Single-molecule spectroscopy on RC-LH1 complexes of Rhodopseudomonas acidophila strain 10050.

We have revisited the RC-LH1 complex from Rhodopseudomonas (Rps.) acidophila for single-molecule spectroscopy. For the current study the pigment-protein complexes were stabilized in the detergent buffer solution using a relatively mild detergent (dodecyl-β-D-maltoside (DDM) instead of lauryldimethylamine N-oxide (LDAO)). This leads to a significant reduction of the fraction of broken/dissociated RC-LH1 complexes with respect to previous studies and has allowed us to investigate a sufficiently large sample of individual RC-LH1 complexes. For most of the complexes the fluorescence-excitation spectra exhibit a narrow spectral feature at the red end of the spectrum. Analysis of the statistics of the spectral properties yields a close resemblance with the results obtained on RC-LH1 complexes from Rps. palustris for which a low-resolution X-ray structure is available. Based on this comparison we come to the conclusion that for both species the RC-LH1 complex can be described by the same structural model, that is, an overall elliptical assembly of pigments that features a gap.