Binding studies on a mouse-myeloma immunoglobulin A having specificity for beta-D-(1 yields 6)-linked D-galactopyranosyl residues.

The free energies of binding between immunoglobulin A J539 (Fab') and methyl 6-O-acetyl beta-D-galactopyranoside (1) and 6-O-beta-D-galactopyranosyl-1, 2:3, 4-di-O-isopropylidene-alpha-D-galactopyranose (2) have been measured. The values found suggest that bulky substitution on O'-6 or O-1, O-2, O-3, and O-4 in the hapten 6-O-beta-D-galactopyranosyl-D-galactose (3) does not interfere with effective binding of that ligand and the immunoglobulin. This conclusion supports the postulations that (a) the ligand 3 binds only on one side of the molecule, and (b) the combining site of the immunoglobulin J539 appears to be located on an exposed surface area.