THE INFLUENCE OF METAL ION COMPLEXATION ON31P-NMR PARAMETERS OF PHOSPHOROTHIOATES OF GUANOSINE NUCLEOTIDES

Abstract
An extension of methods already used for the preparation of adenosine phosphorothioates was used to prepare guanosine phosphorothioates in quantities sufficient for extensive 31P-NMR studies. Guanosine phosphorothioates were used to exemplify the influence of Cd2+ and Mg2+ complexation on 31P-NMR parameters as pK-values, chemical shifts, and P[sbnd]O[sbnd]P coupling constants. pKa values decrease in general in the order pK(α-S) a > pK(β-S) a > pK(γ-S) a; the magnitude of the pKa decrease on metal complexation is also in this order. The chemical shifts for the P[sbnd]S resonances are changed downfield on Mg2+ complexation and upfield on Cd2+ complexation. P[sbnd]O[sbnd]P coupling constants are increased by about 7 Hz on sulfur substitution at one of the contributing phosphorus atoms; this difference is at least partially ineffective in the Cd2+ complexes.