Systematic investigations of anthocyanin-metal interactions by Raman spectroscopy

The FT-Raman spectra of the chloride salts of pelargonidin-3-glucoside, cyanidin-3-glucoside and delphinidin-3-glucoside, their structures at pH 5.0 and their interaction with aluminium and ferric ions are presented for the first time and discussed with regard to their spectroscopic response. Two marker bands at approximately 1510 and 1330/1350 cm−1 and one band at approximately 1330/1350 cm−1 were identified being characteristic for the formation of ferric and aluminium chelates, respectively, of cyanidin-3-glucoside and delphinidin-3-glucoside. In contrast, pelargonidin-3-glucoside, exhibiting one single hydroxyl group in the B-ring, did not form metal chelates, which could be clearly demonstrated by missing marker bands. The formation of anthocyanin–metal chelates was also verified in model systems containing commercial sugar beet pectin and a pectic polysaccharide fraction isolated thereof, respectively. In addition, the absence of anthocyanin–metal chelates in systems prepared with citrate buffer was confirmed, and the effects of low and high methoxylated citrus pectins on chelate formation were studied