Uric acid, keto-enol tautomerisamtion and gout.

Dear students.
Sadly I’ve been absent due to an attack of “gout”. Its the first time I’ve had it and it was very painful. It’s caused by deposition of uric acid and, references say, monosodium urate, both of which are very poorly soluble. That’s very unusual as the general solubility rules we learn tell us ‘all sodium compounds are soluble’. Well, the rule is applicable to simple binary or “double salt” inorganic salts, and not so much for more exotic species like the purine based uric acid responsible for gout.

I’ve been reading quite a bit about the biochemistry of uric acid and it’s fascinating.

keto-enol tautomeriwsation of uric acid
Uric acid (which like vitamin C [ascorbic acid]), doesn’t contain any carboxylic acid group, but can change (“tautomerise”) in an equilibrium manner to an ‘enol’ which exhibits proton releasing [or if you prefer ‘proton donating’] properties. The ‘uric acid’ form can undergoes proton exchange to its ‘enol’ form if an atom adjacent to the C=O group [called the ‘alpha position’] contains a proton. The e- pair in the bond between the alpha atom and its H ‘flips ‘ towards the C=O. The pi bond in the C=O breaks and ultimately a =C-OH forms, the H from the alpha atom. This =C-OH is a new functional group (not on the CIE syllabus) because two functional groups are on the same C, and it has intersting biological properties and very useful properties for synthesising other molecules. The proton exchange happens IF a more conjugated system results (alternating pi and sigma bonds). This allows the e- to delocalise [“spread out”] which minimises their repulsion, lowering their energy, which is a stabilizing effect. Ascorbic acid, is a double enol! Try and spot the conjugation in the enol form of uric acid and in vitamin C. [Coincidentally(?), a good intake of vitamin C is is recommended to try and prevent gout.!]

Those painful crystals… Ouch!

Gout crystals
monosodium urate, needle-like crystals.
Source: http://pixshark.com/monosodium-urate-crystals.htm

Unfortunately, I still have the effects today (Wednesday); swollen foot, stiff toe joint, but I intend to be back on Thursday (28th May) and I’ll do the practical investigations in CL3 with you after class hours. I may run two sessions. 5:00-7:00 and then 8:00-10:00.

We will be making a stock solution of carious known compounds and then doing some tests on them. This session is ONLY FOR STUDENTS who are TAKING PAPER 34 on 2nd June.


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