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 (p Ka1 = 4.4 ± 0.1) compared with 1 in the absence of the lignosulfonate (p Ka1 = 3.9 ± 0.1). Furthermore, comparing the hydration ( kh = 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 ( kh = 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.