Glutamic acid cyclization

Since the proline residue in peptides facilitates the cyclization, 3 sublibraries each containing 324 compounds were prepared with proline in each randomized position. Resolutions of 1.05 and 2.06 were observed for the CE separation of racemic DNP-glutamic acid using peptides with proline located on the first and second random position, while the peptide mixture with proline preceding the (i-alamine residue did not exhibit any enantioselectivity. Since the c(Arg-Lys-0-Pro-0-(i-Ala) library afforded the best separation, the next deconvolution was aimed at defining the best amino acid at position 3. A rigorous deconvolution process would have required the preparation of 18 libraries with each amino acid residue at this position. 

In subsequent experiments (66), this locked substrate was used to obtain evidence for the hypothesis (67) that enzyme-bound y-glutamyl phosphate 14 is an intermediate in the enzyme-catalyzed reaction. All attempts to isolate this acyl phosphate 14 have failed (66), presumably because of the marked tendency of this intermediate to cyclize to pyrrolidonecarboxyUc acid, 15, and to hydrolyze to glutamic acid. 

The diverse origin of two structurally similar compounds is exemplified by the (3 -lactam antibiotics isopenicillin N (1) and clavulanic acid (2). While these molecules are structurally and functionally similar, they are derived by quite different routes. Isopenicillin N is formed by the direct cyclization of the tripeptide (3) (B-80MI10400) while clavulanic acid appears to be elaborated directly from a three-carbon intermediate of the glycolytic pathway (possibly phosphoenolpyruvate, 4) and glutamic acid (5) (B-80M110401). 

Peptides can also be cyclized through their side-chain functionalities (e.g. using lysine, cysteine, glutamic acid, or aspartic acid [68-71]), or with the aid of synthetic spacers ([72] see also Table 8.5). Examples have been reported of the preparation of cyclic peptides using safety-catch linkers (e.g. (4-alkylmercapto)phenol [62], sulfonamide [73], or hydrazide [74]), 2-nitrophenyl ester attachment [61], or oxime ester... 

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