Synthesis of the Repeating Unit of the O-speci®c Polysaccharide of Shigella sonnei and Quantitation of its Serologic Activity Aniko ToÂth, a AdeÂl Medgyes, a IstvaÂn Bajza, a AndraÂs LiptaÂk, a, * Gyula Batta, b Tivadar Kontrohr, c Kla ra PeÂtery c and Vince Pozsgay d a Research Group for Carbohydrates of the Hungarian Academy of Sciences and Institute of Biochemistry, Lajos Kossuth University, Debrecen, Hungary, H-4010 b Research Group for Antibiotics of the Hungarian Academy of Sciences, Debrecen, Hungary c Institute of Microbiology, University Medical School, PeÂcs, Hungary d Laboratory of Developmental and Molecular Immunity, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA Received 11 August 1999; accepted 6 October 1999 AbstractÐThe chemical synthesis of the zwitterionic disaccharide 2 is described that corresponds to the repeating unit of the O- speci®c polysaccharide (1) of the Gram-negative human pathogen Shigella sonnei. Passive hemolysis inhibition tests using a hyper- immune rabbit serum raised against S. sonnei showed that the serologic activity of the disaccharide 2 is nearly 2- to 3-fold higher than those of its component monosaccharides. NMR data of 2 are in support of the proposed structure of the O-speci®c poly- saccharide. # 1999 Elsevier Science Ltd. All rights reserved. Shigella sonnei is a Gram-negative bacterium that can cause diarrhea and dysentery in humans. A major cell surface component of this bacterium is its O-speci®c polysaccharide 1,2 (O-SP) that is the outermost, sero- determinant domain of the highly complex lipopoly- saccharide. The O-SP is an essential virulence factor of Shigellae including S. sonnei: only strains that have their O-SP fully expressed are virulent. Additionally, the O-SP is related to host immunity in that protection against infection is a correlate of the IgG antibody level against the O-SP. 3 The O-SP of S. sonnei is a non- immunogenic molecule (MW 25 kDa) that must be conjugated to an immunogenic protein carrier to induce antibodies. Clinical trials have con®rmed that protein conjugates of the O-SP elicited IgG levels in adult volunteers that were similar to those in patients con- valescent from shigellosis. 4 The O-SP of S. sonnei is composed of the zwitterionic disaccharide repeating unit 1 consisting of the rare sugars 2,4,6-trideoxy-2- acetamino-4-amino-d-galactose and 2-acetamido-2- deoxy-l-altruronic acid that are connected through 1,2- trans interglycosidic linkages. 1,2 Our approach to vaccine development is based on the assumption that fragments of O-SPs mimic epitopes of the native polymers and thus may induce poly- saccharide-speci®c antibodies when conjugated to an immunogenic carrier. Recent data con®rmed this assumption and showed that albumin conjugates of chemically synthesized extended oligosaccharide frag- ments 5,6 of the O-SP of S. dysenteriae type 1 elicited higher levels of the homologous O-SP-speci®c IgG antibodies than did the conjugate of the native polysaccharide. 7 As a prelude to chemical synthesis of oligosaccharides corresponding to 1 we have reported the synthesis of the methyl glycosides of its component monosaccharides in their natural glycosidic con®guration. 8 We have also presented evidence that in the native polysaccharide both pyranose residues are preferentially in the 4 C 1 chair conformation and that the polysaccharide exists in the zwitterionic form. 9 Here we describe the ®rst synth- esis of a disaccharide part (2) of the O-SP that to our knowledge is also the ®rst report of a zwitterionic dis- accharide containing free amino and carboxyl groups at alternating monosaccharide residues. The disaccharide 2 was constructed from an appropriately protected altruronic acid derivative (5) that was used as the acceptor and from the activated donor 9. The common 0960-894X/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0960-894X(99)00585-5 Bioorganic & Medicinal Chemistry Letters 10 (2000) 19±21 *Corresponding author. Tel.: +36-52-316-666; fax: +36-52-512-913; e-mail: liptaka@tigris.klte.hu