Photochemistry of norharmane in aqueous solution

Abstract

The photochemistry of norharmane (9H-pyrido[3,4-b]indole) in acidic (pH 5.0 ± 0.1) and alkaline (pH 10.0 ± 0.1) aqueous solutions was studied. The photochemical reactions were monitored by TLC, UV/VIS absorption spectroscopy, high-performance liquid chromatography (HPLC), electronic ionization-mass spectrometry (EI-MS), UV-laser desorption/ionization-time of flight-mass spectrometry (UV-LDI-TOF-MS) and an enzymatic method for H₂O₂ determination. The neutral (nHoN) and the protonated (nHoH⁺) forms of norharmane irradiated under Ar atmosphere were photostable, but they suffered a photochemical transformation in the presence of O₂, yielding as photoproducts norharmane dimers, trimers and tetramers. nHoN shown to be more photostable than nHoH⁺. The nHoH⁺ and nHoN consumption quantum yields were 1.82 × 10⁻³ and 0.51 × 10⁻³, respectively, and the mechanisms involved in its photochemistry are discussed. H₂O₂ and singlet oxygen (¹O₂) were also detected and quantified in irradiated solutions of norharmane, and their role in the photochemistry of norharmane is discussed.