Interaction of Biotin with Mg−O Bonds: Bifunctional Binding and Recognition of Biotin and Related Ligands by the Mg(15-crown-5)2+ Unit

The interaction between biotin and the macrocyclic magnesium complex Mg(15-crown-5)(Otf)2 (15-crown-5 is 1,4,7,10,13-pentaoxacyclopentadecane, Otf- is trifluoromethanesulfonate anion) in solution was studied as a model for metal−biotin interactions that may be important in its speciation and function. Shifts in the solution IR spectrum establish that the interaction is dominated by ligation between the carbonyl oxygen of the ureido ring of biotin and the Mg2+ cation. However, comparative binding studies using NMR spectroscopy and conductivity reveal a substantial enthalpic contribution to binding that arises from interactions between the ureido −NH moiety and the macrocyclic ring. This is interpreted in terms of a weak-to-moderate hydrogen bond formed between the −NH group and an oxygen from the crown, which is simultaneously coordinated to Mg2+. This hypothesis is reinforced by quantitative examination of the binding of N-methylated derivatives of 2-imidazolidone, which shows that N,N’-dimethylation decr...