Re: Peptide Mass changes

Rod Levine (rlevine@nih.gov)
Tue, 28 Apr 1998 21:41:07 -0400

At 12:16 PM 4/28/1998 -0400, VERNON SHOUP wrote:
>In one of my peptide maps, I have the following peptide:
>
> Predicted MW Observed MW (ESI)
> QYFYETK 978.1 977.4 (M+1 = 978.4)
>
>My tryptic digest is adjusted to 1% TFA to quench the digestion. Over two
days at 4C, about 15% of the peptide seems to convert to a peptide with a
mass of 1922.1. m/e
> Minor M+1 1923.1
> Major M+2 961.5
>

You've got an amino-terminal glutamine, which means that it can cyclize to
form pyroglutamate. This would be favored under your acid conditions,
giving an expected mass decrease of 17 -- just what you observed for the
major peaks. While perhaps a little glib, I'd write the minor 1923.1 off
as a non-covalenty linked dimer.

Rod Levine

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