Resonance Raman spectra of β-carotene in solution and in photosystems revisited: an experimental and theoretical study

Abstract

The resonance Raman (RR) spectra of β-carotene have been studied in solution and in the protein complexes of photosystems I and II (PS I, PS II). The experimental studies are complemented by density functional theory (DFT) calculations that allow for a consistent assignment of most of the experimental RR bands in the region between 900 and 1650 cm⁻¹. Thus, it was shown that the prominent peak at ca. 1525 cm⁻¹ is composed of two closely spaced modes, both of which dominated by CC stretching coordinates of the polyene chain. These two modes exhibit different excitation profiles in the region of the allowed electronic S₀→ S₂ transition with maxima that are separated by ca. 1300 cm⁻¹. Calculated RR spectra and excitation profiles obtained by time-dependent DFT in conjunction with the transform method indicate that the enhancement pattern of these modes cannot be rationalised within the Condon approximation (A-term scattering). Furthermore, symmetry considerations rule out enhancement via vibronic coupling. Instead, we suggest that the different excitation profiles of the two modes result from force constant changes and mode mixing upon electronic transition (Dushinsky rotation). RR spectra and excitation profiles are very similar in solution and in the protein complexes. Thus, the excitation-dependent frequency variation of the 1525-cm⁻¹ peak in PS I and PS II is an intrinsic molecular property of β-carotene and does not reflect different pools of the pigments in specific molecular environments.