Light-Induced Trans to Cis Conversion of β-D-Glucosyl o-Hydroxycinnamic Acid in Melilotus alba Leaves
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Isotope studies indicate that o-coumiaric acid glucoside (β-D-glucosyl trans-o-hydroxycinnamic acid) is the immediate precursor of coumarinic acid glucoside (β-D-glucosyl cis-o-hydroxycinnamic acid) in sweetclover (5, 7). Structures of these compounds are as follows: Preliminary reports (3, 4) suggest that this trans to cis conversion is a nonenzymic photochemical reaction. However, the published data do not preclude the possibility that the reaction is effected by a light-sensitive isomerase system. The present paper deals with an investigation of the conversion of o-coumaric acid glucoside to coumarinic acid glucoside in intact sweetclover leaflets and in leaflet extracts. Evidence to be presented supports the conclusion that in sweetclover leaves this trans to cis conversion is nonenzymic and is induced by ultraviolet irradiation.