Effects of Gamma Irradiation on Free Radical Formation, Phenolic Composition and Antioxidant Activity of Prunella vulgaris L.: An EPR and HPLC Study

Gamma irradiation is widely used for microbial decontamination and preservation of plant-derived materials intended for the food, pharmaceutical, and cosmetic industries. The effects of gamma irradiation (1, 5, and 10 kGy) on free radical formation, antioxidant activity, total phenolic content, and polyphenolic composition of Prunella vulgaris L. leaves (LS) and fruit spikes with flowers (FS) were investigated. Electron Paramagnetic Resonance (EPR) spectroscopy was used to characterize radiation-induced free radicals and evaluate free radical scavenging activity (FRSA) by the DPPH assay. Total phenolic content (TPC) was determined spectrophotometrically, and individual phenolic compounds were analyzed by HPLC. Gamma irradiation of P. vulgaris induced a singlet signal superimposed on the semiquinone radical signal, while the characteristic cellulose satellite peaks were not detected. The radiation-generated radicals gradually decayed during storage, approaching the natural radical level after approximately 120 days. Irradiation did not significantly affect the FRSA of leaves but induced a dose-dependent response in fruit spikes, with decreased FRSA at 5 kGy and increased activity at 10 kGy. HPLC analysis revealed irradiation-induced changes in the phenolic profile, including degradation of some phenolic compounds and accumulation of others. The results demonstrate that the effects of gamma irradiation on P. vulgaris are dose- and plant organ-dependent and that antioxidant activity is influenced primarily by qualitative changes in phenolic composition.