Peptide bond preparation

M.M. Hann, P.G. Sammes, P.D. Kennewell, J.B. Taylor, On the double bond isostere of the peptide bond Preparation of an enkephalin analog, J. Chem. Soc. Perkin Trans. 1 (1982) 307-314. 

Thiol esters, which are more reactive to nucleophiles than are the corresponding oxygen esters, have been prepared to activate carboxyl groups for both lactoniza-tion and peptide bond formation. For lactonization S-f-butyl and S-2-pyridyP esters are widely used. Some methods used to prepare thiol esters are shown below. The S-r-butyl ester is included in Reactivity Chart 6. 

Schemes are available, however, that start from the free carboxylic acid, plus an activator . Dicyclohexylcarbodiimide, DCC, has been extensively employed as a promoter in esterification reactions, and in protein chemistry for peptide bond formation [187]. Although the reagent is toxic, and a stoichiometric concentration or more is necessary, this procedure is very useful, especially when a new derivative is targeted. The reaction usually proceeds at room temperature, is not subject to steric hindrance, and the conditions are mild, so that several types of functional groups can be employed, including acid-sensitive unsaturated acyl groups. In combination with 4-pyrrolidinonepyridine, this reagent has been employed for the preparation of long-chain fatty esters of cellulose from carboxylic acids, as depicted in Fig. 5 [166,185,188] ...

In view of these constraints, we recently suggested a different strategy for the improvement of the material properties of synthetic poly (amino acids) (12). Our approach is based on the replacement of the peptide bonds in the backbone of synthetic poly(amino acids) by a variety of "nonamide" Linkages. "Backbone modification," as opposed to "side chain modification," represents a fundamentally different approach that has not yet been explored in detail and that can potentially be used to prepare a whole family of structurally new polymers. 

N-Silylated peptide esters are acylated by the acid chloride of N-Cbo-glycine to N-acylated peptide bonds [11]. Likewise, acid chlorides, prepared by treatment of carboxylic acids with oxalyl chloride, react with HMDS 2 at 24°C in CH2CI2 to give Me3SiCl 14 and primary amides in 50-92% yield [12]. Free amino acids such as L-phenylalanine or /5-alanine are silylated by Me2SiCl2 48 in pyridine to 0,N-protected and activated cyclic intermediates, which are not isolated but reacted in situ with three equivalents of benzylamine to give, after 16 h and subsequent chro-... 

Saudek V, Pivcova H, Drobnik J (1981) NMR-study of poly(aspartic acid). 2. Alpha-peptide and beta-peptide bonds in poly(aspartic acid) prepared by common methods. Biopolymers 20 1615-1623... 

P 15] Diverse protocols for routes for deprotection and peptide bond-forming reactions in micro reactors have been reported [5, 88]. These are needed for preparation of longer chain peptides. 

OS 23] [R 5] [P 15] Deprotection and peptide bond-forming reactions in a micro reactor and their yields have been described [5,88]. Establishing protocols for these reactions paves the way to the preparation of longer chain peptides in micro reactors. 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

TJ Blacklock, R Hirschmann, DF Veber. The preparation and use of /V-carboxyanhydrides and V-thiocarboxyanhydndes for peptide bond formation, in The Peptides Analysis, Synthesis, Biology, Vol. 9, pp 39-102. Academic Press, New York, 1987. 

It is interesting to note that serine peptidases can, under special conditions in vitro, catalyze the reverse reaction, namely the formation of a peptide bond (Fig. 3.4). The overall mechanism of peptide-bond synthesis by peptidases is represented by the reverse sequence f-a in Fig. 3.3. The nucleophilic amino group of an amino acid residue competes with H20 and reacts with the acyl-enzyme intermediate to form a new peptide bond (Steps d-c in Fig. 3.3). This mechanism is not relevant to the in vivo biosynthesis of proteins but has proved useful for preparative peptide synthesis in vitro [17]. An interesting application of the peptidase-catalyzed peptide synthesis is the enzymatic conversion of porcine insulin to human insulin [18][19]. 

Examlnatloi of molecular models revealed that the desired two B-tum conformation might be restored by moving the N-methyl group from D-Ala to D-Phell in 4. This change would permit the key cis peptide bond to form in the proper location in the backbone and should as a result achieve better overall side chain correspondence with 3b. The resultant structure, cyclo- (N -Me-D-Phe-D-Thr-D-Lys-L-Trp-D-Phe-D-Ala) 5, was prepared and in fact, displayed a full biological response and had about 10) of the potency of 3b and 25 of somatostatin Itself. 

Various EADIs have been synthesized using different synthetic routes [30-34], One of the widely used methods of preparation of E-alkene peptide bond isosteres involving ring opening of vinyl aziridines by cuprates has recently been reviewed [35,36],... 

Remark-. Preparation of monofluoroalkenes. Monofluoroalkenes are interesting substrates for cyclopropanation and for Diels-Alder reactions." They are also used as a nonhydrolyzable mimic of the peptidic bond (cf. Chapters 3 and 7). The main preparations of fluorinated olefins are as follows ... 

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