title: Side-chain conformational thermodynamics of aspartic acid residue in the peptides and achatin-I in aqueous solution.
abstract: 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(alpha)-C(beta)-C are analyzed in terms of the standard free energy DeltaG(0), enthalpy DeltaH(0), and entropy -TDeltaS(0). The thermodynamic quantities are obtained by measuring the dihedral-angle-dependent vicinal (1)H-(1)H coupling constants in nuclear magnetic resonance over a wide temperature range. When the carboxyl groups of Asp are ionized, DeltaG(0) in the aqueous phase depends by approximately 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 DeltaG(0) is a result of the compensation for the intramolecular effect by the hydration (= DeltaG(0)-). The DeltaH(0) and -TDeltaS(0) components, on the other hand, exhibit a notable trend at the C-terminus. The C-terminal DeltaH(0) is larger than the N- and nonterminal DeltaH(0) values due to the intramolecular repulsion between alpha- and beta-. The C-terminal -TDeltaS(0) 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.
authors.affiliation: Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan.
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Kimura
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Matubayasi
authors.forename: Masaru
authors.initials: M
authors.lastname: Nakahara
mesh_terms: D000595:Amino Acid Sequence; D001224:Aspartic Acid / Q000737:chemistry*; D008969:Molecular Sequence Data; D009479:Neuropeptides / Q000737:chemistry*; D009842:Oligopeptides / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012996:Solutions; D012997:Solvents / Q000737:chemistry; D013329:Structure-Activity Relationship; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D009479:Neuropeptides; D009842:Oligopeptides; D010455:Peptides; D012996:Solutions; D012997:Solvents; D014867:Water; C059927:achatin I; D001224:Aspartic Acid