THE EFFECT OF MONOVALENT CATIONS OF THE SELF-ASSOCIATION OF CYTIDYLYL-(3'-5')-GUANOSINE AND GUANYLYL (3'-5')-CYTIDINE IN AQUEOUS-SOLUTION

The hydrogen-bonding, base stacking, and formation of extended aggrega tes 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 d one at 2-70 degrees C on aqueous solutions at pD 4 and 8 using H-1 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 degrees C which has the order: TMA(+) > Na+ > K+. This order suggests that the c ations are interacting with the phosphate and interactions with the ba ses are unlikely. The H-1 NMR spectrum for TMACpG at pD 4 has been ass igned and exhibits chemical shift differences from those at pD 8 which are consistent with protonation of the N3 of the cytidine residue. Ba sed on NMR line broadening, CpG at pD 4 has a greater degree of self-a ssociation 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 co mpared to pD 8, but the structures are uncertain. Due to hemi-protonat ion of the cytidine N3, parallel G-G/C-C+ base paired dimers or G-tetr ads may be forming.