I presented this paper here (thanks, @luismartinMQ!) in my group’s last literature meeting. Apart from being a nice paper that also made me learn about exciton-coupled circular dichroism (that’s a story for another day), I realised how bad I am at drawing porphyrins. So what better choice for this weeks’ chemdoodle than to challenge Grumpy Cat a little with some porphin!
Porphin is the basic structure of a group of compounds called porphyrins. Being all conjugated and aromatic (OBEY the Hückel rule!), they usually have intense colours; porphin is deep red but other porphyrins have other brilliant colours. Tetratolylporphyrin is bright blue-purple.
Porphyrin are the bean bags of chemistry. Have you ever sat quite comfortably in a bean bag and then your phone rings, just out of reach? Getting out of a bean bag is hard! Porphyrins and metal ions – similar: metal 2+ or 3+ ions fit snuggly into the centre of the bis-deprotonated (on the nitrogens) porphyrin. Porphyrins tightly bind to their metal and they’re not picky: magnesium 2+, iron 2+, manganese 3+ or zinc 2+ are just a few examples of what can be complexed.
Porphyrins are common in nature; one of the most famous ones being haemoglobin, the red oxygen-carrying protein in vertebrates. It contains the co-factor haem, a porphyrin derivative with an iron II at its centre.
The redox chemistry when oxygen is bound to the haem iron is quite complex. People found out that it might not be the best idea to try to assign formal oxidation states to the metal and oxygen during this process. It looks though as if at least the resonance form closest to the truth involves an iron III when haem binds oxygen.
Haem does not only like oxygen, it actually prefers carbon monoxide by 250 times and never wants to let it go after binding it. This is why CO and other small molecules like SO, NO or CN– are so toxic; they inhibit oxygen-binding in haem. No more respiration!
If the haem-production machinery of the body doesn’t work properly, porphyrin-intermediates can build up to toxic levels, causing an illness called porphyria. Porphyria can be inherited or caused by external factors like exposure to certain toxins. Acute porphyria causes severe mental disturbances either permanently or in episodes. Cutaneous porphyria leads to accumulation of porphyrin precursors in the skin, causing to photosensitivity and skin problems. Some people think that tales of vampires could be based on people suffering from porphyria: the lack of red blood pigment (maybe to be relieved by drinking blood) and the fear of light (causing skin burns) might have given people in earlier times the wrong impression.
So next time you breathe (which is hopefully going to be quite soon) or cut yourself on an NMR tube (which is hopefully going to be not so soon) or even if you see a vampire (the crappy sparkly type doesn’t count though), think of the power of porphyrin.