Up to the present, we have studied chiroptical properties of Dnp derivatives of normal α-amino acids and related compounds, and proposed four rules relating the absolute configuration at the NH_2 bearing asymmetric carbon to the sign of the Cotton effect near 400nm. A new rule concerning N-Dnp-N-Me-amino acids is now proposed (Fig. 1). They show antipodal CD spectra to those of Dnp cyclic α-imino acid (Fig. 2, Fig. 3). This interesting results can be explained by assuming an equilibrium between two conformers as shown in Fig. 4. It has been confirmed as expected that the former four rules can be extended to β-amino acids, too (Fig. 5). Practical application of the method to an iturinic acid has been carried out. Iturinic acids are a general name given for the long chain β-amino acids occurring in some antifungal peptidolipid (Fig. 6) produced by Bacillus subtilis. The CD spectrum of the Dnp-iturinyl-pMA (Fig. 7) indicated the absolute configuration at β to be R. Examples to demonstrate applicability of Dnp-Aromatic rule to aromatic amino acids having various aromatic nuclei are also presented (Fig. 8). Finally, scopes and limitations in applying the five rules to the determination of the absolute configuration of newly found amino acids are discussed. The Dnp method can be applied safely to ca. 60% of the known abnormal amino acids, which has been picked up from the appendix table of ref. 9). Therefore, the method seems quite useful for the purpose on the title as a whole using the five rules appropriately.
