Phototransformation of l-tryptophan and formation of humic substances in water

The photo-transformation pathways of aqueous L-tryptophan were studied under simulated sunlight. It is known that humic substances can be formed from proteins by photo-chemical processes in surface waters, but the role of the single amino acids and their transformation pathways are not yet known. The irradiated L-tryptophan solutions were analyzed by absorption, fluorescence, Nuclear Magnetic Resonance (NMR) and Electronic Magnetic Resonance (EPR) spectroscopies, chromatography, potentiometry and mass spectrometry (MS). The solutions appeared turbid after irradiation, thus nephelometry and Dynamic Laser Light Scattering were used to characterize the suspended particles. About 95% of L-tryptophan was degraded in 8h irradiation, undergoing deamination and decarboxylation of the amino acid moieties to release ammonium and formate. The MS signal at m/z 146 suggested the formation of 3-ethylindole, while pH-metric and NMR data revealed the presence of hydroxylated compounds. The phototransformation intermediates of L-tryptophan had fluorescence and absorption spectra similar to those of humic substances, were able to produce •OH upon irradiation, and tended to aggregate by both ionic and hydrophobic interactions. For the first time we got insight into the nature of the compounds derived by the photo-transformation of L-tryptophan, the behavior of which was quite different from that of the previously studied L-tyrosine, although both compounds produced humic-like materials.