Geometry and binding properties of different multiple-state glycine-Fe+/Fe2+ complexes

Several biologically relevant glycine-Fe+/Fe2+ complexes with three different multiplicities were studied for the first time by using the hybrid three-parameter B3LYP density functional method with different basis sets. Single-point calculations were also carried out at the BHLYP level with a larger basis set to refine and calibrate these energy values. The results show that the most stable glycine-Fe+ isomer is the C-1-symmetric sextet NO-16, in which Fe+ is interacted with both the amino nitrogen and carbonyl oxygen of the glycine ligand. The ground-state structure of Oycine-Fe2+ is the (5)A" state 20-25, which generates from the interaction of Fe2+ with the two oxygen terminus of the zwitterionic glycine. The calculations indicate that the binding energies mainly derive from the contributions of electrostatic effects, for both the monovalent and divalent metal cation-chelated glycine complexes. The differences in binding energies between these different multiple-state glycine-Fe+ isomers with same combination modes mainly derive from their different electrostatic and polarized effects, and those between the isomers of different combination modes with the same multiple states mainly stem from their different deformation effects. The differences in the relative stabilities of these glycine-Fe+ isomers with different multiple states mainly come from the fact that the more electrostatic contribution of the lower spin complex cannot compensate for the loss of energy enhancement of its corresponding metal cation relative to that of the higher spin counterpart. The same is true for the glycine-Fe2+ complexes. Copyright (C) 2004 John Wiley Sons, Ltd.