is optically active, Md-90 (0.288% in methanol). Pepstatin shows no maximum at 220^ 400 mju except for end absorption. It gives positive Rydon-Smith and permanganate tests but negative ninhydrin, Sakaguchi and Ehrlich reactions. It is soluble in methanol, ethanol, acetic acid, pyridine and dimethylsulfoxide, but is slightly soluble or insoluble in ethyl acetate, ether, benzene, chloroform and water. As reported in the next paper10), the following structure is proposed for pepstatin:

12.27, O18. 68; found: C 59.69, H 9.45, N 12.17, O 18.24). Treatment of pepstatin or its methyl ester in pyridine with acetic anhydride gives their mono or diacetyl derivatives. The esters, the amide and the acetyl derivatives of pepstatin showed 50% inhibition of pepsin using the method described above at the following concentrations: pepstatin 0.01 jug/ml; the methyl ester 0.008 Ag/ml; the ethyl ester 0.01 jug/ml; the^>-bromophenacyl ester 0.01 jug/ml; the amide 0.027/zg/ml; the monoacetyl derivative 1.1//g/ml; the diacetyl derivative 4.2