Spectroscopic and DFT techniques on the mechanism of scavenging •OH radicals by crocin

This study employed a combined experimental and theoretical approach to elucidate the mechanism of crocin-mediated hydroxyl radical (·OH) scavenging. UV-vis spectroscopy revealed that crocin achieved 32% DPPH radical scavenging efficiency within 60 minutes. Nuclear magnetic resonance (NMR) analysis demonstrated the merging of C5 and C14 proton doublets into a singlet post-reaction, indicating enhanced symmetry in the chemical environment. Computational results indicated a minimal energy gap (0.12 eV) between the LUMO level of crocin and the HOMO level of ·OH, supporting an electron-transfer mechanism. Electrostatic potential and Fukui function analyses localized nucleophilic active sites at C3 and C5 near the conjugated chain methyl groups. Transition state calculations revealed that the activation energy for C3 (972.22 kcal/mol) was slightly lower than that for C5 (973.00 kcal/mol), with the product energy being more stabilized (-11569.99 vs. -12117.20 kcal/mol), confirming C3 as the predominant reactive site. Collectively, our findings demonstrate that crocin eliminates free radicals via synergistic electron transfer and hydrogen bonding, with C3 exhibiting optimal activity. This work provides a theoretical foundation for developing crocin as a natural antioxidant.