Resonance Raman spectroscopy of the B820 subunit of the core antenna from Rhodospirillum rubrum G9

Abstract

The resonance Raman spectrum in the carbonyl-stretch region (1600–1700 cm −1) of the B820(OG) subunit form is completely different from the original core antenna complex of Rhodospirillum rubrum G9. The observed changes appear to be fully reversible upon reassociation of the B820(OG) subunits into the native-like B873(reassoc). This allows direct conclusions regarding the BChl a binding-site in the antenna complex. In the native complex all 2-acetyl carbonyl groups are hydrogen bonded since only a single resonance peak is observed at 1640 cm −1. When the core antenna is dissociated into its subunit B820(OG) form, half of this vibrational band is shifted to 1625 cm −1, indicating that approx. 50% of the 2-acetyl groups of the BChl a porphyrin rings have changed their hydrogen bond interaction. Besides these changes, there are also effects in the resonance frequency of the 9-keto carbonyl group that indicate that, upon dissociation, a fraction of these groups becomes free from interactions. Similar changes are observed for the B820(ODPS) subunit that is prepared using a different detergent. The spectral shifts upon dissociation cannot be attributed solely to the change in hydrogen bonding, suggesting that an additional perturbation of the BChl a binding-site takes place. Possible models for the hydrogen bonding of these groups are discussed.