The influence of paramagnetic Cu" ions on the proton decoupled ' T n.m.r. spectra of pyrimidine nucleosides and nucleotides has been studied. For 5'-CMP, the C5 resonance is broadened first on the addition of Cu2+ ions followed by the C2, C4, and C1' resonances. From a comparison of the transverse and longitudinal relaxation rates of the base carbon nuclei due to the presence of Cu2+ ions, binding of Cuz+ t o the N3 nitrogen of 5'-CMP is predicted. A similar broadening behavior is observed for 5'-UMP, 5'-TMP, cytidine, and uridine. This indicates that the Cu2+ ion is located near the N3 nitrogen in these Cu2+-nucleoside and -nucleotide complexes.On a CtudiB l'influence des ions Cu2+ pararnagnktiques sur les spectres r.m.n. du "C de nucliosides de pyrimidine et de nucliotides dans lesquels les protons Btaient dBcouplBs. Dans le cas du 5'-CMP, I'addition des ions Cu" tlargit premikrement la rtsonance en C5 suivie par les rksonances en C2, C 4 et Cl'. En se basant sur les temps de relaxation transversale et longitudinale du carbone de la base dfi i la prCsence d'ion Cu" on prtdit que la fixation du CuZ+ se fait sur l'azote N3 du 5'-CMP. On observe un ilargissement semblable des bandes pour les 5'-UMP, 5'-TMP, pour la cytidine et I'uridine. Ceci indique que les ions Cu" se localisent prks de I'azote N3 dans ces complexes entre Cu2+ et les nuclCosides et les nucl6otide.s.[ Copper(I1) ions have significant and different effects on polyribonucleotides and deoxyribonucleic acid (DNA). Polyribonucleotides are degraded into small oligonucleotides when heated with copper ions (1) by the cleavage of phosphate bonds. Cu2+ ions have an influence on the mean temperature of denaturation of DNA (2). The effect of Cu2+ ions on DNA melting has now been extensively studied (3-1 1). Cu2+ ions interact with DNA in two places, namely the phosphate groups and nitrogen bases. 31P n.m.r. shows the binding of Cu2+ ions to the phosphate portion of the nucleotides (12-1 5). Metal interactions with the base portion of the nucleotides have also been extensively studied. Cu2+ ions preferentially broaden the H5 resonance of -5'CMP and 5'-dCMP (13) indicating a binding to N3. The p.m.r. spectrum of 5'-TMP was not affected leading to the interpretation that Cu2+ is not bound to this deoxyribonucleotide. Proton magnetic resonance spectra have also been obtained for a series of deoxyribonucleosides in . The effect of 'Paper number 4 in a series on nuclear magnetic resonance studies of nucleoside-and nucleotide-metal interactions.Cu2+ ions on the carbon-bound protons was generally the same as with the deoxyribonucleotides in D 2 0 . The NH and NH, resonances are relatively unaffected suggesting that the amino group is not involved in the binding in deoxycytidine or cytidine. The N3 proton was unaffected in deoxythymidine indicating no binding to this nucleoside. These studies were extended to an investigation of Cu2+ ion interactions with uridine in DMSO-d6 and 5'-UMP in D 2 0 (16). Broadening of the N3 and C5 proton resonances of uridine and the H5...