The different b-lactamase variants studied in this work were purified using procedures previously described in detail in refs. . Briefly, genes for the His-tagged proteins were cloned into a pET24 vector with kanamycin resistance were cloned into E. coli BL21(DE3) cells, and the proteins were purified by NTA affinity chromatography. Stock solutions for activity determinations and physicochemical characterization were prepared by exhaustive dialysis against the desired buffer.