Two stereoisomers of spheroidene in the Rhodobacter sphaeroides R26 reaction center: a DFT analysis of resonance Raman spectra

Abstract

From a theoretical analysis of the resonance Raman spectra of 19 isotopomers of spheroidene reconstituted into the reaction center (RC) of Rhodobacter sphaeroides R26, we conclude that the carotenoid in the RC occurs in two configurations. The normal mode underlying the resonance Raman transition at 1239 cm(-1), characteristic for spheroidene in the RC, has been identified and found to uniquely refer to the cis nature of the 15,15' carbon-carbon double bond. Detailed analysis of the isotope-induced shifts of transitions in the 1500-1550 cm(-1) region proves that, besides the 15,15'-cis configuration, spheroidene in the RC adopts another cis-configuration, most likely the 13,14-cis configuration.

FIGURE 1

Schematic representation of the spheroidene molecule and our labeling system. The molecule is shown in the all-trans form. The conjugated part of spheroidene is indicated between dashed lines.

FIGURE 2

Resonance Raman spectra of (a) NA spheroidene in petroleum ether, and (b) NA spheroidene and (c) 15,15′-²H₂ spheroidene in the photosynthetic reaction center of Rb. sphaeroides. Peaks denoted with * are argon plasma lines.

FIGURE 3

Mode compositions for the C=C stretch (a) and Me–C=C stretch modes (b) of nonplanar 15,15′-cis spheroidene. Double bonds have been drawn as thicker sticks. Atom sizes and bond lengths have not been drawn to scale. Arrows indicate relative displacement, but are also not drawn to scale. Displacement for hydrogen atoms has been scaled down.

FIGURE 4

The C=C stretch regions of the resonance Raman spectra of reconstituted spheroidene in the Rb. sphaeroides R26 photosynthetic reaction center. (a) NA, (b) 15′-²H, (c) 15,15′-²H₂, (d) 13-¹³C, (e) 13,14-¹³C₂, (f) 10-²H, (g) 10,12-²H₂, and (h) 12,14-²H₂. Calculated frequencies have been indicated above each spectrum by bars. Values calculated for the 15,15′-cis structure are displayed in solid representation and values for the 13,14-cis structure in shaded representation.

FIGURE 5

A comparison of experimental (a) and calculated (b) shifts for the C=C stretch mode of 15,15′-cis spheroidene with respect to the NA values. The experimental NA frequency is 1523 cm⁻¹ and the calculated one is 1525 cm⁻¹. Only for the isotopomers shown is it possible to assign a transition, whereas in the other spectra the C=C stretch and Me–C=C stretch modes show too much overlap.

FIGURE 6

The composition of the normal mode corresponding to the transition calculated to occur at 1242 cm⁻¹ for the 15,15′-cis structure. The mode is composed almost exclusively of the 14′–15′ and 15–14 stretches and the C–H bend vibrations at the 15 and 15′ positions.