Abstract
Liquid chromatography coupled with mass spectrometry is an outstanding methodology for fast analysis of phenolic compounds in biological samples. Twenty two compounds were quickly and accurately identified in the methanolic extract of the Antarctic lichen Ramalina terebrata for the first time using ultra high pressure liquid chromatography coupled with photodiode array detector and high resolution mass spectrometry (UHPLC-PDA-Q/Orbitrap/MS/MS). In addition, the extract and the four compounds isolated from this species were tested for the inhibitory activity of tau protein aggregation, which is a protein involved in Alzheimer’s disease (AD). All compounds showed null activity with the exception of parietin, which it was able to inhibit aggregation process of tau in a concentration range between 3 µg/mL (10 µM) to 28 µg/mL (100 µM). In addition, we show how parietin interact with tau 306VQIVYK311 hexapeptide inside of the microtubule binding domain (4R) with the help of molecular docking experiments. Finally, the constituents present in the methanolic extract could possibly contribute to the established anti-aggregation activity for this extract and this in-depth analysis of the chemical composition of R. terebrata could guide further research into its medicinal properties and potential uses.
Highlights
Lichens are symbiotic associations between heterotrophic fungi and algae and/or cyanobacteria
Usimine A–C and ramalin isolated from Antarctic lichen R. terebrata, displayed activity against B. subtilis but no activity against S. aureus, where the values of MIC ranged from 1–26 μg/mL [40]
The current structural information about tau protein is very limited; our results suggest that parietin could interact with tau fibril-forming motifs VQIINK (PHF6*), with lysine residues
Summary
Lichens are symbiotic associations between heterotrophic fungi and algae and/or cyanobacteria. The models obtained show that both protonation states have similar orientations forming HB interactions with lysine and glutamine side chains. Peak 8 could be tentatively identified as 4-O-dimethylbaemycesic acid (m/z 359.0756) which produced a MS2 ion at m/z 302.0417. Ramalina terebrata Hook and Taylor from the Antarctic is the producer of usnic acid, ramalin, stereocalpin A and usimines A–C [40,41,42,43]. Usimine A–C and ramalin isolated from Antarctic lichen R. terebrata, displayed activity against B. subtilis but no activity against S. aureus, where the values of MIC ranged from 1–26 μg/mL [40]. The antioxidant activity of atranorin, protolichesterinic acid, usnic acid, 2-hydroxy-4-methoxy-6-propylbenzoic acid, homosekikaic acid, sekikaic acid, 2,4-dihydroxy-6-propylbenzoic acid and 2,4-dihydroxy-3,6-dimethylbenzoate isolated from the hexane extract from R. roesleri were assessed by the DPPH method. D-protolichesterinic acid and lobaric acid are considered 5- and 12-lipoxygenase inhibitors [40]
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