Description:
<jats:p>A yellow chromophore has been isolated from <jats:italic>Avena</jats:italic> coleoptiles grown in the dark. It had previously been shown by Zenk [Zenk, M. H. (1967) <jats:italic>Z. Pflanzenphysiol. 56</jats:italic>, 122–140] to be a flavin of still unidentified structure, and had been suggested to be the blue light photoreceptor in this organism. The structure of this flavin (F1‐X) has been identified as 5′‐malonylriboflavin by a combination of physicochemical techniques, and by its identity with a sample obtained synthetically. The 5′‐malonylester linkage is relatively labile towards hydrolysis and photolysis, it isomerizes to an equilibrium mixture containing probably the 4′ isomer. The electronic spectra of 5′‐malonylriboflavin (absorption and fluorescence) are practically identical to those of normal riboflavin (vitamin B<jats:sub>2</jats:sub>).</jats:p>