pH independent, thiol cross-linker?

Kresten kresten at my-deja.com
Wed Nov 3 08:06:37 EST 1999


In article <VSBT3.4$GA4.23 at testbox>,
  "Frank R. Gorga" <fgorga at bridgew.edu> wrote:
> Yoram Gerchman wrote in message
> <381E9849.B4359018 at leonardo.ls.huji.ac.il>...
> >(This is a re-post, sorry)
> >Greetings Netters
> >I'm looking for a pH independent, thiol cross-linker, any idea would
> >be welcome (should work at pH range 6-9)
>
> There is no such thing... because of the basic chemistry of the SH
> group
>
> The deprotonated form (RS-) is much more nucleophilic than the
> protonated
> form (RSH). Fuethermore, the typical thiol has a pKa of around 8.5.
> Thus at
> pH 9 thiols are more than 50% deprotonated, while at pH 6 there is
> essentially no unprotonated thiol.
>
> The bottom line... the reaction of thiols with any reagent will depend
> on
> the pH in the range 6 to 9. Reactions will proceed quite well at pH 9
> and very poorly (if at all) at pH 6.

That must surely depend on the reagent. As you say, the thiolate is
usually the active reagent (as a nukleophile) but if the electrophile
can also be titrated things are not so simple.

I am not sure, but I would expect that in the reaction of thiols with
(2- or 4-)vinylpyridines the active reagent is the protonated pyridine.
If this is the case (and neither R-SH or pyridine-N: will react at all)
then pH optimum (if my algebra is correct) will be:
pH(opt)= (pKa(R-SH)+pKa(vinylpyridine))/2
which would be lower than the pKa of R-SH

Kresten

BTW: As I mentioned in another post, there is a cyanylating reagent
called CDAP which will cyanylate cysteines at low pH and also reducing
agents (eg. TCEP) that will reduce at low pH.


>
> --- FRG
>
> ------------------------------------------
> Frank R. Gorga, Ph.D.
> Dept. of Chemical Sciences
> Bridgewater State College
> Bridgewater, MA 02325
> 508-531-2827
> 508-531-1785 (fax)
> fgorga at bridgew.edu
> http://topcat.bridgew.edu/~fgorga
>
>

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Kresten Lindorff Larsen, Dept. Yeast Genetics


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