bacteriochlorophylls and imidazole

Mg(II)-porphyrin-ligand and (bacterio)chlorophyl-ligand coordination interactions have been studied by solution and solid-state MAS NMR spectroscopy. (1)H, (13)C and (15)N coordination shifts due to ring currents, electronic perturbations and structural effects are resolved for imidazole (Im) and 1-methylimidazole (1-MeIm) coordinated axially to Mg(II)-OEP and (B)Chl a. As a consequence of a single axial coordination of Im or 1-MeIm to the Mg(II) ion, 0.9-5.2 ppm (1)H, 0.2-5.5 ppm (13)C and 2.1-27.2 ppm (15)N coordination shifts were measured for selectively labeled [1,3-(15)N]-Im, [1,3-(15)N,2-(13)C]-Im and [1,3-(15)N,1,2-(13)C]-1-MeIm. The coordination shifts depend on the distance of the nuclei to the porphyrin plane and the perturbation of the electronic structure. The signal intensities in the (1)H NMR spectrum reveal a five-coordinated complex, and the isotropic chemical shift analysis shows a close analogy with the electronic structure of the BChl a-histidine in natural light harvesting 2 complexes. The line broadening of the ligand responses support the complementary IR data and provide evidence for a dynamic coordination bond in the complex.

Topics: Bacteriochlorophyll A; Bacteriochlorophylls; Carbon Isotopes; Chlorophyll; Chlorophyll A; Histidine; Imidazoles; Isotope Labeling; Light-Harvesting Protein Complexes; Magnesium; Magnetic Resonance Spectroscopy; Molecular Structure; Nitrogen Isotopes; Photosynthesis; Rhodobacter sphaeroides; Spectrophotometry, Infrared; Spinacia oleracea

The aim of this study was to investigate the function of betaHis20 in the spectral behavior of the 800-nm bacteriochlorophyll (Bchl) of the Rhodobacter capsulatus LH2 protein. In this context, the 800-nm Bchl of the membrane-linked LH2 was used as an intrinsic probe to follow the reversible, denaturant-elicited unfolding of the neighboring protein region. This band was reversibly shifted to approximately 770 nm by acidic pH, suggesting that the environment of the pigment, responsible for its native red shift, was significantly disturbed by the protonation of a chemical group. The reversible acid-induced blue shift was only observed in the presence of unfolding agents (urea and guanidinium chloride). Thus, dismantling of the protein structure facilitated exposure of the basic group to the medium. The acid-base titrations of the spectral shift indicated an apparent pK approximately 6.1, a value consistent with His imidazole being the protonatable group responsible for the acid-induced band shift. The pK values of free N-terminal amino groups are higher and not expected to be lowered by their local environment in the unfolded state of the protein. A similar blue shift of the 800-nm Bchl band was caused by the modifier diethyl pyrocarbonate, which is known to carboxylate the imidazole group of His and free amino groups. It is also shown that the Fourier transform Raman spectrum of diethyl pyrocarbonate-treated LH2 preparations lacks the weak mode at 1695 cm(-1), suggesting that it should be assigned to the B800 Bchl.

Topics: Bacteriochlorophylls; Binding Sites; Cell Membrane; Circular Dichroism; Diethyl Pyrocarbonate; Enzyme Inhibitors; Guanidine; Histidine; Hydrogen-Ion Concentration; Imidazoles; Light-Harvesting Protein Complexes; Models, Chemical; Photosynthetic Reaction Center Complex Proteins; Protein Binding; Protein Folding; Protein Structure, Tertiary; Rhodobacter capsulatus; Spectroscopy, Fourier Transform Infrared; Spectrum Analysis, Raman; Urea