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Both cytidine and uridine react with (H3N)2Ptn and enPt11 in aqueous solution over a wide pH range. The reactions with cytidine are complete within a few hours, while those with uridine are considerably slower. In both cases mono and bis complexes are formed. In acidic solution, (H3N)2Ptn behaves as a bifunctional electrophile toward cytidine, while at pH 7 it acts as a unifunctional electrophile. The equilibrium constant for [(H3N)2Pt(OH2)2]2+ + Cyd <=± [(H3N)2PtCyd(OH2)]2+ + H20 was estimated from Raman spectra to have the value log K = ca. 2.9 (0.1 M C104~). The hydroxo ligand produced by hydrolysis of [(H3N)2Pt(OH2)2]2+ is an effective blocking group. Raman and and l3C NMR spectra all identify N(3) as the sole binding site. In particular, no interaction occurs at high pH with a deprotonated amine group. With uridine, the Raman and *H and l3C NMR experiments all show that binding occurs at N(3) of the conjugate base UrdH_f.No reaction takes place under comparable conditions with 3-methyluridine. The equilibrium constant for [(H3N)2Pt(OH2)2]2+ + UrdH-f [(H3N)2PtUrdH-1OH2]+ has a value log K > 9.6 (0.1 M C104~), comparable to H3CHgOH2+. The selectivity of the bases in a native polynucleotide for binding the substitutionally inert (H3N)2Ptn and related species is kinetic not thermodynamic in origin as also is the case with alkylating agents. Binding will occur preferentially at nonprotonated sites rather than N(3) of uridine or thymidine or N(l) of guanosine, the principal binding sites for labile heavy metal species.Kinetic and thermodynamic effects governing heavy metal binding are discussed. The binding behavior of labile species such as H+ or CH3Hg+ is of little use in predicting the reactions of the inert platinum(II) complexes. At pH 6-7, the monouridine complex undergoes a change which causes small shifts in the NMR spectrum. Exposure of solutions containing equimolar (H3N)2Ptn and uridine with pH <8 to the atmosphere results in the characteristic platinum-uracil "blue". The Raman spectra suggest that the principal species in these solutions is the colorless [(H3N)2PtUrdH_!OH2]+.

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