Fragmentation reactions amino acids/peptides

A superspiral consisting of two spirals (coiled coil), known as the leucine zip, is formed in this sequence via dimerisation. The condensation reaction, carried out in the aqueous phase, involves two peptide fragments which contain 15 and 17 amino acid residues respectively. Activation takes place via thioester formation (see Sect. 5.3.1). The ligation to a complete GCN4 matrix gives a new 32 amino acid peptide, which can itself serve as a matrix. The autocatalytic reaction exhibits a parabolic increase in the peptide concentration (caused by product inhibition see Section 6.4). 

As the intermediate products resulting from individual synthetic steps cannot be purified, a virtually 100% selectivity is essential for the synthesis of larger-peptide molecules. Even at a selectivity of 99% per reaction step, the purity will drop to less than 75% for a dekapeptide (30 steps) It is practically infeasible to go beyond 10-15 amino acid peptides by using the solid-phase method. In order to prepare larger peptides, individual fragments are first produced, purified, and then combined with the final molecule by liquid phase synthesis. This combination of methods is listed under chemical hybrid in Table 4.2. 

Peptides formed enzymically or by mineral acid hydrolysis or thermal degradation of higher molecular veight protein can also serve as flavor precursors in thermally induced reactions. The reactivity of peptides is evidenced by their behavior during pyrolysis/GC (9), heating in air (10), reactions vith mono- (11), and dicarbonyl (12, 13) compounds and reactions in hot acetic acid (1A). The types of reactions observed for peptides include side-chain thermolysis, fragmentation into amino acids, DKP formation and Halliard reaction vith ambient carbohydrates. 

Abstract This review provides an overview of some of the more recent work directed to exploit radical-based chemistry for the modification of some of Natures most important biomolecules, such as amino acids, peptides, and carbohydrates. Radical reactions are particularly advantageous for carrying out a variety of structural modifications on biomolecules as the reaction conditions are typically compatible with a wide variety of functional groups and solvents. An array of effective synthetic transformations will he discussed including selective side chain and backbone modifications of amino acids and peptides, along with methods for the transformation of carbohydrate substituents, as well as fragmentation and cyclizations reactions for the preparation of either structurally modified carbohydrates or chiral building blocks. 

An alternative approach to peptide sequencing uses a dry method in which the whole sequence is obtained from a mass spectrum, thereby obviating the need for multiple reactions. Mass spec-trometrically, a chain of amino acids breaks down predominantly through cleavage of the amide bonds, similar to the result of chemical hydrolysis. From the mass spectrum, identification of the molecular ion, which gives the total molecular mass, followed by examination of the spectrum for characteristic fragment ions representing successive amino acid residues allows the sequence to be read off in the most favorable cases. 

Importantly, it was demonstrated that no significant racemization occurred in the course of peptide formation. Furthermore, the complete coupling of difficult peptide sequences could be accomplished within a few minutes, and it was determined that peptide fragments have higher reactivity than single amino acid derivatives under microwave irradiation conditions. However, the exact reaction temperature during the irradiation period was not determined. 

The repetitive cycle to identify a sequence of N-terminal amino acids has been automated. In practice, it is limited to about 20-30 amino acids, since impurities build up and the reaction mixture becomes too complex to yield unequivocal results. The usual approach is to break the polypeptide chain into smaller fragments by partial hydrolysis, preferably at positions relating to specific amino acid residues in the peptide chain. There are ways of doing this chemically, and the enzymes chymotrypsin... 

An alternative to the synthesis of proteins by classical fragment synthesis in solution or by solid-phase synthesis on a support is the use of enzyme-catalyzed condensation of amino acids or peptides. This possibility was first demonstrated in 1938 91 with the synthesis of poorly soluble benzoyl-leucyl-leucine anilide by papain catalysis. After many years, this approach was extended to the preparation of peptide hormones such as Leu-enkephalin 92 and dynorphin(l -8).[93 This was made possible by the use of highly purified enzymes and by careful control of reaction conditions. The basic principles of protease-catalyzed peptide bond formation have been discussed.194 ... 

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