On the state of carotenoids bound to reaction centers of photosynthetic bacteria: a resonance Raman study.

The carotenoids bound to reaction centers of wild, Ga and GIC strains of Rhodopseudomonas spheroides, of Rhodospirrillum rubrum, strain S1 and of Rhodopseudomonas viridis, yield very similar, but unusual resonance Raman spectra. Through a comparison with resonance Raman spectra of 15,15'-cis-beta-carotene, these carotenoids are shown to assume cis conformations, while the corresponding chromatophores contain all-trans forms only. These cis conformations likely are identical for all the carotenoids studied. They remain unaffected by variations of temperature from 20 to 300 K as well as by the redox state of P-870. They are unstable, being rapidly isomerised towards the all-trans forms when extracted from the reaction centers. The possible nature of these conformers is discussed on the basis of their electronic and vibrational spectra.