Structure and chiroptical properties of supramolecular flower pigments

Abstract

Research over the last 30 years has shown that at physiological concentrations of ca. 5 x 10(-3) M, flower pigments composed of anthocyanins, either alone or complexed with flavone copigments, and frequently with metals, are self-assembled into non-covalent, chiral supramolecular complexes. This serves several biological functions including color stability, protection against UV radiation and provision for specific colors to attract insects for pollination. Self-association of the monomers takes place under conditions of molecular crowding by precise matching of the pi-pi stacking interactions of the aromatic chromophores and intermolecular hydrogen bonding between the attached sugars. The resulting handedness is controlled by the chiral information provided by the sugars joined glycosidically at certain positions around the periphery of the aromatic nuclei. This review gives an overview of (i) the physicochemical evidence including circular dichroism, (1)H NMR, and X-ray analysis for the structure and supramolecular chirality of these amphiphilic complexes, (ii) the role of the sugars on directing the chirality of the resulting supramolecules, (iii) the energetics of monomer association, and (iv) the possible influence of stacking chirality on insect pollination.

Cyanosalvianin, a supramolecular blue metalloanthocyanin, from petals of Salvia uliginosa

“…The anti-clockwise self-association of two molecules of 3 in 1 was confirmed from the strong exciton-type negative Cotton effect observed at the same absorption maximum wavelength in the VIS spectrum (Fig. 4) as that of commelinin and other metalloanthocyanins (Kondo et al, , 1994a(Kondo et al, , 2001Ellestad, 2006).…”

Recent Advances in Anthocyanin Analysis and Characterization

“…The metals include Fe 3+ , Al 3+ , Mg 2+ , and Ca 2+ and are essential for the blue pigmentation of the flower petals. All known metalloanthocyanin structures show a chiral molecular stacking of the anthocyanins and flavone cofactors, which distinguishes these pigments from other assemblies of compounds [72][73][74].…”

The chemical mechanism for Al3+ complexing with delphinidin: A model for the bluing of hydrangea sepals

“…bluing in a model chemical system, {2} to develop a molecular mechanism for the formation and stabilization of the complex in the model system, and {3} to apply this understanding to explain certain aspects of the bluing of hydrangea sepals in the presence of Al 3+ . The impact of metal ions in initiating color changes in floral pigments is greatly underappreciated [27][28][29]. Understanding the fundamentals of this bluing chemistry in hydrangea sepals may allow the design of novel chromophores (for example, ones that may produce yellow or orange sepals in hydrangea) in floral pigments using metal ions other than Al 3+ .…”

“…The associated flavylium cation (D f(blue) + ) has similar spectral characteristics as the free flavylium cation, with the exception of the bathochromic shift in its spectrum, which enhances the red to blue color change with the addition of Al 3+ . Other metal-based floral chromophores likewise employ the stacking of aromatic pigment constituents to generate color [5,29,30,44,45], in particular the stacking of anthocyanin units [46]. The Al 3+ -delphinidin interaction employs both complex formation (Eq.…”