Phenylalanine esters, copper complexes

Micelle-forming copper complexes were found to effectively discriminate between enantiomers in the hydrolysis of a-amino esters (257). Hydrolysis of (.V)-phenylalanine p-nitrophenyl ester is 14-fold faster than its enantiomer, Eq. 223. Leucine affords 10-fold faster hydrolysis. The authors note that the micellar nature of these systems is extremely important for both rate of hydrolysis and selectivity (258). For example, the /V-mcthyl-dcrivcd ligand 419b leads to inhibition of the hydrolysis process, relative to catalysis by Cu(II) ion alone. 

Hayama et al.132 discussed the catalytic effects of silver ion-polyacrylic add systems toward the hydrolyses of 2,4-dinitrophenylvinylacetate 84 (DNPVA) by using the weak nudeophilicity of carboxylic groups and the change-transfer interactions between olefinie esters and silver ions133Metal complexes of basic polyelectrolytes are also stimulating as esterase models. Hatano etal. 34, 13S) reported that some copper(II)-poly-L-lysine complexes were active for the hydrolyses of amino acid esters, such as D- and L-phenylalanine methyl ester 85 (PAM). They... 

Table 7. Hydrolysis of phenylalanine methyl ester catalyzed by poly(S-lysine)-copper(II) complex (28)...

Several features of the above studies were reinvestigated in a detailed kinetic study of the copper(II) complexes of glycine methyl ester and phenylalanine ethyl ester in glycine buffer at pH 7.3 (26). Glycine was selected as a buffer in this study in order that a small increase in the glycine concentration caused by the hydrolysis reaction would not increase the concentration of copper(II) complexes to a significant extent. It was found that the rate constant for the hydrolysis of the copper(II) complex of DL-phenylalanine ethyl ester was 106 times greater than the rate constant obtained for the alkaline hydrolysis of the free ester (25). 

The catalytic activity of poiy(S-lysine) — copper(II) complex in homogeneous aqueous solution for the hydrolysis of the optical antipodes of phenylalanine methyl ester was examined by Nozawa, Akimoto and Hatano (28). As shown in Table 7, the R antipode was hydrolyzed faster than the S antipode by 2—4 times, at pH = 7. At this pH poly(S-lysine) — copper(II) complex is in random conformation. At higher pH, where the complex assumes a-helical conformation, spontaneous hydrolysis would proceed with higher rate than the catalytic reaction. 

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