Development of Chemical Ligation in Peptide Synthesis

Usually, unprotected peptide segments are readily soluble, can be more easily handled and can be directly characterized by ESM. The most efficient method for the synthesis of unprotected peptides is SPPS, introduced by Merrifield in 1963 [20], and by 1980, it had been developed into a powerful tool for the synthesis, in good yield and high purity, of peptides with up to 50 AAs [18]. 

Two reactive functional groups are usually incorporated into each peptide by chemical synthesis. Their mutual recognition triggers a chemoselective reaction and allows the use of completely unprotected peptide segments, which led to a marked increase in the size of available synthetic peptides in recent years, as presented in Fig. 12.3. The fact that such chemoselective reactions may be carried out in aqueous buffers is of considerable methodological importance. In addition. 

As simple as this methodology can be, it is not without a principal drawback. This is presented by the formation of an unnatural bond at the site of ligation between the two peptide segments. Such bonds include thioester- [25], oxime- [26], thioether- [27], disulfide- [28] and thiazole-forming [29] CL. These unnatural structures, however, are often well tolerated within the folded protein and examples exist of fully active protein molecules synthesized by CL [23]. 

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