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Unraveling the elusive blue of hydrangea
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Resumo(s)
The vivid blue coloration of Hydrangea macrophylla sepals originates from a ternary complex formed by the anthocyanin myrtillin (delphinidin-3-O-glucoside), neochlorogenic acid, and Al3+ in aqueous solution at approximately pH 4 in a 1:1:1 ratio. However, the equilibrium underlying its formation remains insufficiently characterized, particularly the quantitative relationship with component concentration and pH. In this study, we systematically investigated the thermodynamics and kinetics of aluminum complexation with myrtillin and/or neochlorogenic acid to establish a quantitative physicochemical model of the hydrangea-blue system, employing absorption spectroscopic titrations and reverse pH-jump experiments monitored by stopped-flow techniques. Starting with the binary system, Al3+ complexation occurred with the monoanionic species of myrtillin (primarily the quinoidal base, A−), showing association constants of 106.0 M-1. Furthermore, Al3+ complexation with fully deprotonated neochlorogenic acid occurred in both a stronger 2:2 and a weaker 2:1 mode, with overall association constants of 1034 M−3 and 1016 M−2, respectively. For the 1:1:1 ternary complex, the association constant was determined to be 1025 M−2. Formation of the ternary complex appears to originate from the Al3+-myrtillin (A−) species, as neochlorogenic acid dissociates first under acidic conditions. These findings provide a robust thermodynamic and kinetic framework for understanding the complex equilibrium governing the ternary system, offering new physicochemical insights into anthocyanin-mediated color expression in plants.
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Palavras-chave
Aluminum complexation Blue coloration mechanism Hydrangea macrophylla Myrtillin Neochlorogenic acid General Chemical Engineering Process Chemistry and Technology