Ultrasonic relaxation measurements were performed on cyclic adenosine 3′,5′-monophosphate in water and in 7 M urea. The unimolecular relaxation was attributed to the glycosyl C-N conformational equilibrium. The apparent thermodynamics and kinetic properties, calculated from the relaxation data, were different in 7 M urea than in water. A detailed analysis of the results showed that the glycosyl conformational and stacking equilibria are strongly coupled but in 7 M urea the effects of base stacking no longer intervene. The magnitude and sign of the thermodynamic quantities calculated in 7 M urea can be rationalized by a solvation model in which the ribosephosphate moiety is less solvated in the syn than in the anti conformation. The conformational barrier in 7 M urea has a very low ΔH° of ca. 4 kJ/mol. The conformational equilibrium constant is about 4 at ambient temperatures.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry