Oxidation of myrtenol to myrtenal epoxide in a porphyrin-based photocatalytic system - A novel terpene alcohol derivative with antimicrobial and anticancer properties.

Biomimetic catalysis using porphyrins enables gentle oxidation of terpenes with molecular oxygen and light. This study explores the photooxidation of (-)-myrtenol under visible light to synthesize new terpenoid products with promising biological activity. Among the porphyrins tested, tetraphenylporphyrin (H2TPP) exhibited the highest catalytic efficiency and stability in chloroform, producing myrtenal epoxide (ME) as the main product (with a molar conversion of myrtenol of 66.2 %), confirmed by NMR and MS analyses. Other substrates, i.e. perillyl alcohol and trans-pinocarveol, did not yield redox products. The antimicrobial activity of ME was assessed against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Candida albicans using Disk Diffusion, Minimal Inhibitory Concentration, and Minimal Biofilm Eradication Concentration assays (using liquid ME) and the Quantitative Assay for Measuring the Antibiofilm Activity of Volatile Compounds (using volatile ME). Overall, ME displayed higher antimicrobial activity than myrtenol in the majority of the tests applied. The strongest effects were observed against C. albicans, followed by S. aureus, while the weakest activity was exhibited against Gram-negative bacteria. ME also showed cytotoxic effects on human colorectal cancer cells (HT-29) with significantly higher biological activity than that of (-)-myrtenol. Notably, ME at lower concentrations (5-50 µg/ml) promoted proliferation of normal cells while inhibiting the viability and proliferation of cancer cells. Porphyrin-based photooxidation is a sustainable method for converting biorenewable terpene feedstocks into new compounds that can be used in cancer treatment and antimicrobial therapy.