title: Evaluating hydrogen bond interactions in enzymes containing Mn(III)-histidine complexation using manganese-imidazole complexes.
abstract: It is often difficult to control hydrogen bond interactions in small molecule compounds that model metalloenzyme active sites. The imidazole-containing ligands 4,5-dicarboxyimidazole (H(3)DCBI) and 4,5-dicarboxy- N-methylimidazole (H(2)MeDCBI) allow examination of the effects of internal hydrogen bonding between carboxylate and imidazole nitrogen atoms. A new series of mononuclear manganese imidazole complexes have been prepared using these ligands: Mn(III)(salpn)(H(2)DCBI)(DMF) (1), Mn(III)(salpn)(HMeDCBI) (2), Mn(III)(dtsalpn)(HMeDCBI) (3), [Mn(IV)(dtsalpn)(HMeDCBI)]PF(6) (4), Mn(III)(salpn)(H(2)DCBI) (5), Mn(III)(dtsalpn)(H(2)DCBI) (6), and Mn(IV)(dtsalpn)(H(2)DCBI)PF(6) (8). Complexes 1, 2, 3, 5, and 6 have been prepared by direct reaction of salpn [salpn=(salicylideneaminato)-1,3-diaminopropane)] or dtsalpn [dtsalpn=(3,5-di- t-butylsalicylideneaminato)-1,3-diaminopropane)] and H(3)DCBI and H(2)MeDCBI with Mn(III) acetate, while complexes 4 and 8 were made by bulk electrolysis of complex 3 or 6 in dichloromethane. Complexes 1, 2, and 6 were 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 voltammograms 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.
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA.
authors.forename: M
authors.initials: M
authors.lastname: Rajendiran
authors.forename: T
authors.initials: T
authors.lastname: Caudle
authors.forename: Martin L
authors.initials: ML
authors.lastname: Kirk
authors.forename: Ika
authors.initials: I
authors.lastname: Setyawati
authors.forename: Jeff W
authors.initials: JW
authors.lastname: Kampf
authors.forename: Vincent L
authors.initials: VL
authors.lastname: Pecoraro
mesh_terms: D004563:Electrochemistry / Q000379:methods; D004578:Electron Spin Resonance Spectroscopy; D004798:Enzymes / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry*; D008024:Ligands; D008345:Manganese / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D004798:Enzymes; D007093:Imidazoles; D008024:Ligands; D009942:Organometallic Compounds; D008345:Manganese; D006639:Histidine; C029899:imidazole