糖与肽键合的方式及位点具多样化。首先从糖组分看,既可以由异头碳上的氧原子也可由该碳原子上的氮与肽键合,既可以由单糖形式也可以由双糖及寡聚糖形式与肽键合。从肽的底物结构看,含羟基侧链的残基、含氨基侧链的残基、含羧基侧链甚至含酰胺侧链的残基均可以各自的方式与糖键合(图1)
图1 糖与含羧基肽及含氨基肽的键合
例1 把D-葡萄糖衍生成含噁唑啉结构的中间体,再与固相载体上含Ser、Thr或Tyr残基的肽键合(图2)。
图2糖与含Ser残基肽链的固相键合
例2 肽链上含有多个含侧链一COOH残基而又只对其中的一个(或几个)进行糖化时,可用正交方式保护不同的一COOH。在下列的壳二糖糖肽合成中,对被糖化的ASp给以烯丙酯保护方式(可用Pd脱除)。而旁边的Glu则用叔丁酯形式保护。后者对pd很稳定,不干扰AsP的糖化反应(图3)。
图3含多个侧链一COOH的选择性糖化合成
例3 除了前面的先组装肽链后糖化方式外,还可以利用硅醚Linker先使糖组分与载体相连。随后组装成二糖,后者经活化后再与肽段上的Asp侧链一COOH相键合(图4)。
图4硅醚Linker糖组分与肤的键合
例4 以三烷基硅氯为Linker前体,在碘化钠催化下与保护氨基酸的侧链羟基或糖的羟基形成硅醚键。此Linker结构与固相载体相连后,可以在C端或N端两个不同方向上进行肽链或糖肽的组装合成。基本方式如下所示(图5)。
图5
杭州专肽生物官方微信:allpeptide2
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