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

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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.