Effect of the Substituent Group at the Isothiocyanate Moiety of Edman Reagents on the Racemization and Fluorescence Intensity of Amino Acids Derivatized With 2,1,3-Benzoxadiazolyl Isothiocyanates

Abstract

It is shown that an electron-withdrawing or -donating group at the para-position of aromatic isothiocyanate significantly affects the racemization of 2,1,3-benzoxadiazolylthiazolinone (TZ) derivatives of amino acids, derivatized with newly synthesized benzoxadiazolyl isothiocyanates in Edman sequence analysis. A linear relationship between the logarithms of the TZ-amino acid enantiomer ratio and the para-substituent constants (ς _p ) for the isothiocyanate moiety was obtained, and the D/L configuration of the amino acid residue was retained with an isothiocyanate containing an electron-donating group at the para-position. The para-substitution effect on the racemization of phenylthiohydantoin (PTH) amino acids was also confirmed by several para-substituted phenylisothiocyanate (PITC) reagents, including nitro-PITC, chloro-PITC, PITC, methyl-PITC and methoxy-PITC. The relationship between the fluorescence intensity of the 2,1,3-benzoxadiazolyl TZ amino acid and ς _p was also demonstrated. When the isothiocyanate containing an electron-donating group was used, the fluorescence intensity of the TZ-amino acid decreased while retaining the D/L configuration of the amino acid residues.