Spectrophotometric determination of tryptophan in intact proteins by the acid ninhydrin method

The interaction of tryptophan, lysozyme and tyrosine with ninhydrin in strong acid media has been investigated at 20, 25, 30, and 35 degrees C by spectrophotometry. Second-order rate constants and molar absorptivity values have been evaluated from an analytical point of view. Optimum conditions for the selective estimation of tryptophan, tryptophan residues in intact proteins, and indoles--without the disturbing effect of tyrosine--have been given. Under optimum conditions, in the concentration range from 2.5 X 10(-8) to 3.0 X 10(-7)M, molar absorptivity values and reproducibility data for various reactants have been reported. Molar absorptivity values (Am X 10(-3)/M X cm) of tryptophan (21.35), lysozyme (19.33), bovine serum albumin (21.05), human serum albumin (21.00), casein (17.85), alpha-chymotrypsin (18.28), trypsin (14.43), indole (5.03), and indole-3-acetic acid (13.75) have been measured with a standard error of 2.3% or less for any particular reactant.