Formation of biopolymers owing to the oxidation of esculetine by Cu(II) ions in a Ca-polygalacturonate network

Pectic acids participate in the transport of heavy metal ions in the root apoplasm by establishing interactions that can lead to their partial or total immobilization. The ions accumulated can be mobilized by phenolic compounds and organic acids of the root exudates. In this context, we tested, in aqueous phase, the ability of malic acid and esculetine (ESC) to mobilize the Cu(II) ions accumulated in a Ca-polygalacturonate matrix (Ca-PGA) used as a model of the root apoplasm. The results show that at pH 5.0 and 6.0 malic acid mobilizes about 22% and 34% of the Cu(II) accumulated, respectively, whereas ESC about 12% and 25%. ESC was found to cause the reduction of Cu(II) to Cu(I) with formation of ESC oxidation products. The study of the Cu(II)–ESC binary system evidenced that one molecule of ESC reduces one Cu(II) ion with formation of semiquinonic radicals that couple to form two dimers. The Cu(II) reduction by ESC was found faster in the presence of malic acid.

The esculetine acts as reducing agent towards Cu(II) trapped in a Ca-polygalacturonate network with the concomitant formation of ESC dimmers.Figure optionsDownload as PowerPoint slideHighlights
► Cu(II) reduction by esculetine.
► The reduction of Cu(II) to Cu(I) by esculetine is faster in the presence of malic acid.
► Oxidation of esculetine leads to the formation of oligomers.