Catalytic specificity exhibited by p-sulfonatocalix[n]arenes in the methanolysis of N-acetyl-L-amino acids

Specific acid catalysis of p-sulfonatocalix[n]arenes(n = 4, Calix-S4; n = 6, Calix-S6; n = 8, Calix-S8) was observed in the alcoholysis of N-acetyl-L-amino acids in methanol. The methanolysis rates of basic amino acid substrates (His, Lys, and Arg) were markedly enhanced in the presence of Calix-Sn, as compared with rates observed with p-hydroxybenzenesulfonic acid (pHBS), which is a noncyclic analogue of Calix-Sn. This catalytic effect of Calix-Sn was not observed for the methanolysis of Phe, Tyr, and Trp substrates. On the other hand, H-1 NMR experiments following the effect of Calix-Sn on N-acetyl-L-amino acid substrates in CD3OD showed that the spectrum of a mixture of the His substrate with Calix-Sn was significantly different from the combined spectra of the respective compounds. These changes in spectra support the formation of an inclusion complex of Calix-Sn with basic amino acids. Furthermore, it was obvious that methanolysis of the His substrate catalyzed by Calix-S4 and Calix-S6 obeyed Michaelis-Menten kinetics. These results indicate that the catalytic activity of Calix-Sn originates from its forming a complex with specific substrates (basic amino acids), similar to enzymatic reactions.