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

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 (sigma(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 sigma(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 oil configuration of the amino acid residues.