Blue dadadi

The color blue is familiar to most people and is, in fact, most peoples favorite color (2). But it may be surprising to some that blue is actually almost never found organically in nature.  The blue found in animals is mostly caused by reflection of blue light by tiny structures called schemochromes, which are structural differences that cause light to interact with the structures in a certain way, causing them to appear a certain color (3).  This is most apparent with blue birds such as blue jays or hyacinthine macaws.  These feathers of these birds have air pockets within the bard that act as tiny particles which scatter light refracted by the oxygen in the air (3). Some plants can produce a blue pigment in this way as well but most of the blue found in plants is due to a class of chemical compounds called anthocyanins.  These compounds are responsible for many pigments in nature including reds and purples, the anthocyanin responsible for the blue pigment is called delphinidin (4).

So why is it so hard to find blue in nature? The short answer, is that the chemical is not favorable so it is not common.  In order to emit blue light, red light must be absorbed.  As it stands, most molecules absorb ultraviolet photons, there are not as many that molecules that exhibit lower energy π → π* and n → π* transitions that allow for absorption in the visible light range (1).  Even fewer still are molecules that have all the criteria required to absorb right light (emit blue light). Simply having linear π conjugation is not enough, most red absorbing molecules have to have at least two or more of the following criteria in order to emit blue; π bond system, aromatic ring, heteroatoms, ionic charges and metal ions (1).  These usually large and complex compounds are difficult to find in nature, which is why they are typically not common. 

This lack of blue natural pigment is actually somewhat of a problem in the food industry, having natural, consumer safe and vibrant pigments in all colors is something food and beverage companies search for.  Colorants are typically used to replace pigment lost by packaging and storage, or adding color to otherwise colorless products such as soda or frosting (1)

1 .Newsome, Andrew G., et al. "Nature's Palette: The Search for Natural Blue Colorants." Journal of Agricultural and Food Chemistry, vol. 62, no. 28, n.d., pp. 6498-6511. EBSCOhost, ezproxy.shsu.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edswsc&AN=000339226700002&site=eds-live&scope=site. Accessed. November 16, 2017

2 . https://www.livescience.com/34105-favorite-colors.html Accessed. November 16, 2017

3. http://scienceblogs.com/grrlscientist/2007/10/16/schemochromes-the-physics-of-s/ Accessed. November 16, 2017

4. http://www.ddwcolor.com/natural-colours/anthocyanins/ Accessed. November 16, 2017

5. Photo credit: https://pubchem.ncbi.nlm.nih.gov/compound/Delphinidin#section=Top Accessed November 16, 2017