7 OPTICAL ROTATORY DISPERSION OF DNP-AMINO ACIDS AND ITS APPLICATION TO PEPTIDE CHEMISTRY
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ORD-spectra of DNP-amino acids have two distinct features. The first is that all the DNP-amino acids so far measured showed very strong rotation near 220nm, especially in 4%-NaHCO_3aq. The big rotation was applied to know the configuration and further the D:L-ratio of amino acids, in a very small scale, and the hydrlysate of Gramicidine S was analyzed successfully. The second feature is that some DNP-amino acids have strong Cotton effect in their visible region spectra. They all have two DNP-chromophores in a molecule except DNP-phenylalanine. This interesting behaviour can be attributed to some interaction between the two DNP-chromophores. Similar phenomena are found in cases of steroid dibenzoates reported by K. Nakanishi et al. and of dinucleotides pointed by I. Tinoco, Jr. et al. If the strong Cotton effect means the spatial proximity of the two DNP-chromophores, it can be applied to the conformational analysis of peptides. Bis DNP-gramicidine S was prepared and showed a strong Cotton effect in the same region of its ORD spectrum. The conformation having the two delta-amino groups of the ornithine residues near enough to effect mutual interacion is suggested to be probable on the basis of this result, and is consistent with that proposed for Gramicidine S by L. C. Craig et al. and by R. Schwyzer et al.
