A number of peptide intermediates and derivatives have been synthesized from both optically-pure and racemic amino acids. Carbobenzoxy-DL-alanyl chloride was coupled with L-leucine methyl ester and the mixed, crystalline carbobenzoxy-DL-alanyl-L-leucine methyl ester was isolated. Carbobenzoxy-DL phenylalanyl chloride and carbobenzoxy-DL phenylalanyl azide were coupled with L-leucine methyl ester and two products were separated by fractional crystallization. One of the products has been identified as carbobenzoxy-L-phenylalanyl-L-leucine methyl ester. A series of synthetic, dipeptide derivatives, containing some of those peptide bonds (present in the phenylalanyl chain of insulin) that Sanger and coworkers (31,32) found to be split by pepsin, were subjected to the action of pepsin at pH 2.0 and at pH 4-0. Proteolysis was detected by paper chromatography of the enzyme-substrate solutions. Of the compounds tested, carbobenzoxy-DL-phenylalanyl-DL-phenylalanine ethyl ester and carbobenzoxy-DL-phenylalanyl-L-tyrosine methyl ester were found to be hydrolysed by pepsin at pH 2.0 but not at pH 4.0. Synthetic compounds containing the peptide bonds - phenylalanyl-valyl, leucyl-valyl, glutamyl-alanyl, alanyl-leucyl, and glycyl-phenylalanyl -were resistant to pepsin even though it had been found (31,32) that, in insulin, these peptide bonds were pepsin-sensitive. These results indicate that, in the case of pepsin, observations on the action of pepsin on synthetic, dipeptide-type substrates should not be used to predict, or to explain, the specificity of pepsin on proteins or on high molecular weight peptides. / Medicine, Faculty of / Biochemistry and Molecular Biology, Department of / Graduate