page_title: Basics of blue flower colouration
section_title: Mechanisms
hierarchical_section_title: Basics of blue flower colouration / Mechanisms
caption_attribution_description: English: Hydrangea with different flower colours
context_page_description: Blue flower colour was always associated with something unusual and desired. Blue roses especially were assumed to be a dream that cannot be realised. Blue colour in flower petals is caused by anthocyanins, which are members of flavonoid class metabolites. We can diversify three main classes of anthocyanin pigments: cyaniding type responsible for red coloration, pelargonidin type responsible for orange colour and delphinidin type responsible for violet/blue flower and fruits coloration. The main difference in the structure of listed anthocyanins type is the number of hydroxyl groups in the B-ring of the anthocyanin. Nevertheless in the monomeric state anthocyanins never show blue colour in the weak acidic and neutral pH. The mechanism of blue colour formation are very complicated in most cases, presence of delphinidin type pigments is not sufficient, great role play also the pH and the formation of complexes of anthocyanins with flavones and metal ions.
context_section_description: Self-association is correlated with the anthocyanin concentration. When concentration is higher we can observe change in the absorbance maximum and increase of colour intensity. Molecules of anthocyanins associate together what results in stronger and darker colour.
  Co-pigmentation stabilizes and gives protection to anthocyanins in the complexes. Co-pigments are colourless or have slightly yellow colour. Co-pigments usually are flavonoids (flavones, flavonols, flavanons, flavanols), other polyphenols, alkaloids, amino acids or organic acids. The most efficient co-pigments are flavonols like rutin or quercetin and phenolic acids like sinapic acid or ferulic acid. Association of co-pigment with anthocyanin causes bathochromic effect, shift in absorption maximum to higher wavelength, in result we can observe change of the colour from red to blue. This phenomenon is also called bluing effect. We can diversify two types of co-pigmentation, intermolecular and intramolecular. In the first type co-pigment is bound to anthocyanin by non-covalent bond (hydrogen bond, hydrophobic interaction, ionic interaction). In the second type, we can observe covalent acylation of the glycosyl moiety of anthocyanin. Intramolecular co-pigmentation has stronger effect on the colour. This type of protective stacking is called also a sandwich type stacking and is a very common mechanism in the formation of blue flower colour. Tirantin present in Clitoria ternatea (butterfly pea) and phacelianin present in Phacelia campanularia are examples of pigments with intramolecular co-pigmentation.
  Metal complexation: Often blue flower colour is correlated with presence of anthocyanins in the complexes with metals ions. Metalloanthocyanins are composed from anthocyanins, flavones and metal ions in the stochiometric amounts of 6:6:2, respectively. Typical metals in anthocyanin complexes are iron (Fe), Magnesium (Mg), aluminium (Al), copper (Cu), potassium (K) and tin (Sn). Anthocyanins that can form complexes with metal ions are only cyanidin or delphinidin type, because at least two free hydroxyl groups in the B-ring are necessary to be present
  Examples of metalloanthocyanins: Commelinin isolated from Commelina communis contains malonylawobanin (delphinidin type), flavocommelin (as a co-pigment) and magnesium ions in stochiometric ratio 6:6:2.
   
  Protocyanin is a blue pigment of cornflower (Centaurea cyanus). It is composed from succinylcyanin (anthocyanin), malonylflavone (co-pigment), iron and magnesium anions, and two calcium ions to stabilize the complex. Interesting is that the same anthocyanin when is not in complex with the metal ions is present in red rose petals. 
  Fuzzy metal complex pigments: In the blue coloured flowers, much often instead than metalloanthocyanins we ca can find non – stochiometric metal – complex pigments stabilized by co-pigmentation. Those pigments show blue colour only in aqueous solution and are less stable than metalloanthocyanins. Example of this kind of pigment is present in hydrangea sepals. Main anthocyanin here is delphinidin-3-glucoside what should result in the blue flower formation, but cultivars with red and pink flowers are also present. It is known that acidification of soil can cause change of the hydrangea flower colour from red/pink to blue/violet. Explanation of this phenomenon we can find in the molar ratio of co-pigment (acylquninic acid) to anthocyanin, which is much higher in the blue cells, also the level of Al3+ ions is higher in the blue flowers. Additionally, the pH of blue cell is around 4,1 and red is lower around 3,3. This supramolecule is relatively unstable and easily can fall apart in result of change of component concentration or pH conditions, so this can explain why blue colour in hydrangea sepals has low stability.
   
  Vacuolar pH influence on flower colour: pH theory was the first concept, that tried to explain the mystery of blue colour formation in flower petals. First observation showed that cyanine extracted from blue cornflower changes the colour in aqueous solution in different pH. In the 
caption: Hydrangea with different flower colours
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