Novel stable fluorophore, 6-methoxy-4-quinolone, with strong fluorescence in wide pH range of aqueous media, and its application as a fluorescent labeling reagent

Abstract

6-Methoxy-4-quinolone (6-MOQ, 1), an oxidation product derived from 5-methoxyindole-3-acetic acid, is a novel fluorophore, which has several useful characteristics for biomedical analysis. Compound 1 has strong fluorescence with a large Stokes’ shift in aqueous media, and the maximum fluorescence excitation and emission wavelengths are 243nm and 374nm, respectively. The molar absorptivity at the maximum excitation wavelength and fluorescence quantum yield in aqueous 10% (v/v) methanol are 32600Lmol−1cm−1 and 0.38, respectively. The fluorescence intensity of 1 is scarcely affected by changing the medium pH, showing strong fluorescence from pH 2.0 to 11.0. In addition, 1 is highly stable against light and heat, and no degradation was observed at 60°C for 3 days with exposure to daylight. As a fluorescent labeling reagent, [(6-methoxy-4-oxo-1,4-dihydroquinolin-3-yl)methyl]amine (6-MOQ-NH2, 2) was synthesized, and determination of carboxylic acids was demonstrated; 50pmol of standard propionic acid and isobutyric acid were derivatized, and the obtained S/N ratios for 10fmol (injection amount) of these two acids were 206 and 164, respectively.