Synthetic analogues of the histidine-chlorophyll complex: a NMR study to mimic structural features of the photosynthetic reaction center and the light-harvesting complex

Abstract

Mg(II)-porphyrin-ligand and (bacterio)chlorophyl-ligand coordination interactions have been studied by solution and solid-state MAS NMR spectroscopy. H-1, C-13 and N-15 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 H-1, 0.2-5.5 ppm C-13 and 2.1-27.2 ppm N-15 coordination shifts were measured for selectively labeled [1,3-N-15]-Im, [1,3-N-15,2-C-13]-Im and [1,3-N-15,1,2-C-13]-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 H-1 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.