Water soluble chlorophyll binding protein of higher plants: a most suitable model system for basic analyses of pigment-pigment and pigment-protein interactions in chlorophyll protein complexes.

This short review paper describes spectroscopic studies on pigment-pigment and pigment-protein interactions of chlorophyll (Chl) a and b bound to the recombinant protein of class IIa water soluble chlorophyll protein (WSCP) from cauliflower. Two Chls form a strongly excitonically coupled open sandwich dimer within the tetrameric protein matrix. In marked contrast to the mode of excitonic coupling of Chl and bacterio-Chl molecules in light harvesting complexes and reaction centers of all photosynthetic organisms, the unique structural pigment array in the Chl dimer of WSCP gives rise to an upper excitonic state with a large oscillator strength. This property opens the way for thorough investigations on exciton relaxation processes in Chl-protein complexes. Lifetime measurements of excited singlet states show that the unusual stability towards photodamage of Chls bound to WSCP, which lack any protective carotenoid molecule, originates from a high diffusion barrier to interaction of molecular dioxygen with Chl triplets. Site selective spectroscopic methods provide a wealth of information on the interactions of the Chls with the protein matrix and on the vibronic structure of the pigments. The presented data and discussions illustrate the great potential of WSCP as a model system for systematic experimental and theoretical studies on the functionalizing of Chls by the protein matrix. It opens the way for further detailed analyses and a deeper understanding of the properties of pigment protein complexes.