Marcus,
I typically reduce the volume of cleavage cocktail to
about 10-20% of the initial volume before
precipitating peptides in ether.
Since there are no protecting groups, I am assuming
that you are using a very simple cocktail of
TFA/water?
Another try might be to extract the TFA/cleavage
adducts out using ether and water. Your tetra/penta
Leu is definately hydrophobic and therefore you can
get rid of the cleavage byproducts prior to isolation.
This may also aid in precipitating the sequence. If
it doesn't come out, then lyophilize.
Hope this helps.
Regards,
Steven Johnson, B.S. Chem.
Research Associate, Process Development Chemistry
Biomeasure, Inc. Milford, MA, USA
--- Marcus Macht <Marcus.Macht@uni-koeln.de> wrote:
> Dear colleagues,
>
> We are currently confronted with a precipitation
> problem in peptide
> synthesis. For a customer we synthesized a
> Leu-pentamer. After cleavage
> from the resin I acquired a MALDI spectrum of the
> raw product which showed
> a mixture of the tetra- and pentamer (approx. 1:1).
> When we tried to
> precipitate the peptide with diethylether after
> cleavage nothing happend.
> We still have a nice clear solution even after
> storing it at -80ƒC over night.
> Does anybody of you have an idea about how to
> precipitate the peptide?
> Would eventually addition of something more
> hydrophobic (e.g. pentane or
> hexane) help or will this lead to a phase separation
> between the organic
> phase and the TFA? Has anybody of you already tried
> to synthesize this
> sequence and how did you circumvent the problem?
> Since the synthesis itself
> of the sequence worked better than I expected, we
> will also try to sequence
> (Leu)10 for the same customer. Does anybody have any
> experience with this
> sequence as well? Should we maybe try to synthesize
> a pentamer, cleave of
> halve of it, protect the free peptide with Fmoc-OSu
> and use this as
> building block to finish the decamer?
>
> Any hints are greatly appreciated! Thanks in
> advance.
>
> Marcus Macht
>
>
********************************************************
> Dr. Marcus Macht
> University of Cologne
> Centre for molecular medicine - Service laboratory
> Joseph-Stelzmann-Str. 52
> 50931 Cologne, Germany
> Tel.: +49 221 478-6995
> Fax: +49 221 478-6977
> e-mail: Marcus.Macht@uni-koeln.de
>
**************************************************************************
>
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