Binding to Sulfur-Containing Groups

Many metals bind to the sulfur of sulfhydryl groups in proteins. Metals that bind sulfur in preference to oxygen not only form strong cr bonds with the readily polarizable ligands, but also tt bonds by back-donation of electrons from metal drr to ligand dir orpir orbitals. The electronegativity 

Thioether sulfur atoms have smaller polarizabilities and are weaker donors than S (sulfhydryl) atoms, but they have fewer base-pair electrons and therefore should be better electron acceptors. Methionine generally binds d and d configuration atoms Pd(II), Pt(II), Pb(II), Ag(I), Cu(I), and Hg(II) (Dickerson etai, 1969 Freeman and Golomb, 1970 Blundell and Johnson, 1976 Sheriff elal., 1987). 

Metal Binding to Main-Chain Carbonyl Croups 

An analysis of metal binding to peptide carbonyl groups (Chakrabarti, 1990), mainly calcium ions in protein crystal structures, shows that the cations tend to lie in the peptide plane near the C=0 bond direction. Generally, this binding occurs in turns in proteins or in regions with no regular secondary structures. Ca---0 distances range from 2.2 to 2.5 A, and metal ions do not deviate by more than 35° from the peptide plane. Thus, metal ions in proteins do not, Chakrabarti observed, bind in lone-pair directions. 

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