[radical886]

Ergothioneine (EGT) is a thiohistidine, and presumed to be an antioxidant produced by microorganisms. EGT exhibits tautomerism between the thiol (-SH) and the thione forms; the thione form predominates at neutral pH. The thione forms can also form resonance structures where the =S is somewhat thiolate (please refer to the attached figure)

Almost any review paper on EGT will say that this predominance of the thione tautomer results in an increased 'stability' of EGT. By stability, it is generally referred to the fact that EGT doesn't undergo auto-oxidation in the presence of transition metals, unlike other thiols (cysteine is very vulnerable to this, GSH less so but still gets oxidised over prolonged periods, generating ROS in the process).

But its not clear to me why the predominance of the thione tautomer results in this stability or lack of auto-oxidation. I suspect this stability has something to do with the thiolate intermediate being a necessary step before oxidation/reaction with electrophiles, and maybe this thiolate intermediate is stabilized because of the resonance forms of the thione? Am I somewhat on the right path with this guess? But then, wouldn't this mean that EGT is also a 'weaker' antioxidant compared to say GSH or any other thiol which doesn't exhibit tautomerism?

Thanks for any ideas!

[Babcock_Hall]

I certainly don't have an answer.  However, maybe we can work on framing the question.  One problem I had when reading your question is whether or not you are interested in thermodynamic stability.  The relative stabilities of the reduced an oxidized forms can be approached by examining the half-cell reduction potential.  I am also not sure what you mean by auto-oxidation.  Do you mean reaction with molecular oxygen.  D you mean that this is a metal ion-catalyzed process?  This might be a kinetic question more than a thermodynamic one.

[radical886]

Yes in auto-oxidation I'm referring to metal-ion catalyzed oxidation of thiols by molecular oxygen. EGT has a standard reduction potential of -0.06V compared to -0.24V for the GSH-GSSG redox couple. Also the thiol pKa of EGT is ~10.8 compared to 8.9 for GSH.

I'm not interested in thermodynamics or kinetics of the oxidation of the two thiols per se, but am trying to come up with an explanation as to why EGT is considered to be more resistant/stable to metal ion catalysed oxidation. And what this can mean about the general antioxidant function of EGT. (As you can tell, I'm more interested in a biologically relevant explanation, as that's my background and research area). But if the process requires an examination of thermodynamic stability of the two thiols, I'm interested in doing so.

I thought about the thermodynamics angle before, but my knowledge of chemistry wasn't sufficient to integrate the reduction potential values, the pKa, and the tautomerism of EGT into one consistent explanation on the increased stability of EGT. Does the thione tautomerism of EGT make reaching the thiolate intermediate harder (compared to thiolate from thiol isomer), and this increases the reduction potential compared to GSH? Or is the increased reduction potential a separate attribute of EGT unrelated to its tautomerism? Similarly, I'm not sure how the higher pka of EGT ties in. Higher pKa indicates a lesser willingness of the EGT thiol to deprotonate to the thiolate, and I guess this is because the thione tautomer generally predominates, which is not able to able to deprotonate to the thiolate unlike the thiol form? Does that mean the higher pKa is directly linked to the tautomerism, and also to the increased reduction potential, or again, are they separate attributes?

Regarding you comment on kinetics, I'm not sure about it, as I haven't seen kinetic parameters generally in the review papers I've read of thiols (its generally the pKa and reduction potential that's the focus). Happy to hear any further details on this angle though.

[Babcock_Hall]

I can give you two ideas to think about, but they may or may not be helpful.  One is that the lower the pK_a of a thiol, the more reactive it sometimes is in reduction reactions (other reaction as well).  That makes sense if the rate depends upon the fraction in the thiolate (conjugate base) form.  Two is that redox potentials may or may not be useful in this instance, but they are certainly helpful in to explain certain phenomena.  For example the reduction potential of DTT is approximately -0.32 v at the biochemical standard state (which is pH 7).  Most other thiols (glutathione, 2-mercaptoethanol) fall in the range of -0.20 to -0.30 v.  This can be simply explained using entropy:  Making GSSG from GSH proceeds with a loss in entropy (all else held equal). However, making oxidized DTT from reduced does not, because the reaction is intramolecular.  It might be worth exploring the unusual reduction potential of EGT.  Was it determined under the conditions of biochemical standard state?

There are certainly studies out there (Clarence Suelter's book Practical Enzymology comes to mind) that show that the rates of oxidation of particular thiols depend upon pH and the presence or absence of particular metal ions.  I don't know much about how in-depth these studies are, in terms of determining a mechanism, for instance.