Date: Wed, 8 Jan 1997 11:21:39 -0600
Message-Id: <v01530504aefa85ad8fa8@[128.252.197.88]>
To: abrfhyp@cco.caltech.edu
From: rskubish@pharmdec.wustl.edu (Richard Skubish)
Subject: racemization on Ac-Trp coupling
Yes, you can expect racemization of Ac-Trp-OH when you couple it using
TBTU with HOBt as additive. The occasion arises as you stated because you
are coupling an N-acylamino acid and not a urethane-protected amino acid.
But it is also because the TBTU requires that you effect the coupling in
the presence of a very basic tertiary amine. The tert-amine is required
to initiate and drive the reaction that occurs between the carboxy-anion
and the reagent. I will get arguments about this, but my opinion is that
you will get superior results using a coupling method that does not
involve the excess of tertiary amine, in other words, the old reliable DCC
with HOBt. And it is also likely that HOObt/HODhbt
(3-hydroxy-4-oxo-3,4-dihydro-benzotriazine) or HOAt (the 7-aza-anologue of
HOBt) would be superior to HOBt. The possible superiority of HOObt vs
HOAt is not predictable. And also if you are able to, the coupling
should be done in the least polar solvent possible, such as
dichloromethane.
Leo Benoiton,
University of Ottawa,
benoiton@aix1.uottawa.ca
On Thu, 7 Nov 1996, A. Tossi wrote:
> Can anyone tell me if I should expect racemization coupling Ac-Trp-OH to
> a peptide using HOBT/TBTU activation? I did a solution coupling to a
> diaminodiol central group and the resulting pseudotetrapeptide shows two
> peaks in RF-HPLC (3:1) both with the correct mass. I am worried that the
> absence of the urethane bond as present in Fmoc-AAcids may result in
> increased racemization even if HOBT is present.
>
> Thanks,
> Alex Tossi
> Dept. Biochemistry
> Univ. Trieste
>
>
Richard Skubish
rskubish@pharmdec.wustl.edu
314-362-0283
Washington University Medical School
Box 8103 - PNACL
660 S. Euclid
St. Louis, MO 63110