Recent Advances in Polyphenol Research. Группа авторов

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et al. 2018). It is based on the reverse pH jumps defined above, followed by stopped flow. In Figure 1.3 the stopped flow traces of the model compound 4’‐hydroxyflavylium are shown. The initial solutions should be equilibrated or pseudo‐equilibrated. The reverse pH jumps consist of the addition of acid to make the solutions with pH=1, where flavylium cation is the sole species. In both cases of Figure 1.3 the initial absorbance is due to the quinoidal bases (independently on their protonation state) that give flavylium cation (absorption at 450 nm) during the mixing time of the stopped flow together with some flavylium cation present at the initial equilibrium (at lower pH values) prior to the jump; see also Scheme 1.3. This is the reason why the mole fraction distribution of the flavylium cation and quinoidal bases are represented together in eq. (22). At the final very low pH jump (pH=1) the hydration reaction becomes faster than the tautomerization because it is directly proportional to the proton concentration (Pina 2014b). Therefore, the faster trace is due to the conversion of B into AH+. The slower trace is the formation of more flavylium cation from Cc via B (Scheme 1.4) (Mendoza et al. 2019).

Graphs depict stopped flow traces 4’-hydroxyflavylium after a reverse pH jump from pH equals 6.45 (a) and pH equals 8.9 (b) to the final pH equals 1.0. Schematic illustration of the energy level diagram of the compound 4’-hydroxyflavylium and the kinetic processes after a reverse pH jump to pH less than or equal to 1.

      with

      (28)equation

      and

      (29)equation

Graph depicts the mole fraction distribution of the compound 4’-hydroxyflavylium on the basis that the reverse pH jumps at pseudo-equilibrium.
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