Sodium and Potassium Ion-Promoted Formation of Supramolecular Aggregates of 2′-Deoxyguanylyl-(3′-5′)-2′-Deoxyguanosine

Abstract

Guanine mono–, oligo–, and polynucleotides, including the guanine-rich telomeric sequences found at the ends of chromosomes, have been shown to form self-associated species which contain cyclic tetramers of hydrogen-bonded guanines (G-tetrads). In this study the effect of the tetramethylammonium (TMA⁺), Na⁺, and K⁺ ions on the self-aggregation of 2′- deoxyguanylyl-(3′-5′)-2′-deoxyguanosine, d(GpG), in aqueous solution has been studied by ¹H NMR and FTIR spectroscopy. Although just a dinucleotide, it was found that d(GpG) self-associates to form extremely large assemblies in the presence of Na⁺ or K⁺ ions, especially the latter. The observed cation order for self-aggregation is TMA⁺ ≪ Na⁺ < K⁺, with TMA⁺ having only a weak effect. Assuming a two-state model, the T_m for Na[d(GpG)] is 22 °C and for K[d(GpG)] is 42 °C, as determined by ¹H NMR. Below the melting temperatures a large loss in intensity of the NMR signals was observed for these two salts, indicating that very large aggregates are forming in aqueous solution at pD 8. The intensity loss has been estimated to be 85% at 2 °C for Na[d(GpG)] and 88% at 24 °C for K[d(GpG)]; there is no observable signal for K[d(GpG)] at 2 °C. Incremental addition of KCl to 8 mM Na[d(GpG)] shows that at a mole ratio of d(GpG):KCl of 1:1 at 25 °C the total intensity loss is 98%. The presence of additional salt, especially a K salt, increases the formation of the supramolecular aggregates. ⁺H NMR of 9 mM Na[d(GpG)] in 90% H₂O/10% D₂0 at 7 °C suggest that there are at least two different species present, one of which has a G-tetrad structure, or that there are two different environments for the N1H in the G-tetrads. NOESY spectra of Na[d(GpG)] suggest that the glycosidic conformation is anti for both bases and that the dinucleotide units are stacking in a parallel fashion. Variable temperature FTIR spectroscopy in the 1750–1500 cm⁻¹ region corroborates the cation-effect order found by NMR and shows that base-stacking and base-base hydrogen bonding are occurring in the aggregated species.