Metal ion effects in isotopic hydrogen exchange in biologically important heterocycles.

The binding of metal ions to heteroatomic centers of biomolecules has been utilized as a probe of metal ion effects in living systems. This article focuses on the effect of N-coordination by transition metals, especially Pt(II), Co(III), Cr(III), on isotopic C(2)-H or C(8)-H exchange of imidazoles, thiazoles, and purines. The usual reactivity trend, protonated >> metalated >> neutral substrate, is excepted for Cr(III)/1-methylimidazole, where Cr(III) activates stronger than H(+). An interplay of factors is considered, including metal-to-ligand back-bonding, electronic structure of metal ions, and differences in crystal field stabilization energy.