A specific purple–blue color reaction between tryptophan and tyrosine in concentrated acidic medium

Electron transfer between tryptophan (Trp) and tyrosine (Tyr) is often accompanied with transient emission of visible blue-purple light, which is very common in numerous laser-induced photolysis experiments. However, recent experiments have shown that in the absence of pulse radiolysis, by mixing Trp and Tyr in concentrated aqueous hydrochloric acid can also produce a blue–purple color as it was observed in photolysis experiments. Unexpectedly, this color is quite stable in strong acid media (it can be kept at room temperature for at least a week). It is proposed that after initial formation of Trp radical products (TrpH ▪+ /Trp ▪ ) by the solvent radical oxidation, charge-transfer complexes were generated via electron transfer reactions between Trp radical products and phenol of Tyr or their directly hole-trapping mechanism, which exhibit a blue-purple color. In this work, HPLC analysis and density functional calculations show that there are loosely reversible associations in the charge-transfer complexes which can be stabilized by strong hydrogen bonding. A Trp radicals signal was observed but the intensity was not so strong as expected. This color reaction exhibits high selectivity towards Trp and Tyr, and thus it can be a convenient method for naked-eye identifying the coexistence of these two amino acids or protein modifications of related to them.