F. Bertoncin et al., KINETIC AMPLIFICATION OF THE ENANTIOSELECTIVE CLEAVAGE OF ALPHA-AMINO-ACID ESTERS BY METALLOMICELLES, Langmuir, 14(5), 1998, pp. 975-978
The enantioselective cleavages of p-nitrophenyl esters of phenylalanin
e (PhePNP), phenylglycine (PhgPNP), and leucine (LeuPNP) catalyzed by
Cu(II) complexes of homochiral ligands have been investigated in condi
tions of very fast change of pH (''pH-jump'') at the beginning of the
reaction. Using lipophilic ligands comicellized with hexadecyltrimethy
lammonium bromide and decreasing the pH from 9 to 5, a remarkable ampl
ification of the enantioselectivity has been observed when compared to
the same reaction performed at constant pH (from 24 to 58 for PhgPNP,
as the ratios of the rate constants measured for the faster and slowe
r reacting enantiomers). This has been correlated with the changes in
the ligand/Cu(II) complex concentration induced by the change of pH. I
n fact, it has been established that under the conditions employed the
faster enantiomer of the substrate reacts with a higher concentration
of catalyst than the slower one: in the first case the reaction occur
s before decomplexation takes place and in the second case after decom
plexation is virtually complete.