Evaluating hydrogen bond interactions in enzymes containing Mn(III)-histidine complexation using manganese-imidazole-complexes

摘要

It is often difficult to control hydrogen bond interactions in small molecule compounds that model metalloenzyme active sites. The imidazole-containing ligands 4,5-dicarboxymidazole (H_3DCBI) and 4,5-di-carboxy-N-methylimidazole (H_2MeDCBI) allow examination of the effects of internal hydrogen bonding between carboxylate and imidazole nitrogen atoms. between carboxylate and imidazole nitrogen atoms. A new series of mononuclear manganese imidazole complexes have been prepared using these ligands: Mn(III)(salpn)(H_2DCBI)(DMF) (1), Mn(III)(salpn)(HMeDCBI) (2), Mn(III)(dtsalpn)(HMeDCBI) (3), [Mn(IV)(dtsalpn)(HMeDCBI)]PF_6 (4), Mn(III)(salpn)(H_2DCBI) (5), Mn(III)(dtsalpn)(H_2DCBI) (6), and Mn(IV)(dtsalpn)(H_2DCBI)PF_6 (8). Complexes 1,2,3,5,and 6 have been prepared by direct reaction of salpn [(salpn = (salicylideneaminato)-1,3-diaminorpropane)] or dtsalpn [dtsalpn = (3,5-di-t-butylsalicylideneaminato)-1,3-diaminopropane)] and H_3DCBI and H_2MeDCBI with Mn(III) acetate, while complexes 4 and 8 were made by bulk electrolysis of complex 3 or 6 in dichloromethane. Complexes 1, 3, and 6 wee characterized by X-ray diffraction. The impact of hydrogen bonding interactions of the complexes has been demonstrated by X-ray diffraction, cyclic voltammetry, and EPR spectroscopy. In all complexes the central metal ion is present in a six-coordinate geometry. Magnetic susceptibility measurements confirm the spin and oxidation states of the complexes. The cyclic voltam-mograms of 3 and 6 in dichloromethane reveal single, reversible redox waves with E_(1/2) = 600 mV and 690 mV, respectively. The X-band EPR spectrum of 4 shows a broad signal around g = 4.4 and the corresponding complex 8 possesses a broad signal at slightly lower field (g = 5.5) than 4. These studies demonstrate that even small changes in the effective charge of the imidazole ligand can have a profound impact on the structure, spectroscopy, and magnetism of manganese (IV) complexes. We use these observations to present a model that may explain the origin of the g = 4.1 signal in the S_2 state of photosystem II.