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, Na(2)(5'-GMP) and K(2)(5'-GMP) formed a cholesteric phase, but Rb(2)(5'-GMP) did not. Evidence for antiparallel base stacking (stacked with opposite polarity; head to head) was observed for Rb(2)(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 K(2)(5'-GMP) and Na(2)(5'-GMP) but was much less intense in the latter case. K(2)(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 Rb(2)(5'-GMP). (1)H NMR spectroscopy was used to assist in the interpretation of the CD spectra.