2,3-Dioxo-5-indolinesulfonic acid, a new highly specific reagent for modification of tryptophan in peptides and proteins.

ABSTRACT

A new reagent, 2,3-dioxo-5-indolinesulfonic acid (DISA), has been investigated for its specificity to react with tryptophan and tryptophan residues in peptides and proteins. On reaction of 19 amino acids with 1 M excess of DISA in 0.1 M acetic acid (pH 2.9), considerable (50%) modification of tryptophan was obtained within 50 min and no other amino acid was modified. After reaction for 5 h, only proline showed very slight (7%) modification. On reaction in the presence of 5 M excess of DISA, tryptophan was very rapidly modified. Modification of proline became appreciable in the presence of this molar excess of reagent and cysteine modification, although much smaller, became detectable (12 h, 7%). However, proline modification was completely prevented when this amino acid was engaged in a peptide linkage, even after reaction for 45 h in the presence of 5 or 10 M excess of DISA per proline residue. Reaction of egg albumin with 50 M excess of DISA was entirely specific for tryptophan and showed no modification of proline or cysteine residues. The reagent offers the advantages of stability, easy handling, high water solubility and high specificity. It affords protein and peptide derivatives that are completely water soluble because of the polar nature of the added group. The yellow color (lambda max, 367 nm) of the derivatives offers advantages of easy determination of the extent and location of the modification.