Sequence-specific conformer of peptide sequence Asn-Pro-Asn-X in dimethyl sulfoxide: Synthetic and 1H NMR studies on peptides having Asn-Pro-Asn-Ala sequence and their gly-substituted analogs

A series of small peptides consisting of the repeating sequence Asn‐Pro‐Asn‐Ala, and their Gly‐substituted analogs, having one of the Asn residues replaced by Gly, have been synthesized to investigate the role of Asn side‐chain amido groups in stabilizing the secondary structure of the Asn‐Pro‐Asn‐Ala tetrapeptide unit. Analyses of the 1H NMR data obtained in dimethyl sulfoxide (Me2SO) revealed that the secondary structure of the tetrapeptide sequence is stabilized by virtue of the two Asn residues preceding and following Pro. The sequence‐specific turn structure, which is proposed here as the most probable conformer for the tetrapeptide unit, contains at most three intra‐molecular hydrogen bonds: between backbone CO of Asn preceding Pro and backbone NH of Asn following Pro (inverse γ turn), between side‐chain NHE and backbone CO of the same Asn following Pro, and between side chain CO of Asn preceding Pro and backbone NH of X(Ala).