How Chain Length and Hydroxyls Shape GA Ester Antioxidant Activity

Researchers created 14 ester versions of gallic acid (GA) to reduce instability caused by its carboxyl group and then tested their antioxidant behavior. Results showed that simple alkyl esters lost radical-scavenging activity as their carbon chains grew, falling from 91.9% to 55.6%. By contrast, hydroxyl-bearing esters kept high activity (over 90%) while replacing the carboxyl group. In an oil test, several esters lengthened the time before oxidation began, with GA-C3 giving the largest increase of 2.15 hours and certain hydroxylated esters increasing induction time by about 1.92–2.03 hours. The work links molecular structure to antioxidant performance and points to directions for designing tailored antioxidants.

What the study examined

This study explored how changing the chemical structure of gallic acid affects its antioxidant performance. Fourteen ester derivatives were made by replacing the carboxyl group with different alkyl chains and with alkyl chains that include hydroxyl groups. The goal was to see how those changes influence two measures of antioxidant behavior: radical-scavenging activity in a DPPH assay and the ability to delay oxidation in an oil system.

Key findings

The experiments revealed a clear pattern for simple alkyl substitutions: as the alkyl chain got longer, radical-scavenging activity dropped substantially. Activity fell from 91.9% for the short-chain derivative GA-C3 to 55.6% for the longest chain GA-C30. This shows a steady decline in that specific antioxidant measure as chain length increases.

In contrast, esters that carried hydroxyl groups on their side chains preserved strong radical-scavenging activity. Three hydroxyl-functionalized derivatives—GA-EG, GA-GL, and GA-PT—retained antioxidant activity above 90% while substituting the carboxyl group.

When tested in an oil system, all tested derivatives extended the oxidation induction time compared with a baseline. The best-performing simple alkyl ester, GA-C3, increased the induction time by 2.15 hours. The hydroxyl-functionalized esters GA-EG, GA-GL, and GA-PT also showed notable efficacy in this environment, extending induction time by approximately 1.92 to 2.03 hours.

Why it matters

The findings connect chemical structure to practical antioxidant effects. Short alkyl chains preserve radical-scavenging activity, while adding hydroxyl groups to the side chain keeps high activity even after replacing the original carboxyl group. Both types of modifications can also slow oxidation in an oil medium, with specific derivatives giving the largest increases in induction time.

These structure–activity relationships provide a framework for choosing or designing antioxidant molecules for particular uses. By showing which modifications keep or reduce key antioxidant properties, the results offer direction for developing replacements that balance chemical stability with performance in different systems.