Effect of serine O-glycosylation on cis-trans proline isomerization

In order to examine the ability of an O-glycosylated serine residue preceding proline to stabilize a cis amide conformation in a fashion similar to that observed with aromatic amino acid residues, we prepared a series of glycosylated analogs of a small linear peptide which we have previous reported to contain a cis conformation of an amide bond between tyrosine and proline. The glycopeptides were prepared by incorporating glycosylated N alpha- (fluoren-9-yl) methoxycarbonyl (Fmoc) amino acids into a standard solid phase peptide synthesis protocol. The peptides and glycopeptides were analyzed using 2-dimensional NMR spectroscopy. Unlike the case where the residue preceding proline was tyrosine, no signals corresponding to a cis proline conformation were detected in the spectra of the two glycopeptides containing serine O-glycosylated with either beta-linked N-acetyl glucosamine or alpha-linked N-acetyl galactosamine in the position preceding proline.