ROLE OF SULFUR SITE IN METAL-BINDING TO THIOPURINE AND THIOPYRIMIDINENUCLEOSIDES

Thiolation of uridine and purine bases leads to the formation of very effective ligands for Cd(II) and Ni(II) ions. Potentiometric and spect roscopic studies have shown that the substitution of oxygen fy sulfur in uridine, which is a poor ligand for most metal ions, allows thiouri dine to strongly bind both Cd(II) and Ni(II) ions. The {N,S} chelation is observed for thiopurines and thiouridines, even if the {S,N(3)} ch elation in uridine derivatives leads to the formation of a thermodynam ically unfavorable four-member chelate ring. Metal complexes with thio purine with a five-member chelate ring are, however, considerably more stable (up to 3.5 orders of magnitude) than those of thiouridines. (C ) 1997 Elsevier Science Inc.