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

Mg(II)–porphyrin–ligand and (bacterio)chlorophyl–ligand coordination interactions have been studied by solution and solid-state MAS NMR spectroscopy. 1H, 13C and 15N 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 1H, 0.2–5.5 ppm 13C and 2.1–27.2 ppm 15N coordination shifts were measured for selectively labeled [1,3-15N]-Im, [1,3-15N,2-13C]-Im and [1,3-15N,1,213C]-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 1H 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.