Production of melanin pigments by chemical and enzymatic oxidation of tetrahydroisoquinolines.

Tetrahydropapaveroline (THP) oxidation was studied in various experimental conditions by absorbance spectroscopy. THP was found to be easily oxidized by mushroom tyrosinase, giving rise to the formation of a chromophore (THP-chrome) with absorption maxima at 308 and 470 nm. The oxidation further proceeds leading to the formation of a melanin-like pigment. The use of periodate as oxidant at pH 7.4 allows the visualization of the THP-chrome, as well. Other tetrahydroisoquinolines bearing a catechol moiety, such as salsolinol, laudanosoline and apomorphine, have been found to be easily oxidized in the same conditions, giving rise to pigmented derivatives. The products of THP oxidation are able to copolymerize with dopa or opioid peptides in the presence of tyrosinase, generating mixed-type melanins.