The photophysics and functions of some naturally occurring carotenoids

Abstract: Carotenoids are one of the most widespread and important group of pigments in Nature and they can act as light-harvesting agents, photoprotectors and antioxidants. Mechanisms of their multiple functions are not fully understood mainly because their complicated photophysics involving ?dark? excited states. The central theme of this thesis is characterization of the lowest dark S1 state of carotenoids in solution or bound to proteins by means of time-resolved pump and probe spectroscopy. In solution, the relaxation pattern of carotenoids was studied, with the vibrational relaxation in the S1 state as a main focus. In a more extended study of the relaxation pathways in the carotenoid zeaxanthin, the excitation wavelength dependence was also elucidated. The observed results showed a different relaxation pattern after excitation with higher energy and direct generation of the zeaxanthin triplet and radical. Due to the fact that a protein environment may affect the spectroscopic properties of a carotenoid molecule, the spectroscopic properties of two carotenoid-protein complexes were studied in this thesis. First, the interaction between a photosynthetic protein and a carotenoid was explored. The results showed that the efficiency of the energy transfer between carotenoids and bacteriochlorophyll within a light-harvesting complex from purple bacteria was decreased when the number of conjugated double bonds of the carotenoids was increased. Second, the interaction between a protein found in the human eye and the xanthophylls, zeaxanthin and lutein, was studied. By a comparison of the spectroscopic properties of the xanthophylls in solution, a specific binding between the xanthophylls and the protein was found. Finally, a way of controlling the formation of the aggregated forms of zeaxanthin was explored. Further, the time-resolved measurements showed different spectroscopic properties, for example longer S1 lifetime, in the aggregates compared to monomeric zeaxanthin.