Crystal molecular structures of two tripeptides related to the sequence coding for N-glycosylation

The crystal structures of two tripeptides related to the sequence coding for N-glycosylation of peptides have been solved: Boc-Asn(Me)-Ala-Ser-OMe, 1, and Boc-Asn(Me)-Pro-Ser-NHMe, 2. Both molecules contain an “Asx-turn” characterized by a hydrogen bond between the Ser-NH and Asn-CγOγ sites. Moreover, the Pro-Ser sequence is βI-folded in 2. The Ser hydroxyl group is also intramolecularly hydrogen-bonded in both molecules, but two different hydrogen bondings are observed. In 1, the Ser-Oγ H bond interacts with the Asn-CγOγ carbonyl, in good agreement with one of the mechanisms which have been proposed for the N-glycosylation of peptides. In 2, the Ser-OγH bond turns out to be involved in an intra-residue interaction with Ser-C'O, and this conformational change is essentially the consequence of chemically different C-terminus functions.