Shi-Sheng,
I would guess that your 1227.5+++ peak is monoisotopic, so the MW you
calculated from that, 3680, is monoisotopic. The MW you calculated for
residues 1-37, 3683, is the average molecular weight (and what you saw in
the MALDI). (I get 3684, considering 4 disulfide bridges, close enough) The
monoisotopic mass for 1-37 is 3681--closer to what you saw in the
electrospray.
As for the 204 ion in the MS/MS, a few other internal fragments also
correspond to that mass. (Check out MS-Product at
http://prospector.ucsf.edu/) I would suggest maybe the C-terminal K isn't
there.
Kara Pearson
Post-doctoral Fellow
NHLBI/NIH
Bethesda, MD 20892
301-435-4490
>From: "Shi-Sheng LI" <chembiology00@hotmail.com>
>To: Recipients of ABRF List <abrf@aecom.yu.edu>
>Subject: MS & ProtSeq
>Date: Thu, 07 Dec 2000 18:52:30 -0000
>
>Dear Friends:
>
>By Edman chemistry, One polypeptide with 38 AA residues follows the
>sequence
>as:
>WSGCSPCPGNECCSKYGYCGLGGDYCGAGCQSGPCYGK
>It fits quite well with the AAA results. The problem is:
>1) Maldi MS showed the molecular weight 3684, in the case of ESI Q-tof it's
>3680, that's ok.
>2) If we calculate the mw from the sequence above mentioned, we'll get
>3811 (4 disuphide bridges assumed)
>So, what's going on with the 131 Da part (3811-3680) during the MS?
>
>If the C-terminal K was lost during ionization (? it seems not reasonable?
>any idea?), probably, the ion with mw 3683 may be observed. But
>three-charge
>ion (1227.5) for 3680 was visualized in ESI MS, if K was lost, it should
>not
>be able to get three charges, am I right? And also MS/MS on the native
>peptide (1840.7++) gave reasonable y2 ion 204.14.
>
>Any comments are appreciated!
>
>
>Shi-Sheng Li, Ph. D.
>Biomedical Center, Box 579
>Uppsala University
>751 23 Uppsala
>Sweden
>Emails: chembiology00@hotmail.com; rockylee86@yahoo.com
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