Complexes of carboxylato pillar[6]arene with Brooker-type merocyanines: Spectral properties, pKa shifts and the design of a displacement assay for trimethyl lysine

The supramolecular complexes of three strongly solvatochromic dyes, Brooker's merocyanine (M1) and its two derivatives (M2, M3) with carboxylato pillar[6]arene (WP6) were studied in aqueous solutions. The dye-WP6 mixtures were described in terms of four equilibrium reactions: the acidic dissociations of the pyridinium phenols into the zwitterionic phenolates, the acidic dissociations of the complexed phenols, the bindings of the phenol form dyes to WP6 and the bindings of the phenolates to WP6. The equilibrium constants were determined by an analysis of the absorption spectra. It was found that the acidity of the phenol form merocyanines were largely reduced on complexation, pK(a) shifts of 1.1-1.6 units were observed. In neutral solutions, the complexes of the phenol forms of M1 and M2 were dominant, in contrast to the more acidic M3 (a dibromo derivative), of which the phenolate complex was more stable. Comparing the spectral properties, the binding constants and the pK(a)-s of the dye-WP6 complexes, the complex M3.WP6 was chosen to be tested as a displacement assay. It was demonstrated that this complex functioned as a colorimetric indicator displacement assay which discriminated trimethyl lysine from other lysine derivatives. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

The study investigated the supramolecular complexes of three strongly solvatochromic dyes with carboxylato pillar[6]arene in aqueous solutions. Equilibrium constants were determined by analyzing the absorption spectra, which revealed shifts in acidity upon complexation. The M3.WP6 complex was chosen for a displacement assay, functioning as a colorimetric indicator to discriminate trimethyl lysine from other lysine derivatives.