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Side-Chain Conformational Thermodynamics of Aspartic Acid Residue in the Peptides and Achatin-I in Aqueous Solution

Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan

Correspondence: Address reprint requests to Masaru Nakahara, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan. Tel./Fax: 81-774-38-3070; E-mail: nakahara{at}scl.kyoto-u.ac.jp.

Sequence-position dependence of the side-chain conformational equilibrium of aspartic acid (Asp) residue is investigated for both model Asp peptides (di- to tetra-) and neuropeptide achatin-I (Gly--Phe-Ala-Asp) in aqueous solution. The trans-to-gauche conformational changes on the dihedral angle of C–C–C_ß–C are analyzed in terms of the standard free energy G⁰, enthalpy H⁰, and entropy -TS⁰. The thermodynamic quantities are obtained by measuring the dihedral-angle-dependent vicinal ¹H-¹H coupling constants in nuclear magnetic resonance over a wide temperature range. When the carboxyl groups of Asp are ionized, G⁰ in the aqueous phase depends by 1–2 kJ mol^-1 on the sequence position, whereas the energy change in the gas phase (absence of solvent) depends by tens of kJ mol^-1. Therefore, the weak position dependence of G⁰ is a result of the compensation for the intramolecular effect by the hydration (= G⁰–). The H⁰ and -TS⁰ components, on the other hand, exhibit a notable trend at the C-terminus. The C-terminal H⁰ is larger than the N- and nonterminal H⁰ values due to the intramolecular repulsion between - and ß-. The C-terminal -TS⁰ is negative and larger in magnitude than the others, and an attractive solute-solvent interaction at the C-terminus serves as a structure breaker of the water solvent.

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