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| - Enthalpy and Entropy Decomposition of Free-Energy Changes for Side-ChainConformations of Aspartic Acid and Asparagine in Acidic, Neutral, and Basic AqueousSolutions
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| - 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 1H NMR in acidic, neutral, andbasic aqueous solutions over a wide range of temperature (5−90 °C). The standard free-energy changes ΔG0were obtained for the trans to gauche conformational variations on the Cα−Cβ bond with respect to theα-carboxyl group and the β-carboxyl group in Asp (β-amide in Asn) and were decomposed into enthalpicΔH0 and entropic −TΔS0 components. The hydration of ionic and polar groups in Asp competes against thelarge intramolecular electrostatic repulsion energy and stabilizes the gauche more than the trans conformer incorrespondence 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 intramolecularrepulsion and leads to a negative ΔH0. The fact that the hydration almost cancels the intramolecular electrostaticrepulsion contradicts the widely accepted view that the trans preference in the conformational equilibrium isdue to the intramolecular repulsion between α-CO2- and β-CO2- (or β-CONH2).
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