Solvent-modulated antioxidant activity and molecular binding of a heterocyclic sulfur dipeptide: A spectroscopic and theoretical study

A novel sulfur-containing heterocyclic dipeptide (compound 3) was synthesized and preliminarily evaluated for its therapeutic potential in combination with bioactive plant extracts. The antioxidant activity of the dipeptide was assessed using DPPH radical scavenging and phosphomolybdate assays. In the DPPH assay, compound 3 exhibited dose-dependent activity, with IC50 values of 2.39 f 0.24 mg/mL in methanol and 2.84 f 0.29 mg/mL in DMSO, indicating greater efficacy in methanol. Conversely, the phosphomolybdate assay revealed higher total antioxidant capacity in DMSO, suggesting solvent-dependent behavior. Antibacterial activity was tested against Escherichia coli ATCC 25922 using the Kirby-Bauer diffusion method. While compound 3 alone showed no inhibitory effect at concentrations up to 50 mg/disk, its combination with methanolic extracts of Paeonia daurica subsp. mlokosewitschii (leaves and roots) resulted in a dose-dependent modulation of the plant extracts' antibacterial activity, indicating an interaction that may influence phytotherapeutic efficacy. Independently, the plant extracts demonstrated measurable antibacterial activity, with inhibition zones of 13.61 f 0.17 mm (leaves) and 13.75 f 0.05 mm (roots) at 200 mu g/disk. Circular dichroism (CD) spectroscopy using bovine serum albumin (BSA) revealed specific peptide-protein binding interactions, supporting the biological relevance of the dipeptide. These findings were further supported by Density Functional Theory (DFT) calculations, molecular docking, and CD spectral deconvolution, providing insight into the compound's structural and interaction properties. Overall, the study highlights the potential of compound 3 as a modulator of plant-derived bioactivities and its relevance in the development of synergistic therapeutic strategies.