Circular Dichroism Study of Supramolecular Assemblies of Guanosine 5′-Monophosphate

Concentration and temperature dependent studies of the circular dichroism of dianionic guanosine 5′-monophosphate (5′-GMP), where the cation was Na+, K+, or Rb+ ion, were done to obtain information regarding the nature of the self-assembled 5′-GMP species in aqueous solution, including G-quartets and other structures. Concentrations in the 0.05–0.85 M range and temperatures in the 5–50 °C range were used. At the highest concentrations and 5 °C, Na2(5′-GMP) and K2(5′-GMP) formed a cholesteric phase, but Rb2(5′-GMP) did not. Evidence for antiparallel base stacking (stacked with opposite polarity; head to head) was observed for Rb2(5′-GMP) but not for the Na+ or K+ salts. This structure, believed to be G-quartets, had a melting temperature of 15 °C and dissociated into a second associated species as the temperature increased. The latter was present to the greatest extent at ∼40 °C, and it is characterized by a prominent negative CD band at 306 nm, which may be indicative of an X-DNA type of structure (an expanded G-quartet) or base-stacked monomers or dimers. The same negative band appeared at 310 nm in the CD spectra of K2(5′-GMP) and Na2(5′-GMP) but was much less intense in the latter case. K2(5′-GMP) also formed a noncholesteric phase containing at least two different species, one more stable at low temperatures and the other more stable at higher temperatures, similar to Rb2(5′-GMP). 1H NMR spectroscopy was used to assist in the interpretation of the CD spectra.