Fluorescence of carotenoids: Probing binding site interactions and conformational motion in carotenoproteins

The function of carotenoids in carotenoproteins is optimized by the electrostatic and steric interactions between the carotenoid and its surrounding binding site. Binding to the protein distorts the conformation of the carotenoid and induces a charge-transfer character. This chapter shows how the line shape of the fluorescence spectrum, the fluorescence quantum yield, and the fluorescence anisotropy of the second excited singlet state of a carotenoid, S₂, can be used to probe the structure and dynamics of carotenoids in carotenoproteins. The experimental approach and a brief introduction to the theory we used to detect hydrogen bonding interactions with the active ketocarotenoids in the orange carotenoid protein are discussed here as an example. Fluorescence anisotropy is then introduced as a probe of the conformational motions that follow optical excitation of a carotenoid using results from a study of β-carotene in solution over a range of temperatures.