Structural Scaffold of 18-Crown-6 Tetracarboxylic Acid for Optical Resolution of Chiral Amino Acid: X-Ray Crystal Analyses of Complexes of D- and L-Isomers of Serine and Glutamic Acid

Abstract

(+)-18-Crown-6 tetracarboxylic acid (18C6H₄) has been used as a chiral selector for D/L-amino acids in HPLC, where L-isomer is usually eluted prior to D-isomer, except for the case of serine. To clarify why serine exhibits the reverse order for the elusion, the chiral interactions of D- and L-serines with (+)-18C6H₄ were investigated by the X-ray single crystal analyses, together with the case of D- and L-glutamic acids, which exhibit the usual elution order in HPLC. The backbone structures (amino, Cα-H and carboxyl groups) of these four amino acids showed the nearly same interaction with (+)-18C6H₄ despite their different chirality. In contrast, the hydroxyl group of L-serine side chain formed a hydrogen bond with the carboxyl group of (+)-18C6H₄, whereas such a interaction was not formed for the side chain of D-serine and D- and L-glutamic acids. Thus, it was shown that the exception of D/L-serine from the first elution rule of L-isomer in HPLC is due to the presence and absence of a hydrogen bond formation of its side chain OH group.