Three-Dimensional Arrangement of Sugar Residues along Helical Polypeptide Backbone. 2. Synthesis of Periodic N-Glycosylated Peptides by Polymerization of Tripeptide Active Esters Containing α,α-Disubstituted Amino Acid

New type of N-glycosylated peptides having periodic sequence of −[X-Gln(β-D-GlcNAc)-Aib]− [X = L-Glu(OMe), L-Lys(Ac), L-Ala; Aib = α-aminoisobutyric acid] were synthesized by polymerization of glycosylated tripeptides with an active ester methods using Cl^-+H₃N−L-Glu(OMe)-Gln[β-D-GlcNAc(Ac)₃]-Aib-ONp (Np=p-nitrophenyl) (13a), Cl^-+H₃N−L-Lys(Ac)-Gln[β-D-GlcNAc(Ac)₃]-Aib-ONp (13b), and Cl^-+H₃N−L-Ala-Gln[β-D-GlcNAc(Ac)₃]-Aib-ONp (13c) as the monomers. Polymerizable glycosylated tripeptides were prepared by stepwise N,N-dicyclohexylcarbodiimide (DCC)/1-hydroxybenzotriazole (HOBt) method. Polymerizations of 13a−c were initiated by triethylamine and proceeded in DMSO at 50 °C for 5 days in the presence of 1-hydroxy-7-azabenzotriazole (HOAt) as the activator (conversions were 25−75%). The glycopeptides were deacetylated by hydrazine monohydrate in methanol to afford periodic glycopeptides 14 (12−27 residues) without racemization (yield, 35−89%). CD spectra in methanol, trifluoroethanol, and water of deacetylated glycopolymers 14a, 14b, and 14c showed double minima (206 and 222 nm) of negative Cotton effect indicating that N-glycoside (N-acetyl-d-glucosamine) was arranged three-dimensionally along the α-helical peptides in water as well as in organic protic solvents. The helix content depends on the solvent, peptide sequence, and spacer between peptide backbone and sugar. Interaction of the glycopeptides with wheat germ agglutinin (WGA) lectin was investigated by fluorescence measurement.