Blood group antigens: synthesis of TN glycopeptide related to human glycophorin AM

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Abstract
Three 2-acetamido-2-deoxy-α-d-galactopyranose residues attached to Ser2, Thr3 Thr4 of the amino-terminal portion of glycophorin AM are responsible for the so-called TN blood group specificity. In a continuation of earlier work, this report describes the first chemical synthesis of the triglycosylated pentapeptide H2N-Ser1-Se*r2-Th*r3-Th*r4-Gly5-OH, in which (*) represents the 2-acetamido-2-deoxy-α-d-galactopyranosyl residue. This compound constitutes the G1-G5 sequence of the amino-terminal portion of human glycophorin AM, the main erythrocyte membrane glycoprotein. The above compound was obtained by a stepwise peptide coupling strategy alternatively using aminoacids and adequately protected and/or activated O -glycosyl-aminoacids. Since the desired sequence possesses both unglycosylated and glycosylated serine this route was preferred to that in which the glycosylation is carried out on the preformed pentapeptide H2N-Ser-Ser-Thr-Thr-Gly-OH. Carbohydrate residues were introduced into the sequence as 2-azido-2-deoxy-α-d-galactopyranosyl-l-serine and l-threonine derivatives. The azido functions were further converted into the corresponding acetamido groups by treatment of the final triglyco-pentapeptide with sodium borohydride in the presence of nickel chloride followed by acetylation.