Note

Synthesis of 2,3-epoxypropyl O-β-d-galactopyranosyl-(1→6)-β-d-galactopyranoside☆

Disaccharide-peptide conjugates were obtained in yields of 30–50% from o-nitrophenyl β-d-galactopyranoside by employing β-d-galactosidase from E. coli as catalyst. Two series of β-d-galactosyldipeptides were examined as galactosyl acceptors. They both contain an l-serine residue β-linked to the anomeric carbon of galactose. In the first series, serine is in the N-terminal position of the dipeptide; in the second series, serine is in the C-terminal position. The second amino acid is l-alanine or glycine. Some of our substrates gave a high yield of β-(1 → 3)-digalactosyldipeptide derivatives and all gave very little of the β-(1 → 6) regioisomer. The conditions and the limitations of the transgalactosylation reaction are discussed.

A stereoselective synthesis of pyruvic 4,6-acetals in their naturally occurring configurations on α-d-glucopyranose, α-d-mannopyranose, and β-d-galactopyranose residues is based on the preferential formation of 4,6-[1-(3,4-dimethoxyphenyl)ethylidene] acetals bearing equatorial methyl groups. 2,3-Di-O-acyl derivatives of these acetals are oxidized with ruthenium tetraoxide to yield the corresponding 4,6-(1-carboxyethylidene) acetals. Synthesis of allyl 4,6-O[1(S)-1-methoxycarbonylethylidene]-3-O-methyl-β-d-glucopyranoside has been achieved with protection of the allyl glycoside by epoxidation and subsequent regenerative deoxygenation with 3-methylbenzothiazole-2-selone. The allyl glycoside has been subjected in sequence to saponification, ozonolysis, and reductive amination in the presence of bovine serum albumin to furnish a neoantigen.

β-d-Galactosidase induced the formation of allyl, benzyl, and trimethylsilylethyl β-d-galactopyranosides on a 1–20-g scale from lactose and allyl alcohol, benzyl alcohol, and trimethylsilylethanol, respectively. Similarly, α-d-galactosidase catalysed the formation of allyl α-d-galactopyranoside from raffinose and allyl alcohol. The galactosides were used as acceptors for the preparation of the following disaccharide glycosides: β-d-Gal-(1→3)-β-d-Gal-OCH₂CHCH₂, β-d-Gal-(1→6)-β-d-Gal-OCH₂CHCH₂, β-d-Gal-(1→3)-β-d-Gal-OBn, β-d-Gal-(1→6)-β-d-Gal-OBn, β-d-Gal-(1→3)-β-d-Gal-OCH₂CH₂SiMe₃, and α-d-Gal-(1→3)-α-d-Gal-OCH₂CHCH₂. The β-d-galactosidase-catalysed reactions were efficient enough to allow the one-pot preparation of the various β-linked mono- and di-galactosides from lactose and alcohol.

3-Azi-1-methoxybutyl β-d-galactopyranoside, prepared from 2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl bromide in four steps, had an affinity constant for antigalactans IgA (Fab′) J539 and X24 of 1550 and 1730 m⁻¹, respectively. 3-Azi-1-methoxybutyl β-d-(4-³H)galactopyranoside, when photolysed in the presence of IgA X24, specifically labelled the galactan-binding area of the immunoglobulin.

The constants of association of seven monoclonal antibodies—each capable of binding a tetrasaccharide fragment of a linear β(1,6)-d-galactopyranan—were measured with a series of galactosylligands some of which carried deoxy-fluoro groups at selected locations. In these oligosaccharide ligands, the galactosyl residues bearing a fluorineinstead of a hydroxyl-group, cannot bind to the highest-binding subsite, which requires hydrogen-bonding. This forces a shift in the saccharide contact-residues, and in this way the relative affinities of the antibody subsites for individual galactosyl residues could be evaluated and compared with those of the four substites investigated earlier. Correlation of sequence data, spatial structure of J 539 and binding behaviour leads to the exclusion of the third complementarity determining region (CDR) of the H-chain as partaking in the binding, and shows that the galactopyranan antigen probably binds along the lower periphery of the H-L interface of the antibodies, and does so in a groove-type fashion. Each of the seven antibodies has four subsites C, A, B and D in going from the H-to the L-chain, and the relative affinity for “their” galactosyl residue decreases in the order A > B > C > D. The single sugar-binding subsite A accounts for ca 50% of the total binding free energy of the maximally binding tetrasaccharide determinant in all cases.