Peptide synthesis disadvantages

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

Sequencing by both Edman sequence analysis and mass spectrometric analysis can be complimentary and where one may fail in yielding information concerning the synthesis problem, the other may succeed. This is because every peptide has different properties and each technique has advantages and disadvantages. However, sequence analysis is expensive and time consuming, which must be taken into consideration when solving the problems of peptide synthesis, especially those of routine peptide synthesis. 

This peptide synthesis, although hampered by some disadvantages, already featured most of the key elements of solid-phase peptide synthesis that have ultimately led to the development of the powerful synthetic protocols used today. 

The first solid-phase peptide synthesis reported by Merrifield had some disadvantages, which were corrected in later versions of this synthesis. The main improvements were the replacement of benzyloxycarbonyl protective groups by the TFA-labile Boc protection, and the use of TFA-resistant linkers cleavable by HF. 

In its original form the DCC procedure for peptide synthesis has one basic disadvantage, which you have already learned about as side reaction B —> E in Figure 6.15 When acyl isoureas are exposed to a poor nucleophile, they rearrange to form unreactive A-acyl ureas. These mismatched reactivities characterize the situation encountered in the peptide syntheses in Figures 6.31 and 6.32. The example given in Figure 6.31 uses 0,0-di-fert-butylserine as a... 

Solid phase peptide synthesis is the only widely practiced technique of facilitation but several other methods have also been proposed for the rapid construction of long peptide chains. Some of these alternative approaches have certain advantages over solid phase synthesis reactions carried out in solution are not affected by the rate-limiting control of the gel. Also, where isolation of intermediates is possible, these can be analyzed and purified. A major disadvantage, however, common to the various techniques discussed below, is that they are less conducive to mechanization and automation than the Merrifield method. 

The equilibrium-controlled peptide synthesis has the main advantage that all kinds of proteases can be used, independently of their catalytic mechanism. However, a serious disadvantage of the method is the low reaction velocity and low product yield of the peptide synthesis. As already mentioned, to overcome this problem, low water reaction mixtures like reverse micelles, which favor the shift of the reaction toward peptide synthesis, can be used. 

Thiols. 5-Acetyl derivatives can be prepared by the reaction of acetic anhydride and a thiol in the presence of potassium bicarbonate. Several disadvantages to the 5-acetyl group in peptide synthesis include /3-elimination upon base-catalyzed hydrolysis. Also, sulfur to nitrogen acyl migration may be problematic. 

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