Novel Hybrid Indole-Based Caffeic Acid Amide Derivatives as Potent Free Radical Scavenging Agents: Rational Design, Synthesis, Spectroscopic Characterization, In Silico and In Vitro Investigations

Abstract

Antioxidant small molecules can prevent or delay the oxidative damage caused by free radicals. Herein, a structure-based hybridization of two natural antioxidants (caffeic acid and melatonin) afforded a novel hybrid series of indole-based amide analogues which was synthesized with potential antioxidant properties. A multiple-step scheme of in vitro radical scavenging assays was carried out to evaluate the antioxidant activity of the synthesized compounds. The results of the DPPH assay demonstrated that the indole-based caffeic acid amides are more active free radical scavenging agents than their benzamide analogues. Compared to Trolox, a water-soluble analogue of vitamin E, compounds 3a, 3f, 3h, 3j, and 3m were found to have excellent DPPH radical scavenging activities with IC50 values of 95.81 +/- 1.01, 136.8 +/- 1.04, 86.77 +/- 1.03, 50.98 +/- 1.05, and 67.64 +/- 1.02 mu M. Three compounds out of five (3f, 3j, and 3m) showed a higher capacity to neutralize the radical cation ABTS(center dot+) more than Trolox with IC50 values of 14.48 +/- 0.68, 19.49 +/- 0.54, and 14.92 +/- 0.30 mu M, respectively. Compound 3j presented the highest antioxidant activity with a FRAP value of 4774.37 +/- 137.20 mu M Trolox eq/mM sample. In a similar way to the FRAP assay, the best antioxidant activity against the peroxyl radicals was demonstrated by compound 3j (10,714.21 +/- 817.76 mu M Trolox eq/mM sample). Taken together, compound 3j was validated as a lead hybrid molecule that could be optimized to maximize its antioxidant potency for the treatment of oxidative stress-related diseases.