A Comparison Study on Fluorescence Properties of Formononetin and Ononin

Abstract In this work, reasons for the spectral difference between two isoflavones, Formononetin (F) and ononin (FG), are explained in the viewpoint of molecular structure through a comparison study of the fluorescence features of the two. The fluorescence enhancement of FG in hot alkaline condition is reported for the first time. For F, there was almost no fluorescence under acidic conditions, but when pH>5, its fluorescence began to increase with increasing pH due to the proton ionization of 7-OH. In the range of pH 9.3-12.0, the anion form of F produced a fairly strong and stable fluorescence with maximum excitation wavelength (λex) of 334 nm and emission wavelength (λem) of 464 nm, its fluorescence quantum yield (Yf) was measured to be 0.042. And for FG, its aqueous solution fluoresced weakly in a wide pH range until it was placed under hot alkaline conditions, which was presumed to the cleavage reaction of the γ-pyrone ring in FG by observing a significant fluorescence at λex / λem =288 / 388nm, and Yf was determined to be 0.020. The fluorescence sensitization methods of F and FG both exhibit low limits of detection (2.60 ng·mL-1, 9.30 ng·mL-1) and wide linear ranges (0.0117-1.86 μg·mL-1, 0.0146-2.92μg·mL-1). Although the structural relationship between F and FG is glycoside and aglycone, FG cannot be translated to F by glucoside hydrolysis under hot alkaline condition, the fluorescence enhancement mechanisms of the two are essentially different. The fluorescence difference between the two under different experimental conditions lays the foundation for future fluorescence quantitative analysis.