Carotenoid Structures and Environments in Trimeric and Oligomeric Fucoxanthin Chlorophyll a/c2 Proteins from Resonance Raman Spectroscopy

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/c(2) 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 nu(1) 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 (FCPolig) forms of FCP. These Fx's are broadly classified into two each of high (Fx(blue)) and low (Fx(red)) energy, and 1-2 of intermediate (Fx(green)) energy that are allied to their location and function in the protein. The C=C 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 Fx(red)'s exhibiting signs of nonlinearity. Additionally, red-edge excitation of Fx in solution, and in the FCPs, exhibits the effect of mixing between the two lowest-energy, 2(1)A(g)*(-)-like and 1(1)B(u)*(+)-like, excited states, which underpins the hi-fi light-harvesting and energy-transfer efficiency of the Fx(red)'s. RR spectra also reveal differences between FCPatrim and FCPolig complexes, such as the greater prevalence of Ddx in FCPolig. 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.