Chemical Synthesis and Antigenicity Evaluation of Shigella dysenteriae Serotype 10 O‑Antigen Tetrasaccharide Containing a (S)‑4,6‑O‑Pyruvyl Ketal

• Published in: Journal of the American Chemical Society 2022-11, Vol.144 (46), p.21068-21079
• Main Authors: Qin, Chunjun, Li, Lingxin, Tian, Guangzong, Ding, Meiru, Zhu, Shengyong, Song, Wuqiong, Hu, Jing, Seeberger, Peter H., Yin, Jian
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.2c05953

Shigella is the second most common etiologic pathogen responsible for childhood acute diarrhea. An anti-Shigella vaccine is still eagerly awaited due to the increasing drug resistance of this pathogen. The Shigella O-antigen is a promising vaccine target. To identify the immune epitopes of the glycan, the first total synthesis of Shigella dysenteriae serotype 10 O-antigen tetrasaccharide containing a (S)-4,6-O-pyruvyl ketal was completed. The 1,2-trans-β-glycosylation & C2 epimerization and conformational locking strategies facilitated the construction of two 1,2-cis-β-glycosidic linkages. The reactivities of both the glycosyl donor and acceptor were improved by adding electron-donating benzyl groups, enabling an efficient assembly of the tetrasaccharide. The (S)-4,6-O-pyruvyl ketal was introduced at the final stage due to its influence on the glycosylation stereospecificity and efficiency. In addition, (R)-4,6-O-pyruvylated and nonpyruvylated tetrasaccharides and three further fragments were synthesized. Glycan microarray screening revealed that the tetrasaccharide repeating unit is the key antigenic epitope of the O-antigen. Moreover, the (S)-4,6-O-pyruvyl ketal is an essential structural feature of this antigen for designing carbohydrate-based vaccines against S. dysenteriae serotype 10. The comparison of the (S)-4,6-O-pyruvylated glycan and its (R)-epimer will set an example for biological evaluation of other bacterial glycans containing pyruvyl ketals.