Stark absorption spectroscopy on the carotenoids bound to B800-820 and B800-850 type LH2 complexes from a purple photosynthetic bacterium, Phaeospirillum molischianum strain DSM120

Stark absorption spectroscopy was applied to clarify the structural differences between carotenoids bound to the B800-820 and B800-850 LH2 complexes from a purple photosynthetic bacterium Phaeospirillum (Phs.) molischianum DSM120. The former complex is produced when the bacteria are grown under stressed conditions of low temperature and dim light. These two LH2 complexes bind carotenoids with similar composition, 10% lycopene and 80% rhodopin, each with the same number of conjugated C=C double bonds (n = 11). Quantitative classical and semi-quantum chemical analyses of Stark absorption spectra recorded in the carotenoid absorption region reveal that the absolute values of the difference dipole moments vertical bar Delta mu vertical bar have substantial differences (2 [D/f]) for carotenoids bound to either B800-820 or B800-850 complexes. The origin of this striking difference in the vertical bar Delta mu vertical bar values was analyzed using the X-ray crystal structure of the B800-850 LH2 complex from Phs. molischianum DSM119. Semi-empirical molecular orbital calculations predict structural deformations of the major carotenoid, rhodopin, bound within the B800-820 complex. We propose that simultaneous rotations around neighboring C=C and C-C bonds account for the differences in the 2 [D/f] of the vertical bar Delta mu vertical bar value. The plausible position of the rotation is postulated to be located around C21-C24 bonds of rhodopin. (C) 2014 Elsevier Inc. All rights reserved.