A VIBRATIONAL SPECTROSCOPIC STUDY OF THE SELF-ASSOCIATION OF 5'-GMP IN AQUEOUS-SOLUTION

The Raman spectra of highly concentrated solutions of 5'-GMP at neutral and acid pH were recorded in order to better characterize the structure of the self-aggregates formed in these solutions and their melting behavior. Vibrational coupling of the C = O stretching vibrations in tetrameric units at neutral pH is shown to yield a characteristics pattern of two Raman bands at ca. 1730 and 1680 cm-1 (1708 and 1664 cm-1 in D2O), and an iractive mode at 1678 cm-1 in D2O. From the intensity of the 1730-cm-1 band, proportional to tetramer concentration, and that at 1485 cm-1, which reflects the stacking of the bases, the thermal stability of the self-associates formed at neutral pH is shown to be higher for stacked tetramers. At acid pH, the melting of the helical aggregates responsible for the formation of a gel is preceded by the freeing of the hydrogen-bonded phosphate groups, accompanied by a change of conformation from C3'-endo to C2'-endo in some of the associated ribose units. Previous spectroscopic results suggesting the formation of tetramers as an intermediate step in the melting of the gel were not reproduced in this study.