MOLECULAR RECOGNITION .10. A NOVEL PROCEDURE FOR THE DETECTION OF THE INTERMOLECULAR HYDROGEN-BONDS PRESENT IN A PROTEIN-BULLET-OLIGOSACCHARIDE COMPLEX

The relative potencies of both the monodeoxy and mono-O-methyl derivatives of the Le(b)-OMe tetrasaccharide (alpha-L-Fuc-(1 --> 2)-beta-D-Gal-(1 --> 3)-[alpha-L-Fuc-(1 --> 4)]-beta-D-GlcNAc-OMe) as inhibitors of the complexation of a Le(b) artificial antigen by the lectin IV of Griffonia simplicifolia are interpreted in terms of the X-ray crystal structure at 2.5 angstrom resolution of the GS-IV . Le(b)-OMe complex. Both kinds of derivatives maintain high potencies when the hydroxyl groups involved appear, in the crystal structure, to be in contact with the aqueous phase. Hydroxyl groups situated at the periphery of the combining site and hydrogen bonded to the protein can also be deoxygenated without important loss in potency. However, their methylation leads to a strong decrease in the stability of the complex, because the steric bulk of the introduced methyl group causes loss of complementarity. In contrast, the hydroxyl groups that form hydrogen bonds with the protein along the base of the shallow amphiphilic cleft of the combining site can neither be deoxygenated nor methylated without virtually complete loss of binding activity. Thus, the procedure can provide an appreciation of the various kinds of hydrogen bonds that are present in a protein . oligosaccharide complex. Hard-sphere calculations supported these contentions since an energetically favorable orientation was indicated for a methoxy group at any one of the five positions that were,expected to remain in contact with the aqueous phase. However, the calculations, as expected, showed the introduction of strong destabilizing nonbonded interactions when the methylation involved hydroxyl groups that are hydrogen bonded to the protein in the complex. The results are in accord with the previously made rationalization of the near linear enthalpy-entropy compensation found for the active deoxy congeners.