Trans and gauche conformational equilibria in the side chains of aspartic acid (Asp) and asparagine (Asn) were investigated by measuring the vicinal spin-spin coupling constants of H-1 NMR in acidic, neutral, and basic aqueous solutions over a wide range of temperature (5-90 degreesC). The standard free-energy changes DeltaG(0) were obtained for the trans to gauche conformational variations on the C-alpha-C-beta bond with respect to the alpha-carboxyl group and the beta-carboxyl group in Asp (beta-amide in Asn) and were decomposed into enthalpic DeltaH(0) and entropic -TDeltaS(0) components. The hydration of ionic and polar groups in Asp competes against the large intramolecular electrostatic repulsion energy and stabilizes the gauche more than the trans conformer in correspondence to a larger degree of separation of positive and negative partial charges. In the neutral solutions, where both the carboxyl groups are negatively ionized, the hydration part even overwhelms the intramolecular repulsion and leads to a negative DeltaH(0). The fact that the hydration almost cancels the intramolecular electrostatic repulsion contradicts the widely accepted view that the trans preference in the conformational equilibrium is due to the intramolecular repulsion between alpha-CO2- and beta-CO2- (or beta-CONH2).