Scheme 1.7 Sketch representing the intramolecular copigmentation in polyacylated anthocyanins; CPK models of heavenly blue anthocyanin.
Source: Mendoza et al. 2018.
Table 1.1 Equilibrium constants of heavenly blue anthocyanin and their derivatives.
Source: Mendoza et al. 2018.
| pK’a | pK”a | pK^a | pKa | pKh | K t | |
|---|---|---|---|---|---|---|
| HBA1 | 3.5 | 7.3 | 3.6 | 3.8 | 4.6 | 1.1 |
| HBA2 | — | — | 2.92 | 4.23 | 3.1 | 0.35 |
| HBA3 | — | — | 1.95 | 4.19 | 2.1 | 0.37 |
| K i | pK ^^a | pK A/A‐ | pK B/B‐ | pK Cc/Cc‐ | pK Ct/Ct‐ | |
| HBA1 | 4.0 | 7.35 | 7.35 | 7.5 | 7.25 | 7.36 |
Estimated error 10%.
In Table 1.1 the equilibrium constants of the non‐acylated, di‐acylated and tri‐acylated derivatives of heavenly blue anthocyanin are also reported (Scheme 1.8). HBA2 and HBA3 behave as common anthocyanins, being relatively stable only in acidic medium, preventing the calculation of the data regarding the anionic species at equilibrium.
The mole fraction distribution for HBA1 of the several species is represented in Figure 1.5. This distribution is in line with the previous observation (Yoshida et al. 1995) that the buds of heavenly blue anthocyanin are purple while the petals are blue. In fact the pH of the vacuoles in buds is around 6.6, while in petals pH=7.7 (Yoshida et al. 1995). In that pH region a small pH change gives different contributions of the quinoidal base and anionic quinoidal base and significant color changes can be observed.
Scheme 1.8 Energy level diagrams of HBA1 (black), HBA2 (blue), and HBA3 (red). In HBA1 there is an inversion of the relative stability between the quinoidal base and hemiketal. The energy levels of Ct cannot be measured with the necessary accuracy due to some decomposition in HBA2 and HBA3 for longer reaction times.
Source: Mendoza et al. 2018.
Figure 1.5 Mole fraction distribution of heavenly blue anthocyanin.
Source: Mendoza et al. 2018.
The rate constants of the kinetic steps of HBA1 and derivatives were also determined and are reported in Table 1.2, together with the data for HBA2 and HBA3. The isomerization is very slow and some decomposition was observed, preventing the determination of the isomerization constants with accuracy.
Inspection of Table 1.2 shows that the hydration constant of HBA1 decreases 35‐fold when compared with HBA3, while the dehydration increases c. nine‐fold. This kinetic effect is compatible with the previous explanation considering a π‐π stacking effect of the caffeoyl residues that protect positions C2 and C4 from the water attack. This kinetic effect also has thermodynamic consequences, because the equilibrium constant Kh increases 306‐fold. In addition, the equilibrium constant of the quinoidal base does not change significantly. The most interesting feature in HBA1 is the inversion of the energy levels of the quinoidal base relative to the hemiketal (Scheme 1.8).
Table 1.2 Rate constants between AH+ and CB (estimated error 10%). Reproduced from Mendoza et al. (2018), with permission.
Source: Mendoza et al. 2018
| k h / s‐1 | k ‐h / M‐1 s‐1 | k t / s‐1 | k ‐t / s‐1 | k i / s‐1 | k ‐i / s‐1 | |
|---|---|---|---|---|---|---|
| HBA1 | 0.01 | 377 | 0.09 | 0.086 | 2x10‐6 | 5x10‐7 |
| HBA2 | 0.12 | 145 | 0.08 | 0.23 | — | — |
| HBA3 | 0.35 |
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