[wizzify]

http://i.imgur.com/zuVmX.jpg

I cannot reason the logic behind the Enol-form of the Pyruvate being more stable than the Keto, since there is no resonance difference.

In the case of Vitamin C, it'S because of the resonance, right? The negative charge and the H+ can pass from one O to another, having 3 resonance forms to stabilize it, while in the Keto form there are only 2.I just want to make sure I understand it correctly.

In the case of Pyruvate, there'S really no reason I can think of.

Thanks!

[discodermolide]

I think you are correct in what you say.
Pyruvate enol is certainly not more stable than the keto form.

[wizzify]

It can't be though. It's in the professor's notes. I mean, sure it it possible that it's wrong, but are you sure?

[discodermolide]

Yes.
Ask him to explain it

[wizzify]

Could it be because the enol form can dimerize?

[Babcock_Hall]

The keto form is more stable than the enol form of pyruvate.  See Figure 13-13 in Nelson and Cox, Biochemistry, 5th edition.

[orgopete]

The keto form is more stable than the enol form of pyruvate.  See Figure 13-13 in Nelson and Cox, Biochemistry, 5th edition.

Does it give the ratio?

[Dan]

A quick google uncovered this old paper from 1926 in which the equilibrium is plotted over pH 4-8 (see fig. 3). It would appear that the enol form predominates above pH 5.8.

While I follow the concept, I know little about the methods used and cannot give an authoritative interpretation of the results myself. If I had to explain why the enol form is the major form of the pyruvate ion, on first inspection I would probably go for an intramolecular hydrogen bond between the OH and the carboxylate (which will not be present in the keto form). How significantly stabilising this would be in aqueous (protic) solution though, I am not sure.

[Babcock_Hall]

The keto form is more stable than the enol form of pyruvate.  See Figure 13-13 in Nelson and Cox, Biochemistry, 5th edition.

Does it give the ratio?

I was hoping that it did, but I don't see one.  The party line has always been that the enol to keto isomerization is what explains why the free energy of phosphoryl group transfer in phosphoenolpyruvate is so negative (ΔG°' is about -60 kJ/mol), as David Metzler discussed on p. 510 of Biochemistry, second edition.  However, I don't have a citation for the equilibrium constant for the tautomerization itself.

[Babcock_Hall]

A quick google uncovered this old paper from 1926 in which the equilibrium is plotted over pH 4-8 (see fig. 3). It would appear that the enol form predominates above pH 5.8.

While I follow the concept, I know little about the methods used and cannot give an authoritative interpretation of the results myself. If I had to explain why the enol form is the major form of the pyruvate ion, on first inspection I would probably go for an intramolecular hydrogen bond between the OH and the carboxylate (which will not be present in the keto form). How significantly stabilising this would be in aqueous (protic) solution though, I am not sure.

I am a little pressed for time today, but I think I found a relevant reference:

Keto-enol equilibria in the pyruvic acid system: determination of the keto-enol equilibrium constants of pyruvic acid and pyruvate anion and the acidity constant of pyruvate enol in aqueous solution

Y. Chiang , A. J. Kresge , P. Pruszynski
J. Am. Chem. Soc., 1992, 114 (, pp 3103–3107
DOI: 10.1021/ja00034a053n

It looks to me that pK_E for pyruvate is about 5 and for pyruvic acid is about 3.2.