2003; Karrasch et al. 1995). The LH1 structural inhomogeneity similarly induces broadening of the lines in the NMR spectra and in an earlier study on Rhodospirillum rubrum LH1 αβ subunits reconstituted with 13C–15N-labeled BChls, only one set of BChl NMR signals was assigned without distinction between the α- and β-BChl
MK-8776 supplier (Wang et al. 2002). In our recent work, NMR assignment of the two types of LH1 BChls was achieved in intact LH1-RC core complexes, using the LH2 spectra as a “template” for the assignment. Two sets of signals were observed, corresponding with the electronic S3I-201 structures of the α- and the β-bound BChls in LH1 that also form a ring of dimers, similar to LH2. By overlay of the LH1 and LH2 2D-NMR spectra, the BChl ground-state electronic structures of the homologous LH2 and LH1 antenna rings were directly compared, revealing differences and similarities in their conformation or local protein environment with atomic selectivity (Fig. 2). This method circumvents referencing to monomeric BChl in an organic solvent, of which chemical shift values are biased by the solvent polarity. Fig. 2 Comparison of the ground-state electronic structures of Rps. acidophila LH1 and LH2 B850 BChls. Left Side chain atoms with similar values for the LH1 and LH2 BChls are highlighted
in gray and differences are highlighted in yellow. Right 13C-13C NMR homonuclear correlation spectra of the LH1-RC protein (green), overlaid on the spectrum of LH2 (red) obtained SIS3 nmr under similar conditions The LH1 and LH2 BChl NMR chemical shift patterns on the one hand could not be modeled by the effects of hydrogen bonding, side chain out-of-plane rotation and long-range electrostatic see more interactions, suggesting that the BChl electronic structures in the ground state are mainly shaped by macrocycle deformation (Pandit et al. 2010a). Chlorophyll macroaromatic cycles are readily distorted, which makes for a very flexible electronic structure of the porphyrin ring, where the electronic densities follow
the distortions imposed upon the structure due to the predominant electron–phonon coupling. The effect of structural deformation of the chromophores, however, was omitted in prediction of the site energies and corresponding excitonic couplings of the pigments inside the major light-harvesting complex II (LHCII) and of the Fenna–Mathews–Olson (FMO) complex, due to uncertainties in the crystal structures used for the calculations that otherwise could lead to overestimation of the transition dipoles (Muh et al. 2010; Adolphs et al. 2008). Also, here the NMR data thus complement the crystallographic data and eventually may be combined in a synergistic way for more accurate prediction of pigment site energies.