I do not at all disagree that a few equivalents of trifluoroacetic acid
might get displaced by lots of equivalents of acetic acid. But, from my
'knowledge' and recollection of views on this network, swamping a peptide
with acetic acid and evaporating it does not displace/force out all of the
trifluoracetic acid.
Leo B.
At 02:22 PM 98/7/20 -0400, you wrote:
>At 09:52 AM 7/20/1998 -0400, Leo Benoiton wrote:
>>
>>
>>TFA is a strong acid. It will protonate (create the TFA salt) any amino
>>group. The same goes for HCl.
>>Acetic acid is a weak acid. It will only protonate more-basic amino groups
>>such as that on the side chain of lysine and of course the guanidino group
>>of arginine. As such, AcOH cannot displace TFA from a salt...
>>
>>
>>The only way to remove the TFA is to displace it (evaporate in the presence
>>of) with a stronger acid - Hydrochloric acid will displace TFA ...
>
>On an equimolar basis I'm sure you're right. But I wonder if things are a
>bit different if one has first dried the peptide to remove excess TFA, then
>redisolved/resuspended in acetic acid?
>
>This principle (mass action) is the basis of the "TFA fix" introduced by
>Alex Apffel and his colleagues for dealing with the suppresion caused by
>TFA when introducing an LC eluate into an electrospray mass spec. The
>eluant (with the usual 0.05-0.10% TFA) is mixed with half its volume of a
>mixture consisting of 1:1 acetic acid:acetonitrile. It is impressively
>good in displacing TFA and improving the signal in the mass spec.
>
>See: Apffel A, Fischer S, Goldberg G, et al: Enhanced sensitivity for
>peptide mapping with electrospray liquid chromatography-mass spectrometry
>in the presence of signal suppression due to trifluoroacetic
>acid-containing mobile phases. J.Chromatogr.A. 1995; 712:177-190.
>
>Rod Levine
>
>NIH
>Bldg 3, Room 106 MSC 0320
>Bethesda, MD 20892-0320
>
>email: rlevine@nih.gov
>voice: 1 (301) 496-2310
>fax: 1 (301) 496-0599
>
N. Leo Benoiton
Department of Biochemistry
University of Ottawa
Ottawa, Ontario, Canada K1H 8M5
Tel: 613 562 5800, Ext. 8216
Fax: 613 562 5440
eMail: benoiton@uottawa.ca