D-β-aspartyl (Asp) residues are found in aged human-lens αA-crystallin. To explore why the uncommon D-β-Asp is accumulated, the stability of L-α-, D-α-, and D-β-Asp residues is compared in view of the staggered side-chain conformers. By using αA-crystallin fragment, T(55)VLD(58)SGISEVR(65), composed of Asp58 isomers, the vicinal spin-spin coupling constants of Asp58 Hα-Hβ1 and Hα-Hβ2 are quantified by high-resolution solution NMR. The trans conformer is most preferred in the D-β-Asp side-chain, whereas gauche+ and gauche- are abundant in L-α- and D-α-Asp. The close distance between Asp58 carboxylate carbon (CCOO-) and Ser59 nitrogen (N) in gauche+ and gauche- is advantageous to the intramolecular cyclization to form succinimide intermediate, followed by the transformation from α- to β-Asp. The cyclization is not allowed in the trans conformer because of the long distance between CCOO- and N, to keep D-β-Asp stable. The change from gauche to trans conformer in D-β-Asp is exothermic and enthalpy-driven.
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