1. The replacement of the phenolic proton of tyrosine by alkyl, aryl or acyl groups completely abolishes the chymotryptic hydrolysis of tyrosine esters. Similarly, chymotrypsin fails to hydrolyze the tyrosyl bonds of dinitrophenyladrenocorticotropin.2. Several model substrates have been synthesized and characterized. The action of chymotrypsin was followed potentiometrically in buffer containing some alcohol. The hydrolysis of the analogous compounds unsubstituted a t the phenolic group was normal.3. We conclude that inhibition is due, not to an inductive effect, but to a blocking effect which is markedly affected by replacement of the phenolic proton, even with a group as small as a methyl group.In the course of our effort to develop an isotopic chemical method for the assay of adrenocorticotropin by dinitrophenylation with fluor~dinitro-[~H]-benzene and subsequent hydrolysis with chymotrypsin, it has been observed that no bis(dinitropheny1)-seryltyrosine could be identified from the digested mixture. We inferred that the o-dinitrophenyl derivative of tyrosine may interfere with the action of chymotrypsin of tyrosyl peptide bonds [l]. The rates of hydrolysis of the ester bond of L-tyrosine derivatives having the general formula I with R' = H are somewhat increased or decreased depending upon the nature of the substituents on the amino group [2], but very limited work has been done on the esterase activity of chymotrypsin when the phenolic group of tyrosine is blocked by alkylation [3] or acylation. I n order to verify our hypothesis, model substrates of structure I were synthesized, and the esterase action of chymotrypsin on them was studied.

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Ein Malein‐/Fumarsäuregemisch in Ammoniumhydrogencarbonatlösung wird an einer Sephadex‐G‐10‐Säule in die Isomeren getrennt.