DONOR ACTION LEADING TO HIGHER COORDINATION IN CYCLIC PHOSPHORUS-COMPOUNDS

Sulfur, oxygen, and nitrogen ligands, prominent at enzyme active sites , are incorporated in flexible ring systems and exhibit donor action a t phosphorus centers. X-ray studies reveal an increase in coordination geometry for phosphorus atoms in phosphites, phosphates, and oxyphosp horanes that model substrates and enzyme activated complexes. The incr ease in coordination results in trigonal bipyramids for the tri- and t etra-coordinated phosphorus compounds and octahedral geometries for th e pentacoordinated ones. Using sulfur as an example, the degree of coo rdination as measured by the displacement toward the more highly coord inated geometry follows the order: oxyphosphoranes > phosphites > phos phates. With the use of an oxygen donor in place of sulfur in flexible ring systems, a lesser degree of coordination is indicated by the sma ller displacement toward the more highly coordinated geometry. Nitroge n donor systems are also illustrated. The possible role of donor atom coordination from nearby residues at active site environments of phosp horyl transfer enzymes in enhancing substrate reaction rate is discuss ed.