Medium-sized peptides

Despite the high lysine and arginine content of most antimicrobial peptides, tryptic digests typically yield medium-sized peptides, as often a proline residue follows these basic residues eliminating the cleavage site. 

Convergent methods avoid the cumulative disaster of linear synthesis. Access to large peptides can be provided by use of medium-sized peptide segments, easily available by solid-phase synthesis. There are two distinct approaches. One involves the synthesis and puri-... 

The polymer reagent method has been used in peptide synthesis by other investigators also 95 -97). The method has been applied to the synthesis of a number of medium-sized peptides and biologically active protein sequences 98). 

The polymeric active ester method has been used successfully for the preparation of several small- to medium-sized peptides in very pure form the potentiality of the method has also been illustrated by the synthesis of a number of biologically active protein sequences like bradykinin 106), thyrotropin-releasing hormone 107), ACTH sequences 105), and LH—RH 108> in good overall yields. 

Automation of the liquid-phase synthesis, at its present stage, is advantageous only when the optimum reaction conditions for the synthesis of a specific peptide have been elucidated in advance, and the synthesis of small- to medium-sized peptides appears to be within the scope of this automated method. 

These findings can be of considerable impact for the conventional strategies of peptide synthesis. The segment condensation of medium-sized peptides with low solubility to larger peptide chains may result in the increased solubility of the reaction product due to conformational transitions. Whenever it is possible, amino acid residues with low tendencies for p-strueture formation should be inserted towards the middle of the peptide segments. In this respect, the prediction of secondary structures can be a useful tool in establishing the most efficient path for peptide synthesis. To this end, experimental elucidation of the conformational preferences of side-chain-protected trifunctional amino acids is of much relevance 250). 

A simple approach to protein description consists of representing a protein by a sequence of properties of its constituent amino acids. Each amino acid is described by one ore more properties and therefore the total number of protein descriptors is given by the product of the number of amino acids in the protein and the number of selected amino acid properties. As this number of descriptors increases very fast with the size of proteins, this approach is usually applied to small- and medium-size peptides. Moreover, in QSAR studies that require uniform-length descriptors, it can be used only to describe a series of peptide analogues, vhich are peptide sequences with the same length. To enable QSAR studies of peptide sequences with different length, some method is required that is able to translate the peptide sequences into vectorial descriptors with the same number of variables. For example, ACC transforms were applied to compress information about principal properties of amino acids into peptide sequences with different length. 

M8. Mabuchi, H., and Nakahashi, H., Medium-sized peptides in the blood of patients with uremia. Nephron 33, 232-237 (1983). 

Solid phase peptide synthesis[6, 7] (SPPS) is now an established methodology for the synthesis of small- to medium-sized peptides. Since the first reports on this technique in the early 1960s [3] several improvements in protecting groups, coupling reagents and solid supports have allowed access to a broad range of peptides in a routine and automatic manner. However, the synthesis of complex peptides... 

Kemp et al have now published complete details on the use of this reagent for peptide synthesis. The second step is best carried out with a tetraalkylam-monium salt of an amino acid or with an amino acid (or peptide) in DMSO containing 1 eq. of tetramethylguanidine, [(CH3)2N]2C=NH. The method has been shown to be useful with all the common amino acids except arginine and histidine. Medium-sized peptides have been prepared. Kemp concludes that this method is comparable to the azide, carbodiimide, and mixed-anhydride methods of peptide synthesis. Difficulties with yield and purification have been observed with fairly large peptides. 

The chemical route is often a better technological option than the biotechnological methods of recombinant DNA and biocatalysis for the synthesis of medium size peptides that comprise most of the pharmaceutically relevant molecules. The synthesis of peptides was originally performed in solution. However, since the introduction of solid-phase synthesis by Merrifield (1986), this technology has gained more relevance (Stewart and Young 1984). 

NMR spectroscopy has developed to such an extent that it has become the most useful method for the elucidation of the conformation of peptides in solution. It can also identify the amino acid sequence in medium size peptides. The method is based on the magnetic properties of the nuclei of isotopes with spin numbers I = 1/2. Of these the proton ( H), the carbon isotop C, and to a lesser extent are of practical importance. 

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