LC-MS and NMR guided isolation of monoterpene dimers from cultivated Thymus vulgaris Varico 3 hybrid and their antityrosinase activity

Thymus vulgaris (Lamiaceae), is an important culinary herb and medicinal plant used for the treatment of productive cough and upper respiratory congestion [1],[2]. Thymus vulgaris L. Varico 3 is a novel hybrid with high yield in essential oil and at least 60% in thymol. As a part of our research program for quality control studies and pharmacological activities on Greek cultivated medicinal plants the chemical investigations on the non volatile constituents of Thymus vulgaris Varico 3 were undertaken. Preliminary analyzes of the leaves revealed the presence of biphenyl constituents, which are known to possess deodorant, antioxidant and antiplatelet activities [3],[4]. These compounds, which are thymol variants, possessing ortho-hydroxyl groups are structurally similar to tyrosinase substrates. Aims of this work were (i) to investigate the biphenyl content of previously unexplored hybrid “Varico 3” and (ii) to test the monoterpene dimers for their antityrosinase activity. Targeted isolation based on a combination of NMR and HPLC-PDA-MS of the dichloromethane extract afforded one new p-cymene dimer (1) together with two known p-cymene derivatives (2, 3), as well as thymol, oleanolic acid, ursolic acid, cirsimaritin and xanthomicrol. The structural elucidation of all compounds was performed by NMR spectroscopic analyses and HRESIMS experiments. The biphenyls were assayed for their inhibitory activity on tyrosinase. Compounds 2 and 3 showed negligible activity, while compound 1 effectively inhibited the enzyme with 35% (± 0.3) inhibitory activity, higher than the inhibition of the reference compound kojic acid (18.6 ± 0.02).

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• References

• 1 Basch E, Ulbricht C, Hammerness P, Bevins A, Sollars D. Thyme (Thymus vulgaris L.). Thymol J Herb Pharmacother 2004; 4: 49-67
• 2 EMA. EMA/HMPC/342332/2013. . Community herbal monograph on Thymus vulgaris L. and Thymus zygis L., herba, 2003
• 3 Dapkevicius A, van Beek TA, Lelyveld GP, van Veldhuizen A, de Groot A, Linssen JPH, Venskutonis R. Isolation and structure elucidation of radical scavengers from Thymus vulgaris leaves. J Nat Prod 2002; 65: 892-896.
• 4 Rainis G, Ternes W. Identification and Characterization of Dimeric Oxidation Products of p‑Cymene-2,3-diol Isolated from Thymus vulgaris L. J Agric Food Chem 2014; 62: 235-243.