Mimetics of β-turn structures in proteins have been used to calibrate the relative reactivities toward deamidation of asparagine residues in the two central positions of a β-turn and in a random coil. N-Acetyl-Asn-Gly-6-aminocaproic acid, an acyclic analog of a β-turn mimic undergoes deamidation of the asparaginyl residue through a succinimide intermediate to generate N-acetyl-Asp-N-Gly-6-aminocaproic acid (6-aminocaproic acid, hereafter Aca) and N-acetyl-L-iso-aspartyl (isoAsp)-Gly-Aca (pH 8.8, 37°C) ≃ 3-fold faster than does the cyclic β-turn mimic cyclo-[L-Asn-Gly-Aca] with asparagine at position 2 of the β-turn. The latter compound, in turn, undergoes deamidation ≃ 30-fold faster than its positional isomer cyclo-[Gly-Asn-Aca] with asparagine at position 3 of the β-turn. Both cyclic peptides assume predominantly β-turn structures in solution, as demonstrated by NMR and circular dichroism characterization. The open-chain compound and its isomer N-acetyl-Gly-Asn-Aca assume predominantly random coil structures. The latter isomer undergoes deamidation 2-fold slower than the former. Thus the order of reactivity toward deamidation is: Asparagine in a random coil ≃ 3xasparagine in position 2 of a β-turn ≃ 30xasparagine in position 3 of a β-turn.
- Asparagine deamidation
- Structure-reactivity relationship
- β-turn structures
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