Impact of Lignosulfonates on the Thermodynamic and Kinetic Parameters of Malvidin-3‑O‑glucoside in Aqueous Solutions

The interaction of malvidin-3-O-glucoside (1) and a lignosulfonate was studied by UV–visible spectroscopy, and the results obtained showed the formation of a complex between the negatively charged lignosulfonate and the flavylium cation form (AH+) of this anthocyanin at pH 1. The thermodynamic and kinetic parameters of 1 in the presence of a lignosulfonate were determined by UV–visible spectroscopy and stopped-flow techniques. The main differences were observed in the flavylium cation (AH+)/quinoidal base (A) equilibrium, the AH + form being more stabilized than A (pK a1 = 4.4 ± 0.1) compared with 1 in the absence of the lignosulfonate (pK a1 = 3.9 ± 0.1). Furthermore, comparing the hydration (k h = 0.028 s–1) and dehydration (k –h = 40 M–1 s–1) processes of 1 in the presence of the lignosulfonate with the processes of 1 (k h = 0.12 s–1 and k –h = 35 M–1 s–1) show that the hydration process is slower while the dehydration process is practically unaffected in the presence of the lignosulfonate.