Carotenoid Structures and Environments in Trimeric and Oligomeric Fucoxanthin Chlorophyll a/c2Proteins from Resonance Raman Spectroscopy

Abstract

Resonance Raman (RR) spectroscopy is used to characterize the structures and environments of the carotenoid fucoxanthin (Fx), the primary light harvester in the membrane-intrinsic fucoxanthin chlorophyll a/c2 proteins(FCP) from the diatom Cyclotella meneghiniana, thereby building on the findings from Stark spectroscopy and calculations (J. Phys. Chem. B 2008, 112 (37), 11838-11853). Solvent-dependent effects on the RR bands of isolated Fx and the xanthophyll-cycle carotenoid, diadinoxanthin (Ddx), are studied to better characterize the protein-specific environmental factors that affect their geometry and spectral signatures. In addition, excitation-wavelength-dependent (441.6-570 nm) changes in the RR bands of the nu1 and nu 3 modes,as well as the conjugated C8 carbonyl stretch, allow the identification of 5-6 in both the trimeric (FCPatrim)and oligomeric (FCPbolig) forms of FCP. These Fx's are broadly classified into two each of high (Fxblue) and low (Fxred) energy, and 1-2 of intermediate (Fxgreen) energy that are allied to their location and function in the protein. The CdC stretching frequencies (nu 1), which indicate conjugation over at least 7 double bonds, and the low intensity of the nu 4 C-H bending modes attest to their planar all-trans conformations both in the protein and in solution, with the protein-bound Fxred's exhibiting signs of nonlinearity. Additionally, rededge excitation of Fx in solution, and in the FCPs, exhibits the effect of mixing between the two lowest energy, 21Ag--like and 11Bu+-like, excited states, which underpins the high light-harvesting and energy transfer efficiency of the Fxred's. RR spectra also reveal differences between FCPatrim and FCPbolig complexes,such as the greater prevalence of Ddx in FCPbolig. Importantly, the identification of 5-6 Fx's per FCP monomer suggests that there may be more than the four Fx's previously assumed per FCP monomer, or else there is definite heterogeneity in Fx structures and/or binding sites.