Towards elucidating the solar instability of the anti-fungal food preservative natamycin: insights from spectroscopy

Natamycin, a natural antifungal peptide, is widely used in food and medicine. The absorption of natamycin in the ultraviolet region of the electromagnetic spectrum, is attributed to the conjugated linear chromophore present in the molecule. Upon irradiation, natamycin degrades, disrupting its antifungal efficacy. In an effort to understand these photodegradation pathways, the present study uses steady-state, transient electronic absorption and fluorescence lifetime spectroscopies allied with time-dependent density functional theory calculations to unravel the photodynamics of natamycin following absorption of ultraviolet radiation. We find that the photodynamics of natamycin are strongly influenced by the conjugated tetraene moiety, involving initial excitation to an optically bright ππ* state. The excited state population traverses a conical intersection into a second, optically dark, excited state from which it partially repopulates the ground state; the remaining excited state population leads to photoproduct formation. A photoproduct was isolated and characterised, and identified as being formed through a combination of isomerisation and epoxide ring-opening pathways. The insight garnered into the photodynamics of natamycin may facilitate design of next generation UV photostabilisers which protect natamycin against UV-induced degradation.