this antenna is organized in building blocks that are analogous to those of LH2. Our spectra do indeed show multiple broad absorption bands that are similar to those obtained in LH2, but in detail there are significant differences. In addition, we have investigated the properties of CP29, a minor LH complex of green plants. This complex has recently been studied by bulk techniques. However, a complete understanding is still missing and several questions remain, such as the variability of the occupation of the chlorophyll¿s binding sites. Single molecule spectroscopy should be able to bring new insights into these problems. The most recent results in this project will be presented.