Synthesis of deoxy and alanine-substituted derivatives of a T cell stimulating glycopeptide—An investigation of conditions for cleavage from the solid phase and deprotection

Abstract

Little is known concerning how T cells recognize glycopeptides presented by MHC molecules on antigen-presenting cells. In order to probe the specificity of helper T cells elicited on immunization of mice with glycopeptide 1, which has the disaccharide galabiose [Galα(1→4)Galβ] O-linked to serine 56 in the hen egg lysozyme peptide HEL(52–61), we have prepared three sets of glycopeptides. These are: (i) the 6- and 6′-deoxygalabiose analogs of 1, (ii) two glycopeptides in which the galabiose moiety of 1 has been replaced by galactose and lactose, respectively, and (iii) an alanine-scan series of 1 in which all amino acid residues, apart from 54 and 56, were replaced by alanine, one by one. Two deoxygenated galabiose donors, activated either as an anomeric trichloroacetimidate or as a β-acetate, were used for glycosylation of Fmoc-Ser-OPfp. The resulting, and other, glycosylated amino acids were then used as building blocks in solid-phase synthesis of the target glycopeptides. It was found that improved yields of glycopeptides could be obtained if cleavage from the solid phase was performed at 40 °C instead of at room temperature. In the final base-catalyzed deprotection of the carbohydrate moiety, removal of O-benzoyl groups was accompanied by substantial β-elimination. For one of the glycopeptides even deacetylation required carefully controlled conditions in order to avoid β-elimination.