β‐CARBOLINE ALKALOIDS: MECHANISMS OF PHOTOTOXICITY TO BACTERIA AND INSECTS

Phototoxicity of five naturally occurring (3‐carboline alkaloids was assayed against a series of Escherichia coli strains and against the insect Trichoplusia ni (Lepidoptera: Noctuidae). Rank order efficacy of the compounds was comparable in both organisms. Although the bacterial assay demonstrated oxygen dependence, the degree of phototoxicity did not correlate with the relative efficiency of in vitro singlet oxygen or hydrogen peroxide photoproduction by the alkaloids. A better correlation was observed between chromatographic migration distances (lipophilicity) and phototoxicity. Therefore, hydrophobic mechanisms in which the alkaloids diffuse with varying efficiencies into cells or into the vicinity of target molecules may be important modes of phototoxicity for these compounds.