Abstract
Catechols are antioxidants and radical scavengers with a broad medical potential. 4-Methylcatechol (1b) and 4-ethylcatechol (2b) (occurring in some traditional fermented and smoked foods) activate the cell defense against oxidative stress. We examined the biocatalyzed reactions towards 4-n-alkylcatechols with different side chains length, which is a factor important for the biological activities of catechols. 4-n-Alkylcatechols with methyl through heptyl side chains (1b–7b) were obtained in one pot by (i) oxidation of phenols 1a–7a with tyrosinase from Agaricus bisporus followed by (ii) reduction of ortho-quinones (intermediates) with L-ascorbic acid sodium salt. The conversions decreased with increasing side chain length. The preparative reactions were carried out with substrates 1a–5a. The isolated yields of the purified products decreased from 59% in 2b to 10% in 5b in correlation with logP of the substrates. Homology modeling indicated that the affinities of two tyrosinase isoforms (PPO3 and PPO4) to the substrates with side chains longer than C2 decreased with increasing side chain length. This was probably due to steric limitations and to missing interactions of the extended side chains in the active sites. We envisage using the model to predict further substrates of tyrosinase and testing the products, catechols, for radical-scavenging and biological activities.
Highlights
A number of natural ortho-diphenols and their synthetic derivatives (Scheme 1) exhibit biological effects, which can be assigned to free radical scavenging or metal ion binding [1]
The reactionof the reaction was proposed to consist of phenol oxidation by tyrosinase coupled to the reduction was proposed to consist of phenol oxidation by tyrosinase coupled to the reduction of the intermediate quinone (Scheme 2), similar to previous hypotheses on this reaction type [10,13]
Mushroom tyrosinase was purchased from Sigma Aldrich (Merck KGaA, Darmstadt, Germany) or prepared from A. bisporus fruiting bodies purchased at the local market
Summary
A number of natural ortho-diphenols (catechols) and their synthetic derivatives (Scheme 1) exhibit biological effects, which can be assigned to free radical scavenging or metal ion binding [1]. Piceatannol was shown to have radical (HO· and t-BuOO·) and singlet oxygen (1 O2 ) scavenging activities several times higher than resveratrol or Trolox [2] and to exhibit immunosuppressive, anticancer, antiinflammatory and anticolitic effects [3]. Protocatechuic aldehyde (3,4-dihydroxybenzaldehyde) exhibited antioxidant and anticancer effects and prevented experimental pulmonary fibrosis [4]. Carnosic acid and its analogues were found to have, e.g., antiproliferative, antimutagenic, antifungal or antiviral properties [5]. Antiviral activities were reported in brominated catechols such as lanosol [6], a diterpenoid-like catechol [7] or dihydroxychromone catechols [8].
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