Purple-fleshed sweet potato acylated anthocyanins: Equilibrium network and photophysical properties

Abstract

Two anthocyanins from purple-fleshed sweet potato were isolated and characterized by LC-MS and NMR analysis. They were identified as peonidin-3-(6'-hydroxybenzoyl)-sophoroside-5-glucoside and peonidin-3-(6'-hydroxybenzoyl-6″-caffeoyl)-sophoroside-5-glucoside. The acid-base dynamics of these acylated anthocyanins was evaluated by means of pH jump techniques. Equilibrium and kinetic constants were determined and, in general, these anthocyanins demonstrated a higher capacity in retaining the red and blue colors at acidic and basic pH values, suggesting a higher resistance to pH variations compared to the parent anthocyanin, peonidin-3-O-glucoside. The presence of acyl groups and additional glucoside moieties seems to determine this particular characteristic. The fluorescence properties of these anthocyanins were evaluated. Overall, the species present at higher pH values (7-9) showed higher fluorescence intensity for both anthocyanins, with an optimum λ_ex/λ_em pair at λ_ex 610 nm/λ_em 640 nm. The fluorescence characteristics of these anthocyanins were used to evaluate their location in gastric and intestinal cells by fluorescence microscopy.

Keywords: Acylated anthocyanins; Chemical equilibria; Fluorescence; Purple-fleshed sweet potato.