MATRIX AND TEMPERATURE EFFECTS ON ABSORPTION-SPECTRA OF BETA-CAROTENEAND PHEOPHYTIN-ALPHA IN SOLUTION AND IN GREEN PLANT PHOTOSYSTEM-II

Absorption spectra of beta-carotene (beta-Car) and pheophytin (Pheo) a lpha have been recorded in a number of solvents and polymers and the p roperties are compared with those observed in the isolated reaction ce ntre complex of green plant photosystem II and in its core antenna com plex 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 beta-Car> Pheo (Sorer) > 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 liqui ds with similar refractive indices the shift in the maxima of the abso rption bands was found to be proportional to the dielectric permittivi ty function (epsilon-1)/(epsilon+2). Most absorption bands shift stron gly (beta-Car) or weakly (Pheo Sorer and Q(x)) to the red when the pol arity is increased, but the Pheo Q(x) band shifts to the blue. All inv estigated absorption bands also broaden with increasing polarity of th e medium. The temperature dependence of the absorption properties was recorded in poly(vinyl butyral) and polystyrene matrices between 80 an d 295 K. The purely thermal shift of the absorption maxima was calcula ted from the difference between the observed shift and the estimated d ispersive shift. The thermal and dispersive effects cause in most case s blue and red shifts respectively on cooling, but for beta-Car both e ffects lead to red shifts on cooling from room temperature to 110-160 K. The absorption bands of beta-Car in CP47 are remarkably narrow, sug gesting a non-polar and highly uniform environment. The temperature-in duced 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 beta-Car in the isolated photosystem II reaction centre complex ar e distorted because of excitonic interactions but exhibit roughly the same temperature dependence as those in CP47. The Pheo alpha Q(x) abso rption band is considerably red shifted compared with that in solvents and polymers, which is ascribed to specific interactions with the rea ction centre proteins.