Nitrogen-15 chemical shifts for imidazole in aqueous Cd/sup 2 +/ solutions

Paper reports results of an investigation of the complexation of doubly labeled (with /sup 15/N) imidazole by Cd/sup 2 +/ in aqueous solution. Analysis of experimental data, using literature values for stepwise formation constants, gives an average shift value, delta, for each coordinated imidazole, and a corresponding value of anti ..nu.., the average number of imidazoles bound by a given Cd/sup 2 +/ ion. Conclusions from the data are summarized: (1) coordination to Cd/sup 2 +/ produces an observed 8 to 12 ppM diamagnetic shift in the /sup 15/N resonance of imidazole relative to neutral aqueous imidazole; (2) shift data for Cd/sup 2 +/-imidazole solutions show no evidence for coordination of more than 4 imidazole groups within the range of compositions used; (3) calculated values for delta/sup 0/ decrease with increasing number of ligand species, following the reported decrease in overall formation constants. Data seem to follow the pattern of earlier experimentation with the zinc--imidazole system, suggesting a general correlation between magnitude of NMR shift and the free energy replacement of water by imidazole in the first sphere of diamagnetic cations.