The Chemistry of Blood Colours

Blood is a necessary component of the majority of living organisms (all vertebrates and some invertebrates); it carries vital nutrients, oxygen and proteins to body tissues and carries away waste products. Blood, although most commonly red, can also be found in such colours as green, blue, clear, pink and violet. However, the common misconception surrounding the idea that human blood is blue before oxygenation is false. All human blood is varying shades of red; these wild alternate colours are found in other animal species such as certain species of crabs and insects.
This colouring of the blood can be seen as an indicator of oxygen carrying ability or make apparent the metal with the highest concentration in the blood. The colour containing portions of blood are known as respiratory pigments; these pigments are metal containing proteins which combine reversibly with oxygen. Respiratory pigments are found within cells of blood and their primary function is to aid in the transportation of molecular oxygen.
There are four unanimously recognised respiratory pigments, these are; hemoglobin, followed by hemocyanin, then chlorocruorin and Hemerythrin. These four pigments occur in greater percentages and are far more efficient in carrying oxygen than the few other pigments known. These lesser known pigments (not all fully recognised as respiratory pigments) include; vanadium chromagen and pinnaglobin. Haemoglobin, the most common respiratory pigment on earth is the pigment found in all vertebrates (excluding a few Antarctic fish) including humans.

Hemoglobin is located within the platelet component of blood giving the distinctive red colouring associated with blood both when oxygenated and deoxygenated, when oxygenated it is a bright red and by the time it is traveling In the veins back to the heart, blood containing haemoglobin is a dark red in colour. This colour is due to the presence of iron in the haemoglobin. Iron is the central atom of the heme group ( Without iron in the heme group, there would be no site for the oxygen to bind) One molecule of haemoglobin, with iron at the centre, can carry four oxygen molecules.
Fig 1: Hemoglobin structure Image: (Wikipedia, the free encyclopedia, 2013) Fig1. 2: Hemoglobin, human adult, heme group Image: (Wikipedia, the free encyclopedia, 2013) Hemocyanin is the second most evident form of respiratory pigment, found in mollusks, arthropods and some insects. Blood containing Hemocyanin is blue in colour when oxygenated and transparent in appearance when deoxygenated. Oxygenation causes a colour change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form.
This blue colour is due to the presence of Two copper atom at the centre of hemocyanin particles and unlike haemoglobin, only two oxygen molecules can reversibly combine with the metal proteins at the centre of the hemocyanin particles therefore it is four times less efficient as an oxygen carrier than haemoglobin. Fig. 2: Hemocyanin, deoxygenated and oxygenated Image: (htt) Chlorocruorin is an iron, metalprotein, respiratory pigment with many similarities to hemoglobin.
The most notable of the differences between hemoglobin and chlorocruorin is the abnormal heme group structure of chlorocruorin and unlike hemoglobin it floats freely within the plasma of blood rather than being confined to red blood cells. The chemical colour change of chlorocruorin bears resemblance to both hemoglobin and hemerythrin, changing from a green when deoxygenated to red when oxygenated (two oxygen molecules reversibly combine one iron atom). This ration puts chlorocruorin at 25% the efficiency of hemoglobin. Fig. 3: Chlorocruorin structure
Image: (Wikipedia, the free encyclopedia, 2013) Hemerythrin is the third respiratory pigment containing iron, found in marine invertebrates (sipunculids and brachiopods) and used for oxygen transfer and/or storage. Although containing the same metal protein, hemerythrin differs from both haemoglobin and chlorocruorin as it contains two more iron atoms which reversibly combine and are connected by an oxygen molecule. This means that the efficiency of this pigment is 25% as effective as haemoglobin and on par for effectiveness with chlorocruorin.
When the oxygen molecule combines directly (no heme group) with the iron atoms a colour change occurs; deoxygenated hemerythrin is near colourless changing to a pink/violet colour when oxygenated. Fig. 4: Hemerythrin structure found in sea worms Image: (Coleman, 2009) Other than the four major respiratory pigments, giving blood colour, there is dispute over two other pigments, little is known of these pigments. They are; Pinnaglobin, a brown pigment found in the blood of a mollusc of the genus Pinna, this pigment demonstrates similarities to the pigment Hemocyanin but contains manganese as the metal atom in place of copper.
The other proposed pigment (idea is disputed) vanadium chromagen, is said to be light green in colour, contain metal atoms of Vanadium and is found in sea squirts, ascidians and tunicates. Bibliography (n. d. ). Retrieved from http://web. tock. com/kalee/chem32/spec/ (2008, 04 13). Retrieved from http://www. klingon. org/smboard/index. php? topic=1377. 0 Wiktionary. (2012, 11 11). Retrieved from http://en. wiktionary. org/wiki/vanadium_chromagen Coleman, W. F. (2009, 04 11). Dept. f Chemistry, Wellesley College. Retrieved from http://academics. wellesley. edu/Chemistry/Flick/chem341/hemoglobin1. html Department of Biology, Davidson college . (2005). Retrieved from http://www. bio. davidson. edu/Courses/Molbio/MolStudents/spring2005/Heiner/hemoglobin. html Encyclopedia britannica. (n. d. ). Retrieved from http://www. britannica. com/EBchecked/topic/260910/hemocyanin Frey, R. C. (n. d. ). Hemoglobin and the Heme Group. Retrieved from Department of chemistry, Washington University: