Modeling substrate- and inhibitor-bound forms of liver alcohol dehydrogenase: chemistry of mononuclear nitrogen/sulfur-ligated zinc alcohol,formamide, and sulfoxide complexes

Using a mixed nitrogen/sulfur ligand possessing a single internal hydrogen bond donor (N,N-bis-2-(methylthio)ethyl-N-(6-amino-2-pyridylmethyl)amine (bmapa)), we prepared and structurally and spectroscopically characterized a series of zinc complexes possessing a single alcohol ((bmapa)Zn(MeOH)](ClO(4))(2) (1)), formamide ((bmapa)Zn(DMF)](ClO(4))(2) (3), (bmapa)Zn(NMF)](ClO(4))(2) (4)), or sulfoxide ((bmapa)Zn(DMSO)](ClO(4))(2) (7), (bmapa)Zn(TMSO)](ClO(4))(2) (8)) ligand. X-ray crystallographic characterization was obtained for 1.MeOH, 3, 4, 7.DMSO, and 8. To enable studies of the influence of the single hydrogen bond donor amino group of the bmapa ligand on the chemistry of zinc/neutral oxygen donor binding interactions, analogous alcohol ((bmpa)Zn(MeOH)](ClO(4))(2) (2)), formamide ((bmpa)Zn(DMF)](ClO(4))(2) (5), (bmpa)Zn(NMF)](ClO(4))(2) (6)), and sulfoxide ((bmpa)Zn(DMSO)](ClO(4))(2) (9), (bmpa)Zn(TMSO)](ClO(4))(2) (10)) complexes of the bmpa (N,N-bis-2-(methylthio)ethyl-N-(2-pyridylmethyl)amine) ligand system were generated and characterized. Of these, 2, 5, 6, and 9.2DMSO were characterized by X-ray crystallography. Solution spectroscopic methods ((1)H and (13)C NMR, FTIR) were utilized to examine the formamide binding properties of 3-6 in CH(3)CN and CH(3)NO(2) solutions. Conclusions derived from this work include the following: (1) the increased donicity of formamide and sulfoxide donors (versus alcohols) makes these competitive ligands for a cationic N/S-ligated zinc center, even in alcohol solution, (2) the inclusion of a single internal hydrogen bond donor, characterized by a heteroatom distance of approximately 2.80-2.95 A, produces subtle structural perturbations in N/S-ligated zinc alcohol, formamide, or sulfoxide complexes, (3) the heteroatom distance of a secondary hydrogen-bonding interaction involving the oxygen atom of a zinc-coordinated alcohol, formamide, and sulfoxide ligand is reduced with increasing donicity of the exogenous ligand, and (4) formamide displacement on a N/S-ligated zinc center is rapid, regardless of the presence of an internal hydrogen bond donor. These results provide initial insight into the chemical factors governing the binding of a neutral oxygen donor to a N/S-ligated zinc center.