Matrix and temperature effects on absorption spectra of β-carotene and pheophytin a in solution and in green plant photosystem II

Abstract

Absorption spectra of β-carotene (β-Car) and pheophytin (Pheo) a have been recorded in a number of solvents and polymers and the properties are compared with those observed in the isolated reaction centre complex of green plant photosystem II and in its core antenna complex CP47. The peak maxima of the investigated absorption bands shift to the red with increasing polarizability of the medium. The extent of the shift is in the order β-Car > Pheo (Soret) > Pheo (Q x ) > Pheo (Q y ), and it exhibits a linear relationship with the Lorentz-Lorenz function ( n 2 − 1)/( n 2 + 2) of the refractive index of the solvent. The maxima also depend on the polarity of the solvent, and for liquids with similar refractive indices the shift in the maxima of the absorption bands was found to be proportional to the dielectric permittivity function ( ϵ − 1)/( ϵ + 2). Most absorption bands shift strongly (β-Car) or weakly (Pheo Soret and Q x ) to the red when the polarity is increased, but the Pheo Q y band shifts to the blue. All investigated absorption bands also broaden with increasing polarity of the medium. The temperature dependence of the absorption properties was recorded in poly (vinyl butyral) and polystyrene matrices between 80 and 295 K. The purely thermal shift of the absorption maxima was calculated from the difference between the observed shift and the estimated dispersive shift. The thermal and dispersive effects cause in most cases blue and red shifts respectively on cooling, but for β-Car both effects lead to red shifts on cooling from room temperature to 110–160 K. The absorption bands of β-Car in CP47 are remarkably narrow, suggesting a non-polar and highly uniform environment. The temperature-induced band shift is much larger in CP47 than in the polymer matrices, which probably is related to a phase transition of the protein matrix at about 200 K. At room temperature, the effective refractive index of the CP47 host was estimated to be 1.51 ± 0.04. The absorption bands of β-Car in the isolated photosystem II reaction centre complex are distorted because of excitonic interactions but exhibit roughly the same temperature dependence as those in CP47. The Pheo a Q x absorption band is considerably red shifted compared with that in solvents and polymers, which is ascribed to specific interactions with the reaction centre proteins.