Anthocyanin Color Stabilization by Host-Guest Complexation with p-Sulfonatocalix[n]arenes.

Johan Mendoza,Fernando Pina,Luis Cruz,Victor De Freitas,Nuno Basílio

doi:10.3390/molecules26175389

Johan Mendoza, Fernando Pina + Show 3 more

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https://doi.org/10.3390/molecules26175389

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Abstract

Flavylium-based compounds in their acidic and cationic form bring color to aqueous solutions, while under slightly acidic or neutral conditions they commonly bring discoloration. Selective host-guest complexation between water-soluble p-sulfonatocalix[n]arenes (SCn) macrocycles and the flavylium cationic species can increase the stability of the colored form, expanding its domain over the pH scale. The association constants between SCn and the cationic (acid) and neutral basic forms of flavylium-based compounds were determined through UV-Vis host-guest titrations at different pH values. The affinity of the hosts for synthetic chromophore was found to be higher than for a natural anthocyanin (Oenin). The higher affinity of SC4 for the synthetic flavylium was confirmed by 1H NMR showing a preferential interaction of the flavylium phenyl ring with the host cavity. In contrast with its synthetic counterpart, the flavylium substitution pattern in the anthocyanin seems to limit the inclusion of the guest in the host’s binding pocket. In this case, the higher affinity was observed for the octamer (SC8) likely due to its larger cavity and higher number of negatively charged sulfonate groups.

Highlights

Despite being generally represented in flavylium cation form (AH+ ), anthocyanins are involved in a series of reversible pH-dependent chemical reactions that comprise both the colored flavylium cation and the neutral quinoidal base (A) forms, as well as the colorless hemiketal and cis-trans-chalcones (B, Cc, and Ct; see Figure 1) [3,4]
Because the formation of the trans-chalcone (Ct) in anthocyanins is a very slow process, it is possible to define a pseudo-equilibrium between AH+ and CB, with [CB] = [A] + [B] + [Cc], where no significant amounts of Ct are formed [8]
SCnOenin and was the flavylium cation of the model compound 4The and the between anthocyanin investigated at pHform

Summary

Introduction

Despite being generally represented in flavylium cation form (AH+ ), anthocyanins are involved in a series of reversible pH-dependent chemical reactions that comprise both the colored flavylium cation and the neutral quinoidal base (A) forms, as well as the colorless hemiketal and cis-trans-chalcones (B, Cc, and Ct; see Figure 1) [3,4]. The fact that these last colorless species are usually predominant at slightly acidic or neutral pH values strongly hampers more generalized applications of anthocyanins as alternative dyes.

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