Study of the carotenoid interaction in peridinin-chlorophyll-proteins.

Abstract

Different magnetic resonance spectroscopy techniques have been used to investigate the carotenoid triplet state nature and the triplet-triplet energy transfer process among peridinin molecules in the peridinin-chlorophyll-protein (PCP) from the Dinoflagellate alga Heterocapsa pygmeae. The peridinin triplet ODMR signal temperature dependence has been followed in a wide temperature range in this complex and compared with the analogous carotenoid triplet signals of the more traditional LHCII antenna complexes. The LHCII complexes show some common features with the PCP complex even if no optical evidence of triplet excitonic splitting is given for the former antenna system. Some knowledge on kinetic parameters and triplet spin polarization were obtained by X-band time-resolved EPR spectroscopy performed on the photoexcited carotenoid triplet state for both complexes. Furthermore, an investigation on the relaxation times of the carotenoid triplet state has been performed combining the time-resolved EPR experiments with Electron Spin Echo spectroscopy experiments. The use of different magnetic resonance techniques for the investigation on the carotenoid triplet state nature has shown a number of puzzling features in the dynamic behaviour of these pigment-protein complexes, related to some still unexplained general feature of the carotenoid triplet state magnetic properties. Evidence of variation of the triplet state sublevels population with temperature and of anomalous homogeneous relaxation time dependence on the temperature have emerged from this global magnetic resonance picture. The complete analysis of the different magnetic resonance observables, which is still under way, should allow to solve a number of open questions concerning the carotenoid triplet puzzles.