α-Chymotrypsin-catalyzed synthesis of poly-l-cysteine in a frozen aqueous solution

Poly-l-cysteine (PLCys) is drawing attention as a potential sorbent of thiol (SH)-reactive toxic heavy metal ions in the wastewater and polluted soils. However, preparation of PLCys relies on chemically synthesized polymers, in which SH groups must be protected and deprotected prior to use. On the other hand, α-chymotrypsin polymerized l-cysteine ethyl ester in a frozen aqueous solution, provides PLCys with degree of polymerization from 6 to 11 without blocking of SH groups. Kinetic analyses suggested that the acylation of α-chymotrypsin with the initial substrate was a rate-limiting step in the enzymatic polymerization. The peptide yields reached 85% and 65% of SH groups in PLCys were assumed to be free forms. Although detail information on correlation between the state of SH groups and heavy metal adsorption properties of PLCys should be explored in further studies, the present study for the first time proposed an easy method for synthesis of PLCys requiring neither SH-protection nor -deprotection.