15N magnetic resonance of aqueous imidazole and zinc(II)-imidazole complexes. Evidence for hexacoordination

15N NMR chemical shifts of doubly labeled [15N)imidazole permit evaluation of hydrogen bonding, proton association, and Zn(II) complex formation in homogeneous solution. The 15N resonant frequency in aqueous solutions of imidazole at pH 9-12 is independent of imidazole concentration, suggesting insignificant self-association via hydrogen bonding involving the N3 lone pair and the N1 proton of a neighboring molecule. Protonation at N3 (pH less than 5) produces a 31.2-ppM diamagnetic shift and deprotonation at N1 (pH greater than 13) an approximately20-ppM paramagnetic shift relative to neutral aqueous imidazole. Those shifts are very large compared to the approximately +-0.5-ppM uncertainty in the 15N shift measurements. In solutions of Zn2+ and imidazole the 15N resonance in ZnIm/sub i/2+ complexes (Im = imidazole) is diamagnetically shifted by 10 to 20 ppM relative to neutral aqueous imidazole. Over a range of ratios of total imidazole to total zinc such that the average number of complexed imidazole molecules per Zn2+ (anti ν) is approximately 3.5, or less, the shift data are well interpreted by a four-species model (i = 1-4) using stepwise formation constants from the literature. Significant deviations from that model at anti ν greater than 3.5 require that higher species (e.g., ZnIm52+ and ZnIm62+) be considered. A six-species model with reasonable formation constants for the fifth and sixth complexes provides satisfactory interpretation of all data. Implications of those observations with respect to biologically active zinc(II) proteins are considered. 2 tables, 4 figures