article · Heliyon
Thymol is a natural monoterpene phenol found in plant essential oils with recognized antimicrobial properties, often considered as a starting template for the semi-synthesis of therapeutic agents. This research investigated whether chemical modification of the hydroxyl group on carbon one of the thymol nucleus alters its antibacterial activity. Ether-substituted and ester-substituted derivatives were evaluated alongside unmodified thymol and reference antibiotics against three clinical bacterial isolates. Although these chemical derivatives showed measurable growth inhibition compared to the vehicle control, their effectiveness was significantly lower than that of native thymol. Unmodified thymol was twice as potent as either derivative class, requiring half the concentration to inhibit and kill the bacterial isolates. Additionally, increasing the size and bulkiness of the chemical side chains at the carbon one position further decreased bacterial growth inhibition, confirming the functional necessity of the original hydroxyl group.
Identifying the precise chemical components responsible for antimicrobial activity is essential when developing new medicines from nature. Natural plant compounds like thymol provide valuable templates for drug design, but modifying their structure can inadvertently weaken their medicinal effects. Clarifying that the hydroxyl group on carbon one is indispensable ensures researchers do not pursue ineffective modifications, saving time and resources during early antimicrobial drug discovery programmes.
This work provides early-stage structure-activity guidance for medicinal chemists and pharmaceutical developers working on plant-derived antimicrobial agents. By demonstrating that modifying the carbon one hydroxyl group reduces bioactivity, it establishes boundaries for semi-synthetic drug design. Because the findings are derived entirely from laboratory-based minimum inhibitory concentration tests against clinical isolates, any practical application in therapeutic formulation remains at a preliminary, discovery-phase stage far from real-world use.
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BACKGROUND: Thymol, a natural monoterpene phenol is not only relevant clinically as an anti-microbial, anti-oxidant and anti-inflammatory agent but also holds the prospect as a natural template for pharmaceutical semi-synthesis of therapeutic agents. It is a major component of essential oils from many plants. Evidence abound linking overall bioactivity of thymol to its monoterpene nucleus, specifically, the hydroxyl (-OH) substituent on carbon number one (C1) on the monoterpene nucleus. Other studies have posited that the overall bioactivity of thymol is not substantially altered by chemical modification of - OH on the C1 of the monoterpene nucleus. In view of this, it is still unclear as to whether removal or modification of the -OH on C1 of the monoterpene nucleus relates generally or context-dependently to bioactivity of thymol. OBJECTIVE: . MATERIALS AND METHODS: ). Standard anti-biotics used were Thymol Streptomycin and flucloxacillin, while DMSO was used as vehicle for thymol derivatives. MIC and MBC were determined. RESULTS: ) compared to DMSO. Although the growth inhibitory effects of the ester-and-ether derivatives of thymol was significant (P ≤ 0.05) compared to DMSO, it was however insignificant (P ≥ 0.05) compared to thymol and reference antibiotics. Comparatively, at equimolar concentrations, ester-substituted derivatives of thymol, particularly the branched chain derivative (TM1C) produced more effective growth inhibition on the isolates than the ether-substituted derivatives of thymol. Thymol was twice as potent (MIC and MBC, 500 μg/ml) than both ester-and-ether substituted derivatives of thymol (MIC and MBC, > 1000 μg/ml) on all the three clinical isolates. Increase in side chain bulkiness of -OH moiety on the monoterpene nucleus of thymol decreased growth inhibition on isolates. CONCLUSION: Thymol has demonstrated broad-spectrum anti-bacterial effects attributable to the hydroxyl moiety on C1 of the monoterpene nucleus. Structural modification of the hydroxyl moiety on C1 of the monoterpene nucleus of thymol with either ether-or-ester substitutions yielded no significant anti-bacterial effects.
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DOI: 10.1016/j.heliyon.2020.e03492
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