Steve,
The answer is simple. You're making these peptides on
MBHA rink resin without a NLe spacer. Your linker is
coming apart and staying with your peptide on the
C-terminus. What happens is you lose the
2,4-dimethoxyphenyl group changing the dynamic of the
peptidyl bond with the linker so that it won't come
off there. Where it breaks is on the resin side of
the amide at the 4-methylphenyl/benzyl group. If you
have resin with the NLe spacer, you'll find a peak of
plus 276. They're both fairly hydrophobic pieces and
you'll find them running later on RPHPLC.
I found to eliminate or severely control the formation
of the impurity, you need to increase the amount of
water in your cleavage cocktail. I use a cocktail of
TFA/Water/TIPS (triisopropylsilane) in a ratio of
90/8.5/1.5 and very rarely see that impurity. I was
seeing it more when I used less water in my
cocktail(s).
Regards,
Steven R. Johnson B.S. Chem
Research Associate, Chemistry/Process Development
Biomeasure, Inc. Milford, MA, USA
--- "Steve L. Mouton" <moutons@uthscsa.edu> wrote:
> I have synthesized several peptides lately and have
> come up with a peak in mass spec of 163 higher in
> almost every one of these peptides. There has been
> no tyrosine in any of them. This contaminant does
> purify out but it is decreasing my yield. I have
> already checked out all common reagents to these
> peptides and have not found anything that has been
> used in all of them. Also other peptides
> synthesized along with these do not have the 163
> extra mass so it is not in every peptide made. Does
> anyone have any ideas? I would really appreciate
> it.
>
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