Vorgänge bei pH-Änderung und bei Belichtung eines Anthocyanidins

Summary

The anthocyanidin investigated here exists below pH 4 as a cationAH ⁺₂ (wave length of absorption maximumλ _max=459 mµ), between pH 5 and 7 in the neutral formAH (λ _max=492 mµ) and above pH 8 as an anionA ⁻ (λ _max=537 mµ). At pH 5 the freshly dissolved substance is partially converted into a colourless formBH (λ _max=372 mµ) and a chemical equilibrium betweenAH andBH is reached within 1 h. A kinetic study of the process of formation ofBH shows that an intermediate productZ is formed. This process can be reversed by light exposure. It can be concluded from a kinetic investigation by using flash light thatBH is transformed by the absorbing light into a new substanceL (λ _max ⋍ 275 mµ and 225 mµ), andL changes partially intoBH, partially intoZ, which itself is transformed partially intoAH ⁺₂ ,AH,A ⁻, partially intoBH. The reactionZ →AH,AH ⁺₂ ,A ⁻ is proportional to the concentration of protons [H ⁺], the reactionZ →BH independent of [H ⁺]. Thus a photochemical production ofAH ⁺₂ ,AH,A ⁻ fromBH is readily obtained in the presence ofH ⁺ and not obtained in the absence ofH ⁺.