嘧啶核苷:C工-N1糖苷键CIN糖苷键 N 3N 5CH HN CH C、16CH CH N HOH29 HOH2还 OH OH OH OH 胞嘧啶核苷CR 尿嘧啶核苷UR嘧啶核苷：C1´ - N1糖苷键 4 C1'-N1糖苷键 胞嘧啶核苷CR 尿嘧啶核苷UR 1 2 3 5 6