Effect of Temperature on Acidity and Hydration Equilibrium Constants of Delphinidin-3-O- and Cyanidin-3-O-sambubioside Calculated from Uni- and Multiwavelength Spectroscopic Data

Abstract

Delphinidin-3-O-sambubioside and cyanidin-3-O-sambubioside are the main anthocyanins of Hibiscus sabdariffa calyces, traditionally used to make a bright red beverage by decoction in water. At natural pH, these anthocyanins are mainly in their flavylium form (red) in equilibrium with the quinonoid base (purple) and the hemiketal (colorless). For the first time, their acidity and hydration equilibrium constants were obtained from a pH-jump method followed by UV-vis spectroscopy as a function of temperature from 4 to 37 °C. Equilibrium constant determination was also performed by multivariate curve resolution (MCR). Acidity and hydration constants of cyanidin-3-O-sambubioside at 25 °C were 4.12 × 10(-5) and 7.74 × 10(-4), respectively, and were significantly higher for delphinidin-3-O-sambubioside (4.95 × 10(-5) and 1.21 × 10(-3), respectively). MCR enabled the obtaining of concentration and spectrum of each form but led to overestimated values for the equilibrium constants. However, both methods showed that formations of the quinonoid base and hemiketal were endothermic reactions. Equilibrium constants of anthocyanins in the hibiscus extract showed comparable values as for the isolated anthocyanins.

In Situ Stability of Anthocyanins in Lycium ruthenicum Murray

“…In this study, the addition of glucose in the freeze-dried Lycium ruthenicum Murr. prolonged the half-life at 4 • C, but not at 20 • C, or 37 • C. The deprotonation of the anthocyanin flavylium ion to form quinonoid base and hydration to form hemiketal were proven to be endothermic [24]. Thus, elevating the storage temperature brought about more rapid degradation of anthocyanins.…”

Chemistry and Pharmacological Actions of Delphinidin, a Dietary Purple Pigment in Anthocyanidin and Anthocyanin Forms

“…Stability of Dp can be related to the presence of three-OH groups on ring B. The blue tinge in flowers is due to the presence of Dp pigment under alkaline conditions ( 39 ). Dp is a natural pH indicator, and it appears red in acidic pH, blue in basic pH, and purple to magenta in neutral pH ( 23 ).…”

Feasibility of Individual Carotenoid Quantification in Mixtures Using UV-Vis Spectrophotometry with Multivariate Curve Resolution Alternating Least Squares (MCR-ALS)

“…Preprocessing steps consisted in removing the shortwavelength region (250-350 nm) and in normalization. MCR-ALS analysis was done with Matlab ( e Math-Works, Inc., Natick, MA, USA) using the toolbox built by Jaumot et al [41,50]. Briefly, Multivariate Curve Resolution aims to achieve a bilinear decomposition in 2 matrices of an experimental absorbance matrix D(m,n) which corresponds to a combination of spectrophotometric measurements.…”

“…samples. Indeed, MCR-ALS was successfully tested on voltametric signals to quantify organic acids in fruit juices and on UV-Vis spectra to discriminate polyphenols in a hibiscus extract [38][39][40][41]. It was also employed for quantification and spatial distribution of salicylic acid in filmcoated tablets using Raman spectrophotometry, or to get insight into conformational changes and protein folding [42,43].…”