Papain Catalyzed Oligomerization of α‐Amino Acids. Synthesis and characterization of water‐insoluble oligomers of L ‐methionine

Water‐insoluble oligomers were synthesized from L‐methionine ethyl ester with papain as the catalyst. L‐Oligomethionine was obtained in yields of 50% when synthesis was carried out in highly concentrated citrate buffer at pH 5.5. Yields of up to 85% were obtained when the enzymatic synthesis proceeded in distilled water at pH 6.5, the pH being strictly maintained. The insoluble polymer was converted to highly water‐soluble sulfoxide and sulfone derivatives, which consist mainly of an octamer with low amounts of heptamer or hexamer. Most of the carboxyl terminals still contained the ethyl ester group, only a minor part being present in the free acid form. The potential of the enzymatic approach for the synthesis of optically pure and monodisperse oligomers of α‐amino acids is discussed.