Z-D-Ala-Gly-NH2 → (S)-Benzyl (1-((2-amino-2-oxoethyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate

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For accurate comparison, the D-isomers Z-D-Phe-OBn, Z-D-Ala-OBn, Z-D-Glu-OMe, Z-D-Lys-OBn, and Ac-D-Phe-OBn (15-19) of the representative L-amino acids Z-L-Phe-OBn, Z-L-Ala-OBn, Z-L-Glu-OMe, and Z-L-Lys-SBn (1-4) examined in the previous ligation examples were used. The stereoselectivity of SBL-WT for L-amino acids was clear (Table 4), because none of the D-amino acid esters evaluated gave dipeptide products with WT as the catalyst. All of the S166C-MEs yielded dipeptide products containing D-amino acids Z-D-Phe-Gly-NH2, Z-D-Ala-Gly-NH2, Z-D-Glu-Gly-NH2, Z-D-Lys-Gly-NH2, and Ac-D-Phe-Gly-NH2 (20-24). While each of these enzymes still showed a preference for L-amino acids, yields of up to 66% of Z-D-Phe-Gly-NH2, using S166C—SCH2C6H5, over 0% for WT, demonstrated a dramatic improvement in SBL's acceptance of D-amino acids.

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