New β-orcinol depsides from Hypotrachyna caraccensis, a lichen from the páramo ecosystem and their free radical scavenging activity

Abstract

The new hypotrachynin A (1) and B (2) along with the known (+)-(9b-R)-usnic (3) and methylstictic acids (4) were isolated for the first time from Hypotrachyna caraccensis. Additionally, their potency and reactivity as DPPH• scavengers was determined by a kinetic study calculating their EC₅₀ and second-order rate constants (k₂). Considering 1–4 could be dermatological agents, their n-octanol-water partition coefficients and standard molar Gibbs free energies of transfer were calculated as estimation of their lipophilicity and skin penetration. Compounds 1, 3 and 4 were less potent than 2 (EC₅₀ = 3.3014; 1.7540; 2.6652 vs 0.7376) as DPPH• scavengers, in turn 4, was the most reactive with a comparable k₂ to the antioxidant BHT (k₂ = (232 ± 24) × 10⁻² vs (564 ± 12) × 10⁻² M⁻¹ s⁻¹, respectively). Since 2 and 4 had an optimal lipophilicity and permeability for skin penetration, they might be developed as topical ingredients to prevent oxidative damage.

Keywords:

• Lichens
• free radical scavengers
• antioxidants
• skin permeability