Hi Sandie:
Your extra Arg is most likely due to a double addition of Arg when you
performed you "Double Couple" cycle. This can occur when the Fmoc attached
to the Arg added during the first coupling is prematurely removed prior to
the introduction of your second dose of Arg. This effect is enhanced when
there is washing with solvent (NMP or DMF) between the two additions,
especially if you have old or bad solvent containing high residual amine
content. My suggestion...do not double couple. If you do, eliminate any
wash between the two Arg additions for that double couple cycle.
Hope this helps.
Pat Wesdock
Applied Biosystems
Sandie Smith <sandie@mail.utexas.edu>@aecom.yu.edu> on 01/31/2001 02:40:10
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Subject: peptide syn
Dear pepsyn gurus
I have just synthesized a peptide: GARRGKGR-NH2. By mass analysis I
have two species,one is correct, the other has an extra Arg (+156).
The incorrect one is about 70%. There was nothing to indicate an
instrumental error. A Kaiser test indicated only that coupling was
not as efficient at the RR cycles as the other cycles. This has
happened one time before,two to three months ago, again involving a
RR sequence. That time I had done a recouple after the first R. I
redid the synthesis, without a recouple and got the correct mass
only. I put it down to some unknown, never to be explained, problem,
with the recouple. Now it's come up again. Please, does anyone have
a solution to this puzzle?
Thank you,
Sandie
Sandra S. Smith
Research Associate
Protein Microanalysis Facility
Institute for Cellular and Molecular Biology
MBB 1.420
University of Texas, Austin, Texas 78712
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