(a) Explain why two monomethyl esters of N-acetyl-laspartic acid are known. Draw their structures.

(b) Explain why a mixture of these two compounds can be separated by cation-exchange chromatography at pH = 3.0, but not at pH = 7. Your explanation should indicate which of the two compounds would emerge first from a cation-exchange column at pH = 3.0. Explain.