Butterfly Dyes

The colours of some butterfly wings can depend upon two differing mechanisms. The first is optical absorption of some frequencies of the visible spectrum to leave the inverse spectrum reflected; this is a definition of a pigment. Another method utilised by Butterflies is optical interference via the very thin scales in the wings, which causes an iridescence (this is called Structural Colour), where the actual colour reflected may vary with viewing angle, a little like a thin film of petrol on water. Iridescent colours are mostly greens and blues, whereas pigmentation colours in butterflies are mainly in the longer wavelengths of the spectrum, such as yellows and reds. Combination colours, where the colour is due to both mechanisms, are common. Detailed here are only the pigmentation colours, which are due to Pteridines with various side groups attached, the side groups altering the wavelength(s) at which the Pteridines absorb light, being the opposite of the wavelengths that they reflect and the colours you see in their wings.

The pigments based on pteridine actually also fluoresce in ultraviolet light illumination, and this fluorescence un-doubtedly contributes to the brightness and vividness of some butterfly colourings.

PTERIDINE PIGMENTS

Pterin gets its name from the Ancient Greek 'pteron' (πτερον), meaning 'wing', not that the ancients knew that Pterin is a component of certain wings.
The pigments in butterflies wings are based on Pterin, which is based on Pteridine. Pteridine consisting of a pyramidine ring fused to a pyrazine ring. Two additional extra components, an amino group on the 2-position and a keto group on the 4-position gives the molecule called Pterin upon which the butterfly wing pigments are based. [Note that Pteridine should not be semantically nor structurally confused with another molecule similar in shape with the same number of nitrogen atoms (4) within the two fused rings called Purine upon which Xanthine dyes are based. Xanthine should not be confused with the similar sounding Xanthene, which contains no nitrogen atoms, and is the basis of many artificial dyes such as Rhodamine and Fluorescein].

(→UV→)   Leukopterin (or Leucopterin) is a white pigment within butterflies wings and fluoresces pale blue.

(→UV→)   Xanthopterin is a yellow pigment which also fluoresces in the yellow/green part of the spectrum. Xanthopterin also occurs as a breakdown product in the urine of mammals and within the eyes of people suffering from liver failure.

(→UV→)   Isoxanthopterin, which differs from Xanthopterin only in the position of the ketone oxygen atom, is another pteridine colouring found in some butterflies wings, it is white but fluoresces violet in UV light. The bright blue/violet (purple) fluorescing isoXanthopterin occurs in some North American butterflies not only in the wings, but sometimes in the head or body, depending upon particular species, of which over forty have been identified.

(→UV→)   Erothropterin is a red pigment in the wings of butterflies which fluoresces orange/red.

(→UV→)   Sepiapterin is yellow and fluoresces in the orange/yellow part of the spectrum and is found in the wings of the Alfalfa Butterfly (Colias eurytheme) which may not appear in the UK.

(→UV→)   Biopterin, a brown crystalline solid, fluoresces in the blue and is also present in some butterflies. Biopterin is also synthesized in several parts of the human body including the pineal gland, involved in the light-activated synchronisation of the body clock.

Pterorhodin (sometimes known as Rhodopterin), is a pteridine dimer and red pigment which has not only been found in the melanophore of the leaf-frog but is also found in some, but not many, species of butterfly.

Chromophores based upon Pterin are also found in Thale Cress in the form of Chrytochrome 1 and instigate growth on the incidence of blue light.
Pteridines are also found in plants, with Pteridine itself being involved in the production of Folic Acid in chloroplasts (plant 'mitochondria'). Folate molecules consist of Pteridine bonded to para-AminoBenzoate which in turn is bonded to Glutamate. The pteridines are synthesized in the plants cytosol; the para-AminoBenzoic Acid in the chloroplast and the glutamate is found elsewhere...