Synthesis of Peptides and Polypeptides

The synthesis of peptides and proteins in nature is a complex process that you may study in detail in a biochemistry course. However, because short peptides and their derivatives may exhibit useful properties and potent biological activities, chemists in academic and pharmaceutical laboratories have develop ed efficient methods for their synthesis. 

The fundamental problem in the synthesis of peptides is that amino acids must be connected in a defined sequence by specifically forming a peptide bond between the carboxylic acid group of one amino acid and the amino group of another. The potential difficulties arising in such endeavors may be illustrated by considering the synthesis of the simple dipeptide Ala-Phe from the individual amino acids L-alanine (Ala) (3) and L-phenylalanine (Phe) (4), as seen in Equation 24.2. Formation of peptide bonds in a random manner could lead to four different dipeptides. This mixture arises because the carboxylic acid of Ala may react with either the amino 

P Knowledge about the structure-function interrela-i tionships in proteins and peptides has encouraged biochemists to develop techniques for synthesizing peptides and proteins with predetermined sequences. To synthesize a peptide in the laboratory, we must overcome several problems related to preventing undesired groups from reacting. The amino and carboxyl groups that are to remain unlinked must be blocked so must all reactive side chains. 

In this chapter we dealt with some of the fundamental properties of amino acids and polypeptide chains. The following points are especially important. 

Nineteen of the 20 amino acids commonly found in proteins have a carboxyl group and an amino group attached to an a-carbon atom they differ in the side chain attached to the same a carbon. 

All amino acids have acidic and basic properties. 

The ratio of base to acid form at any given pH can be calculated from the pK with the help of the Hen-derson-Hasselbach equation. 

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Determination of Amino Acid Composition of Proteins Determination of Amino Acid Sequence of Proteins Chemical Synthesis of Peptides and Polypeptides... 

Despite the achievements of polypeptide synthesis in solution, currently at the research level, most polypeptides are synthesized by solid phase peptide synthesis (SPPS) [5, 6]. This approach, pioneered by Bruce Merrifield revolutionized the synthesis of peptides and the principles have been applied... 

The synthesis of peptides and proteins involves formation of amide bonds between amino and carboxyl groups of a-amino acids. The synthesis of a polypeptide... 

Merrifield s revolutionary concept of solid-phase synthesis was not limited to peptides, and similar techniques have been developed for the synthesis of nucleic acids and carbohydrates on solid supports. For each application, specialized instrumentation that is computer-controlled is commercially available. Access to such equipment has enabled researchers in areas of biology, medicine, material science, and biomedical engineering to prepare thousands of peptides and polypeptides for study. In the pharmaceutical industry, for example, solid-phase synthesis has been used to prepare relatively large numbers of related molecules, so-called compound libraries, that... 

The preparation and availability of suitable soluble polymer supports are basic requirements for the successful application of liquid-phase synthesis of peptides and nucleotides. Many parameters have to be considered when designing appropriate supports and the route of synthesis and the target product of synthesis must also be considered. Liquid-phase synthesis requires optimum solubility properties of the polymer supports and therefore the adaption of the functional capacity to the solubilizing power of the polymer backbone and side-chains is the determining step before synthesis. The main factors of evaluation are the solubility characteristics of the polypeptide or oligonucleotide to be synthesized. 

How can chemists imitate the synthesis of peptides and proteins that nature manages to carry out so effectively Unfortunately, it s not so easy. If we consider the coupling of two amino acids, 22.33 and 22.34, there are four potential products—each amino acid can couple with an identical molecule, and the coupling between them has two potential directions. Moreover, because they are carbonyl compounds, peptides (amides) epimerize easily. Polypeptides are usually water soluble, but most coupling methods work best in nonaqueous media. 

The rational synthesis of peptide-based nanotubes by self-assembling of polypeptides into a supramolecular structure was demonstrated. This self-organization leads to peptide nanotubes, having channels of 0.8 nm in diameter and a few hundred nanometer long (68). The connectivity of the proteins in these nanotubes is provided by weak bonds, like hydrogen bonds. These structures benefit from the relative flexibility of the protein backbone, which does not exist in nanotubes of covalently bonded inorganic compounds. 

DKPs are simple and easy to obtain and are quite common by-products of synthetic, spontaneous, and biological formation pathways. DKP formation has been well documented as side reactions of solid-phase and solution-phase peptide synthesis. In addition, DKPs have been shown to be decomposition products of various peptides, proteins, and other commercial pharmaceuticals. Cyclic dipeptides were found to be present in solutions of human growth hormone, bradykinin, histerlin, and solutions of agents within the classes of penicillins and cephalosporins. " DKPs are also enzymatically synthesized in several protists and in members of the plant kingdom. Hydrolysates of proteins and polypeptides often contain these compounds and they are commonly isolated from yeasts, lichens, and fungi. ... 

Fischer polypeptide synthesis org chem A synthesis of peptides in which a-amino acids or those peptides with a free amino group react with acid halides of a-haloacids, followed by amination with ammonia. fish-ar pal-e pep,tTd. sin tha sas ) Fischer projection orgchem) A method for representing the spatial arrangement of groups around chiral carbon atoms the four bonds to the chiral carbon are represented by a cross, with the assumption that the horizontal bonds project toward the viewer and the vertical bonds away from the viewer fish-ar pra.jek-shon) Fischer s salt See cobalt potassium nitrite. fish-3rz solt)... 

The treatment of a N-substituted hydrazine hydrobromide successively with the a,a-dicyano-epoxide 91 and with TV-methyl hydrazine results in the racemic a-hydrazino hydrazide 92 (Scheme 29)/111 Repeating the above two reactions allows the elongation of 92 into the racemic (hydrazide) peptide 93/1121 A derived procedure, with ClCH2COCl instead of 91, is applied to the synthesis of the first (hydrazide) cyclotripeptide 94 (R1 = 4-C1C6H4 or 4-MeQlLi) (Scheme 30), by coupling of the N-terminal chloroacetyl and C-terminal hydrazine groups/113 It has been later extended to the synthesis of linear (hydrazide) polypeptides/114 It is noteworthy that the a-carbon is racemic in all of the above cases. 

Synthesis of the protected polypeptide always starting with the carboxyl-terminal amino acid and proceeds with the addition of amino acids one at a time, and ends with the addition of the amino terminal amino acid. The synthesis steps can be performed manually using a fritted glass reaction vessel with a Teflon valve, however, it is more efficient to use a programmable peptide synthesizer, which is capable of performing the steps faster and more reproducibly, than can be accomplished manually. 

Peptide synthesis. This water-soluble diitnide is useful for synthesis of peptides, particularly if it is used in combination with 1-hydroxybenzotriazole. The method was used extensively in a total synthesis of urogastrone, a polypeptide with 53 amino acid residues and three disulfide bonds, which controls human epidermal growth. ... 

In view of these possible complications, the chemical synthesis of peptides is generally performed in the C-to-N direction by a stepwise elongation approach using preferentially A -alkoxycarbonyl-protected amino acid derivatives to produce the target polypeptide chain in solution and on solid supports directly. Specially designed intermediate segments, usually... 

By analogy to the terms co- and posttranslational modifications of peptides and proteins to define these transformations in the in vivo biosynthesis, chemical manipulations at least theoretically can be carried out in a co- or postsynthetic manner. While nature exploits the sequence- and even conformation-dependent regioselectivity of enzymes to expand the molecular and functional diversity of peptides and proteins beyond the genetic code,P l synthetic chemical reactions are insufficient for the required selectivity even with the most advanced conjugation techniques. Therefore, the tactics usually employed involves a cosynthetic approach, i.e. synthesis of polypeptide chains with annino acid derivatives or... 

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