Lichen secondary metabolites putatively protect lichens from a variety of environmental stress factors, but it is unknown whether these substances respond to air pollution. To assess such a possibility, the three major phenolics of two epiphytic lichen species with contrasting tolerance to chronic air pollution from Mexico City were studied by combining experimental reactivity data and measured field contents. The antioxidant activity and antiradical power of boninic (BO), 2-O-methylsekikaic (MA), and usnic (US) acids, isolated from the tolerant Ramalina asahinae and salazinic acid (SA), atranorin (AT), and chloroatranorin (CA), from the sensitive Parmotrema stuppeum, were determined in vitro by kinetic experiments with ozone and the free radical diphenyl picryl hidrazyl (DPPH*), respectively. In addition, the field contents of these phenolics in the lichens, and the potential antioxidant capacity (PAC) they provide, were compared among three forested sites exposed to urban emissions and a similar, relatively clean site. The six phenolics had antioxidant activity and antiradical power according to these trends: CA >> AT > US > SA > or = BO > or = MA for O(3); and CA > AT > US > MA > SA = BO for DPPH*. The three most reactive phenolics are cortical compounds, located in the lichen portion most exposed to the surrounding environment. In contrast, the less reactive SA, BO, and MA are medullary. Such reactivity patterns indicate that some phenolics may provide antioxidative protection at the air-lichen interface. The higher antioxidant power of CA and AT may be due to the reactive hydroxyl groups at positions 2 and 4 of ring A, instead of the less reactive methoxyl at the same positions in both BO and MA. In the field comparisons, total quantified phenolics were significantly higher near Mexico City for both lichens, except for the tolerant R. asahinae at one site. Nevertheless, only the latter species had significantly increased PAC values at all sites near the city. This result is explained by species-dependent changes in individual phenolics. At the polluted sites, R. asahinae had consistently higher contents of its most reactive phenolic, US, with values approximately twice that of the control site. In contrast, P. stuppeum only increased its less reactive SA (26-35%), but this was counteracted by CA and, to a lesser extent, AT degradation. Thus, the substantial increase in US at the polluted sites appears to be associated with the current ecological success of R. asahinae near the city. On the other hand, the inability of P. stuppeum to overcome degradation of its most reactive phenolic (CA) at the same sites seems to partially explain the declining status of this lichen. These results provide evidence for a protective mechanism in lichens against air pollution based on secondary metabolites, which may eventually determine which species survive in forests stressed by oxidative air pollution.

Bioassay-guided fractionation of a phytotoxic extract of Prionosciadium watsoni led to the isolation of three new pyranocoumarins and two pyranochromones. The new compounds were characterized as propionic acid (9R,10R)-9-acetoxy-8,8-dimethyl-9,10-dihydro-2H,8H-benzo[1,2-b:3,4-b']dipyran-2-one-10-yl ester (1), isobutyric acid (9R,10R)-9-hydroxy-8,8-dimethyl-9,10-dihydro-2H,8H-benzo[1,2-b:3,4-b']dipyran-2-one-10-yl ester (2), isobutyric acid (9R)-8,8-dimethyl-9,10-dihydro-2H,8H-benzo[1,2-b:3,4-b']dipyran-2-one-9-yl ester (10), 2-methylbut-(2Z)-enoic acid (3R)-5-methoxy-3,4-dihydro-2,2,8-trimethyl-6-oxo-2H,6H-benzo[1,2-b:5,4-b']dipyran-3-yl ester (11), and isobutyric acid (3R)- 5-methoxy-3,4-dihydro-2,2,8-trimethyl-6-oxo-2H,6H-benzo[1,2-b:5,4-b']dipyran-3-yl ester (12) by spectroscopic and chemical methods. The stereochemistry at the stereogenic centers was established by applying the Mosher ester methodology. The structures of 1 and 2 were corroborated by single-crystal X-ray diffraction studies. The phytotoxic activity of the isolated compounds was assessed on Amaranthus hypochondriacus, Echinochloa crus-galli, and Lemna pausicostata. The phytotoxins also modified the electrophoretic mobility of calmodulin from both bovine-brain and spinach.

The páramo ecosystem is situated in northwest of South America, and it is well known for distribution of lichens. It is documented that lichens produce antioxidant activity, and protect cells from oxidative stress by scavenging free radical. Hence we checked the antioxidant and protective abilities to prevent cell and DNA damage by oxidative stress of Lobariella pallida and Stereocaulon strictum var. compressum extracts. Additionally, the active constituents namely lobariellin (1), methyl orsellinate (2), stereocaullin (3), methyl haematommate (4), methyl β-orcinol carboxylate (5), porphyrilic acid (6) and atranorin (7), were isolated from both extracts and their antioxidant activity was evaluated. The DPPH  , ferric reducing power and inhibition of lipid peroxidation model were used to determined the antioxidant activity of extracts and isolated compounds. The cell and DNA protective effects of extracts and isolated compounds were investigated by exposing keratinocytes, neurons and pUC18 plasmid to hydroxyl radicals, and examined cell viability or DNA cleavage of sample. The findings exhibits that the compounds 1, 4 and 6 scavenged free radicals, inhibited lipid peroxidation and reduced ferric ions at maximum level compared to other isolated compound and extracts. Additionally, 1 and 4 protected cells from oxidative damage and 1 also preserved DNA. These results suggest that 1 and 4 imparts antioxidant activity to prevent oxidative skin and neurodegenerative damage.

The antioxidant and photo-protective efficacy of the extract and isolated compounds from the lichen Usnea rocellina Motyka were evaluated. The antioxidant activity was determined by scavenging the DPPH radicals and ferric reducing power. The photo-protective property of extracts and isolated compound against ultraviolet A (UVA) and B (UVB) radiations was measured in vitro by calculation of their sun protection factor (SPF), critical wavelength and UVA ratio comparing to commercialised sunscreens (avobenzone and benzophenone-3). 3-methoxycarbonyl-2-hydroxy-6-methoxy-4-methylbenzoic acid (1), reported here for the first time, along with (+)-(9b-R)-usnic acid (2) and decarboxythamnolic acid (3) were isolated. The structure of the isolated compounds was characterized by interpreting their spectroscopic (NMR, IR, UV), and spectrometric data. The findings of this study demonstrated that the compound 3 is a better antioxidant compared to other isolated compound and extract, and was similar to standard (BHT and ascorbic acid) ascorbic acid. On the other hand, 1 showed to be an UVB absorbing with a moderate in vitro SPF whereas 2 and 3 showed broad UVB and UVA absorbing properties. The possible skin penetration, determined by calculating octanol/water partition coefficients allowed to confirm this property in 2 and 3. These results suggest 3 as a dual agent with antioxidant and broad spectrum absorbing properties therefore it is able to prevent oxidative skin damage caused by solar radiation.

Two phenanthrene derivatives, characterized as erianthridin (9,10-dihydro-2,7-dihydroxy-3,4-dimethoxyphenanthrene) and gymnopusin (2,7-dihydroxy-3,4,9-trimethoxyphenanthrene), were isolated from an extract of the orchid Maxillaria densa, using phytotoxicity with amaranth (Amaranthus hypochondriacus) to guide fractionation. Gymnopusin and erianthridin inhibited radicle elongation of A. hypochondriacus seedlings with IC 50 values of 330 and 58.2 mM, respectively. The phytoxicity of the two phenanthrene derivatives was also assessed on duckweed (Lemna pausicostata), and compared with mammalian toxicity estimated in vitro with four mammalian cell lines. On duckweed, both phenanthrene derivatives caused electrolyte leakage, chlorophyll loss and photobleaching. Ultrastructural examination of duckweed frond and root tissues treated with gymnopusin (100 mM) revealed membrane damage to the tonoplast after 12 h of exposure. Effects on membrane integrity followed a time course similar to that of electrolyte leakage. Both phenanthrene derivatives exhibited moderate cytotoxicity to all mammalian cells tested, which precludes their use as a bioherbicide. #