Methods for forming peptide bonds

In this section the synthesis of fluoroalkyl (Section 15.1.4.1.3), a,a-difluoroalkyl (Section 15.1.4.2.3), and trifluoromethyl- and perfluoroalkyl ketones are discussed collectively. The second most widely used method for synthesizing peptide fluoromethyl ketones is the Henry nitro-aldol condensation reaction, which involves the use of (3-nitro alcohols to build the fluoromethyl ketones. As with the modified Dakin-West procedure, the Henry reaction has also been used to synthesize mono-, di-, tri-, and extended fluoromethyl ketones, making it another extremely versatile synthetic method.19 12 19 27 29 33 341 However, similar to the Dakin-West procedure, the products of the Henry reaction are not chiral, since an achiral carbanion is involved in the crucial carbon bond forming step. 

The biomolecules are attached directly to the matrix by chemical/covalent linkage, which is not reversed by pH or changes in ionic strength. Carbodiimide coupling to form peptide bonds has been extensively used for the covalent coupling of the enzyme with polymers [126-128]. Since chemical modification is involved, this method results in the drastic loss of activity of the enzymes/biomolecules. Covalently attached redox biomolecules on polymers have been utilised as highly electron transfer mediators in flavin adenine dinucleotide (FAD) centres of oxidases [73]. 

Diphenylphosphinic mixed anhydrides have been utilized to form peptide bonds. Peptides are easier to isolate by this method than by employing 1,3-Dicyclohexylcarbodiimide. These anhydrides are the method of choice for the formation of amides of 2-alkenoic acids (eq 1 ). Carbodiimide and acyl carbonate methods proved to be inferior. Primary amines result in better yields than secondary amines. This activation protocol can be employed to form thiol esters (eq 2) p-Amino acids are readily converted to p-lactams with chlorodiphenylphosphine oxide (eq 3). Secondary amines work best. This activation protocol has been utilized to convert acids to amines via a Curtius rearrangement. Phenols have been generated from diene acids, presumably via base-induced elimination of diphenylphosphinic acid from the mixed anhydrides to form ketenes which spontaneously cyclize. Acids have been converted to ketones via activation followed by reaction with organometallic reagents (eq 4)."... 

The chemistry of the reactions involved in coupling peptides is the same as that for coupling TV-alkoxycarbonylamino acids. However, the oxazolone that is formed by the activated peptide is chirally unstable, it is formed more readily, and there is an added impetus for it to form because the rate of bond formation between segments is lower. In addition, segments usually have to be coupled in polar solvents because they are insoluble in nonpolar solvents, and polar solvents promote the undesirable side reaction. The result is that the number of procedures actually used for coupling peptides is rather small. The methods in question are addressed below. 

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