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 EC₅₀ values of 2.39 ± 0.24 mg/mL in methanol and 2.84 ± 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 reduction of the plant extracts' antibacterial activity, indicating a modulatory interaction that may influence phytotherapeutic efficacy. Independently, the plant extracts demonstrated measurable antibacterial activity, with inhibition zones of 13.61 ± 0.17 mm (leaves) and 13.75 ± 0.05 mm (roots) at 200 μg/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. • Novel sulfur-containing dipeptide (3) synthesized and structurally analyzed. • Dipeptide shows solvent-dependent antioxidant activity via DPPH and TAC assays. • Combined with Paeonia extracts, 3 modulates antibacterial efficacy dose-wise. • CD and DFT studies confirm peptide binding to BSA and structural stability. • Zongertinib-like synergy suggests potential for phytotherapeutic fine-tuning.