The 2, 3, 4, 6‐Tetra‐O‐acetyl‐β‐D‐gluco‐, and β‐D‐galactopyranosides, as well as approximately 4 : 1 anomeric mixtures of α‐ and β‐mannopyranosides of Boc‐X‐Y‐NHCH3 dipeptides (X‐Y = Pro‐Ser, Pro‐D‐Ser, Val‐Ser, Val‐D‐Ser, and Gly‐Ser) have been synthesized. CD and ir spectroscopic studies were performed to characterize the conformation of the glycosylated peptide backbone and examine the possible formation of intrapeptide and glycopeptide intramolecular H‐bonds. It was found that O‐glycosylated peptides containing a D‐serine residue are likely to adopt a type II β‐turn while those with the Pro‐Ser or Val‐Ser sequence feature a type I(III) β‐turn in solution. Glycosylation also increases the magnitude of the CD bands, characteristic of the given type of β‐turns, which can be interpreted as an indication of the stablization of the folded backbone conformation. Infrared data showed that in nonpolar solutions the peracetyl glycopeptides adopt both single‐ and double H‐bonded conformations whose ratio, in some cases, depends on the position at C‐2′ of the H‐bond acceptor acetoxy group. These data suggest that five‐, seven‐, or ten‐membered glyco‐turns may play an important role in fixing the steric orientation of the carbohydrate antennae systems in glycoproteins.
ASJC Scopus subject areas
- Organic Chemistry