Interactions of 6-aminopurine (adenine) in water and aqueous urea solutions

Density (ρ), ultrasonic velocity (U), viscosity (η), and absorbance (A) of adenine in water, 1 M, 3 M, and 6 M aqueous urea solutions have been measured in the presence (and absence) of different concentrations of salts, i.e., NaCl, KCl, and CaCl2. From these measured values, the apparent molar volume (ϕv), experimental slope (Sv), apparent molar compressibility (ϕK), viscosity coefficients A and B of Jones–Dole equation, specific acoustic impedance (Z), hydration number (Sn), internal pressure (Pi), molar absorptivity (ε), hypochromicity (h), and oscillator strength (f) have been calculated at 298.15 K. The increasing and (or) decreasing trends of all these parameters indicate a strong interaction of urea in moderate concentration (1 M and 3 M) with adenine especially at the hydrophilic/polar and ionic sites, particularly by hydrogen bonding. However, the interaction at hydrophobic sites are relatively weaker. Higher concentrations of urea (6 M) have more denaturation effect. The π–π interactions (stacking) in adenine are more prominent in concentrated solutions and are also strengthened by the presence of cations up to an optimum concentration of salts (cut-off point). The self-association of urea and adenine is more significant in concentrated aqueous solutions and becomes negligible on dilution.Key words: interaction, adenine, urea, density, viscosity, and ultrasonic velocity.