Genotoxicity of the Natural Furochromones, Khellin and Visnagin and the Identification of a Khellin‐thymine Photoadduct

Longwave UV irradiation of a frozen aqueous solution of khellin and thymine resulted in the formation of a 2+2 photoadduct between the 2,3 double bond of khellin and the 5′, 6′ double bond of thymine. This behaviour is analogous to that of the furocoumarins and serves as a mechanism to describe the genotoxicity of the furochromones khellin and visnagin. These two compounds are phototoxic towards bacteria and fungi and inhibit mitosis and cause gross chromosomal changes in Chinese hamster ovary (CHO) cells in near UV light. For comparative purposes, 8‐methoxypsoralen was also tested and was found to be more active than visnagin which, in turn, was more active than khellin.