Chemical synthesis elucidates the absolute configuration and key antigenic epitope of <i>Vibrio cholerae</i> serotype O100 O-antigen

The emergence of drug-resistant strains of <i>Vibrio cholerae</i>, coupled with the current limitations of oral vaccines, underscores the urgent need for the development of new vaccines. The O-antigen of <i>V. cholerae</i> serotype O100 has emerged as a promising candidate for vaccine development. To investigate the absolute configuration of 3,5-dihydroxyhexanoyl (dHh) and to evaluate the structure-activity relationship of the O-antigen trisaccharide repeating unit, we completed total synthesis of four potential trisaccharide isomers, along with 11 additional oligosaccharide fragments of the O-antigen. Stereoselective reduction was used for the synthesis of dHh, and the efficient assembly of dHh and (<i>R</i>)-3-hydroxybutanoyl (<i>R</i>Hb) was achieved through a post-glycosylation modification strategy. Through NMR analysis, the absolute configuration of dHh was assigned 3<i>S</i>,5<i>S</i>. Glycan microarray screening indicated that <i>R</i>Hb is essential for the antigenicity of O-antigen. The nonreducing end disaccharide <b>59</b> may serve as the minimal antigenic epitope. These findings are an important step toward the design of semi-synthetic carbohydrate vaccines against <i>V. cholerae</i>.