Reversible bleaching of betalains induced by metals and application to the fluorescent determination of anthrax biomarker

Abstract

Betalains are antioxidant pigments of plant origin that are found in nature in the order Caryophyllales. They are classified into two groups: betacyanins, which present a violet coloration, and betaxanthins, fluorescent betalains with yellow coloration. In this work, the effect of metals, such as copper and europium, on the spectroscopic properties of betalains is studied. For the first time, the reversibility of spectral changes in fluorescence and absorbance is considered after the addition of chelating substances. These results explain the general bleaching of betalains by metals. The reversibility of the betalain-metal complexes is applied to the fluorescent detection of the presence of spores from Bacillus by the interaction of the complexes with the chelator dipicolinic acid, which is the main and characteristic component of endospores. This fast, economic method allows the detection of spores below the infective dose for anthrax by using a flower pigment present in Mirabilis jalapa.