Staggered side-chain conformers of aspartyl residues prerequisite to transformation from L-α- to D-β-aspartate 58 in human-lens αA-crystallin fragment

•Staggered side-chain conformers of Lα-, Dα-, and Dβ-Asp58 isomers in αA-crystallin•1H -1H coupling constants of Asp isomers quantified by high-resolution NMR•Gauche conformers in Lα- and Dα-Asp promote the transformation to uncommon Dβ-Asp.•Uncommon Dβ-Asp is stabilized by the side-chain trans conformer in aged proteins.•Change from gauche to trans conformer is enthalpy-driven and favorable in D-β-Asp.

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, T55VLD58SGISEVR65, 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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