The Effect of Monovalent Cations on the Self-Association of Cytidylyl-(3′-5′)-Guanosine and Guanylyl-(3′-5′)-Cytidine in Aqueous Solution

Abstract

The hydrogen-bonding, base stacking, and formation of extended aggregates has been investigated for salts of guanylyl–(3′-5′)-)-cytidine, GpC, and cytidylyl-(3′-5′)-)-guanosine, CpG, in which the cation was Na⁺, K⁺, or tetramethylammonium (TMA⁺). Variable temperature studies were done at 2–70 °C on aqueous solutions at pD 4 and 8 using ¹H NMR and FTIR. At low temperatures it has been found that at pD 8 both GpC and CpG form Watson-Crick dimers which stack upon each other to form larger species. A slight cation effect is observed below 35 °C which has the order: TMA⁺ > Na⁺ > K⁺. This order suggests that the cations are interacting with the phosphate and interactions with the bases are unlikely. The ¹H NMR spectrum for TMACpG at pD 4 has been assigned and exhibits chemical shift differences from those at pD 8 which are consistent with protonation of the N3 of the cytidine residue. Based on NMR line broadening, CpG at pD 4 has a greater degree of self-association at low temperature than it or GpC have at pD 8. A different type of hydrogen bonding and self- association occur in CpG at pD 4 compared to pD 8, but the structures are uncertain. Due to hemi-protonation of the cytidine N3, parallel G-G/C-C⁺ base paired dimers or G-tetrads may be forming.