A Raman and calorimetric investigation of 3′, 5′ GpG

The Raman spectra of guanylyl (3′-5′) guanosine (GpG) in solution in H₂O and D₂O at pH 3–7 have been recorded at various temperatures between 0 and 80°C. The results are consistent with the existence in the lower temperature range of stable aggregates formed by the stacking of GpG tetramers. The aggregates melt cooperatively near 60°C, which results in important changes in the spectra. Among these, a large increase in intensity of some of the bands assigned to the guanine residues shows that unstacking of the bases occurs at the melting. Also apparent in the spectra are changes in the intensity and frequency of band attributable to molecular groups involved in intermolecular hydrogen bonding between adjacent molecules in the complex. The melting temperature of GpG decreases by approximately 15°C upon lowering the concentration from 5 × 10^?2 to 5 × 10^?4M, as shown by Raman, calorimetric, CD, and uv measurements. The experimentally determined ΔH and ΔS for the melting transition are 9 Kcal/mol and 28 e.u./mol, respectively. The aggregation of GpG in 1.5 × 10^?3M solutions was found to be very slow. The half-time of the process, which roughly follows first-order kinetics, is approximately 3 min at 10°C and 21 min at 35°C. The negative energy of activation associated with this reaction (?143 Kcal) indicated that the process involves intermediates whose concentrations decrease the temperatures raised, thus slowing down the overall process. The rate of disaggregation of GpG upon dilution to very low concentration is also extremely slow, indicating that the GpG aggregates, once formed, are very stable.