Synthesis longer peptides

To avoid the problems with kinetic control of a reaction during peptide bond formation, it is recommended that longer peptide phosphonates be synthesized stepwise by extension in the N-terminal direction, starting with diphenyl l-(aminoalkyl)phosphonates. The longer coupling time will provide peptide products that are usually an equivalent (or close to equivalent) mixture of diastereomers. The synthesis of longer peptides is demonstrated by the synthesis of Boc-Val-Pro-Valp(OPh)2, which is an excellent inhibitor of human neutrophil elastase (kobs/[I] = 27,000 M 1s 1)J38 ... 

A typical random peptide library displays polypeptides usually from 6 to 15 amino acids long. The length of the peptides is chosen based on the type of ligand one wants to select, the number of randomized positions, and the size of the library. For example, ligands for affinity purification should be short in order to diminish the cost of peptide synthesis. A longer peptide would probably work better as a live vaccine. A larger number of randomized positions would increase sequence diversity and the size of the complete library. 

The above observations on the swelling properties of the resin supports in different solvents point to the fact that the extent of swelling and solvation is a very decisive factor for the successful execution of the solid phase synthesis of longer peptides 153). 

Using this method, peptides of up to a hundred amino acids can be synthesized, but the yields of full-length peptides decrease rapidly as synthesis of peptides longer than about 50 amino acids is attempted. In addition, the yield of a peptide is also dependent on the amino acid composition and on the sequence of the peptide. In a typical example, TGF-o, a 50 amino acid peptide was synthesized at an average coupling efficiency of 99.65% at each step (Kent, 1988). 

In addition to the impressive synthetic procedures elaborated for peptidomimetics, an important goal for the synthetic peptide chemist during the 1980s remained the efficient synthesis of longer and longer peptides. Several laboratories were committed to the synthesis... 

Its use in solution synthesis is limited by the presence of sulfur-containing anoino acid residues as these poison metal catalysts, albeit methods have been proposed that partially overcome this serious drawback (see Section 2.1.1.1.1.1.3.1). This linoitation is also bypassed in cases where the target peptide molecule permits acidolysis with strong acids (HBr/AcOH or HF). As an additional limitation, the saponification of Al -Z-protected peptide esters under drastic conditions, such as those required for longer peptide intermediates, was found to induce decomposition of the Al -Z moiety with generation of N-ternninal hydantoins. ... 

To further the scope of this methodology, application to the synthesis of longer peptides, such as A 3(l-42), on a 2-CTC resin was undertaken [73] (see below). Recently, this peptide has also been assembled using a photocleavable Nvoc group [74]. The protecting group was completely removed by photolysis under the conditions used for the O to N acyl shift, thereby obtaining the final peptide in a clean manner. 

The purity of protected amino acids is especially important for the synthesis of longer peptides. Standard techniques such as melting point determination, nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and optical rotation are effective means of characterization. The optical purity can also be evaluated by high-performance liquid chromatography (HPLC) after derivatization with Marfey s reagent [216,217]. The advanced Marfey method refers to analysis by mass spectrometry after derivatization with Marfey s reagent [218-221]. Purification of side-chain protected amino acids by recrystallization is usually sufficient. 

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