Pigment‐Protein Interactions in the Antenna‐Reaction Center Complex ofHeliobacillus mobilis

Membrane fragments ofHeliobacillus (Hc.) mobiliswere characterized using resonance Raman (RR) spectroscopy in order to determine the configuration of the neurosporene carotenoid, the pigment‐protein interactions of the bacteriochlorophyll (BChl)gmolecules, and the Chla‐like chlorin pigments present in the antenna‐reaction center complex constituting the photosynthetic apparatus. Using 363.8 nm excitation, the Raman contributions of the BChlgmolecules were selectively resonantly enhanced over those of the carotenoid and the Chla‐like chlorin pigments. The RR spectrum of BChlgin these membranes excited at 363.8 nm exhibits bands at 1614 and 1688 cm−1, which correspond to a CaCmmethine bridge stretching mode and a keto carbonyl group stretching mode, respectively. Both of these bands are 16 cm−1wide (full width at half maximum, FWHM), indicating that a sole population of BChlgmolecules is being enhanced at this excitation wavelength. The observed frequency of the CaCmstretching mode (1614 cm−1) indicates that the bulk of BChlgmolecules is pentacoordinated with only one axial ligand to the central Mg atom while that of the keto carbonyl stretching mode (1668 cm−1) indicates that these groups are engaged in a hydrogen bond. This homogeneous population of BChlgmolecules bound to the heliobacterial core polypeptides is in contrast to the heterogeneous population of Chlamolecules bound to the core polypeptides of the reaction center of photosystem I ofSynechocystis6803 as observed by the inhomogeneously broadened C9keto carbonyl band in its RR spectrum. The RR spectrum of the Chla‐like chlorin pigments inHc. mobilisexcited at 441.6 nm exhibits a broad keto carbonyl band (43 cm−1FWHM) with components at 1665, 1683 and 1695 cm−1, indicating several populations of these pigments differing in their protein interactions at the level of the keto carbonyl group. Fourier transform (FT) pre‐RR spectroscopic measurements of intact whole cells and membrane fragments at room temperature using 1064 nm excitation indicate that high quality vibrational spectra of the BChlgmolecules can be obtained with no photodegradation. Low‐temperature FT Raman spectra excited at 1064 nm reveals an inhomogeneously broadened 1665 cm−1band corresponding to the C9keto carbonyl stretching mode. Spectral deconvolution and second derivative analysis of this band reveal that it is comprised of components at 1665, 1682 and 1695 cm−1, the latter two most likely arising from BChlgphotoconversion products. Excitation using 885 nm to enhance the preresonance effect of the BChlgmolecules yields an FT Raman spectrum where the keto carbonyl band at 1665 cm−1is narrow, as is the case in the Soret RR spectra, reflecting a sole population of BChlgmolecules, which are engaged in an H bond. The RR spectrum of the neurosporene molecule inHc. mobilismembranes excited at 496.5 nm is compared to that of 1,2‐dihydroneurosporene bound in acisconfiguration in reaction centers ofRhodopseudomona viridisand to that of the same carotenoid in its all‐transconfiguration extracted from these reaction centers in the presence of light. The similarity of this latter RR spectrum with that of neurosporene in theHc. mobilismembranes indicates that it is bound in an all‐transconfiguration.