The crystal structure of the murine Fab S-20-4 from a protective anti-cholera Ab specific for the lipopolysaccharide Ag of the Ogawa serotype has been determined in its unliganded form and in complex with synthetic fragments of the Ogawa O-specific polysaccharide (O-SP). The upstream terminal O-SP monosaccharide is shown to be the primary antigenic determinant. Additional perosamine residues protrude outwards from the Ab surface and contribute only marginally to the binding affinity and specificity. A complementary water-excluding hydrophobic interface and five Ab-Ag hydrogen bonds are crucial for carbohydrate recognition. The structure reported here explains the serotype specificity of anti-Ogawa Abs and provides a rational basis toward the development of a synthetic carbohydrate-based anti-cholera vaccine.

V O L . 28 ml.) was then added and the mutarotation observed at 20'. The specific rotations and first-order rate constants are given in Table 111. Based on an average rate of 0.00016, the "half-life" of the reaction was 1881 min.When mutarotation had ceased, the solvent was removed in vacuo to give a sirup which ww crystallized from ethyl acetatepentane. The short, colorless needles thus obtained were recrystallized from the same solvent mixture 0.2 g. (660/,), m.p. 133-135", [~I~O D +145.4" -* +102.5" (methanol, c 1.27). G. McCann.These values agree with those reported earlier in this paper for 2- 0-benzoyl-8-L-arabinopyranose.Acknowledgment.-We are indebted to Mr. Harry w . Diehl for the preparation of Some starting materials. h a l y s e s were performed by the ~~~l~~ services Unit Of this laboratory under the direction Of Mr. H. Condensation of 2,3,5-tri-O-benzyl-~-arabinofuranosyl chloride with methanol leads predominantly to methyl 2,3,5-tri-O-benzyl-p-~-arabinofuranoside. Condensation of the same halide with N-benaoyladenine and subsequent removal of the protecting groups readily gives 9-p-D-arabinofuranosyladenine, a type of glycoside which is difficultly accessible by other means.

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