The specificity of peptide chain extension by N-carboxyanhydrides

We have used amino acids activated by carbonyldiimidazole to study the enantiospecificity of peptide elongation in aqueous solution. Peptide 'primers' Glu10 and Ala3Glu10 were elongated with the enantiomers of arginine, glutamic acid, asparagine, phenylalanine, serine and valine. The homochiral addition was always the more efficient reaction; the enantiospecificity was large in some cases but very small in others. In every case Ala3Glu10 was elongated more efficiently than Glu10.