The photophysical, photoinduced pro-oxidant and antibacterial properties in vitro of the natural occurring parietin (PTN; 1,8-dihydroxy-3-methoxy-6-methyl-9,10-anthraquinone) were evaluated. PTN was extracted from the lichen identified as Teloschistes flavicans (Sw.) Norm. (Telochistaceae). Results indicate that in chloroform solution, PTN presents spectroscopic features corresponding to an excited-state intramolecular proton-transfer (ESIPT) state with partial keto-enol tautomerization. In argon-saturated solutions, the singlet excited state is poorly fluorescent (Φ = 0.03), decaying by efficient intersystem crossing to an excited triplet state PTN*, as detected by laser-flash photolysis experiments. In the presence of triplet molecular oxygen, thePTN* was fully quenched producing singlet molecular oxygen (O) with a quantum yield of 0.69. In addition, in buffer solutions, PTN has the ability to also generate a superoxide radical anion (O˙) in a human leukocyte model and its production was enhanced under UVA-Vis irradiation. Finally, the in vitro antibacterial capability of PTN in the dark and under UVA-Vis illumination was compared in microbial cultures of both Gram positive and negative bacteria. As a result, PTN showed promising photo-induced antibacterial activity through the efficient photosensitized generation of both O and O˙ species. Thus, we have demonstrated that PTN, an efficient photo-screening pigment in lichens, is also a good photosensitizer in solution with promising applications in antibacterial photodynamic therapy.

In this study, we show that several UV-Vis absorbance, steady-state and time-resolved fluorescence parameters of a series of dissolved humic substances (DHS) from different sources (e.g. terrestrial fulvic and humic acids, and humic acid-like molecules extracted from composted and vermicomposted wastes) correlate with the molar absorptivity at 280 nm per mole of organic carbon (ε(280)), which in turn is proportional to the molecular complexity (e.g. molecular size, aromaticity and oxidation degree) of the DHS. Both absorbance and fluorescence spectral responses were sensitive to the molecular complexity associated with the maturation degree of the DHS. Depending on the DHS, different emitting responses by excitation at the UVA (340 nm) and VIS (460 nm) regions of the absorption spectra were observed. The results were explained in terms of the extent of intramolecular electronic interactions between electron donor groups, such as polyhydroxylated aromatics and indoles, and more oxidized acceptor groups (e.g. quinones or other oxidized aromatics) as the molecular complexity of the DHS increased.