Donor Action Leading to Higher Coordination In Cyclic Phosphorus Compounds

Abstract

Sulfur, oxygen, and nitrogen ligands, prominent at enzyme active sites, are incorporated in flexible ring systems and exhibit donor action at phosphorus centers. X-ray studies reveal an increase in coordination geometry for phosphorus atoms in phosphites, phosphates, and oxyphosphoranes that model substrates and enzyme activated complexes. The increase in coordination results in trigonal bipyramids for the tri- and tetra-coordinated phosphorus compounds and octahedral geometries for the pentacoordinated ones. Using sulfur as an example, the degree of coordination as measured by the displacement toward the more highly coordinated geometry follows the order: oxyphosphoranes > phosphites > phosphates. With the use of an oxygen donor in place of sulfur in flexible ring systems, a lesser degree of coordination is indicated by the smaller displacement toward the more highly coordinated geometry. Nitrogen donor systems are also illustrated. The possible role of donor atom coordination from nearby residues at active site environments of phosphoryl transfer enzymes in enhancing substrate reaction rate is discussed.