The measured molecular masses do correspond to a peptide with the
expected sequence minus the C-terminal Lys and four disulfide
bonds(calculated monoisotpic molecular mass 3680.24 and average mass
3683.06) - I am assumming that the mass measured by ESI with a Q-Tof
instrument (3680) is monositopic whereas the mass measured by MALDI
(3684) is average. I would not put too much emphasis on the AAA data
and as for the Edman data, do you actually see Lys in the last cycle?
Otherwise, I can not imagine how the C-terminal Lys might "fall off"
during ESI and MALDI mass spec analysis.
Regarding the number of charges in ESI, given the size of this peptide
it is not unreasonable to expect to see a triply charged ion even
without the third basic site (C-terminal Lys).
As for the MS/MS data, the ion at m/z 204.14 can, indeed, be assigned
as the y2 ion for the full-length sequence (WSG...YGK) but it can also
be assigned as the GY or YG internal fragment ion minus ammonia. These
ions, at m/z 204 (204.0661 calculated monoisotopic mass and 204.2
calculated average mass, to be exact), would have nominal structures,
respectively, [CH-CO-NH-CH-(CH2-phenol)-CO]+ and
[CH=CH-(phenol)-CO-NH-CH2-CO]+, and given that there are several GY and
YG pairs in the sequence, one would expect this ion to be quite
abundant in the MS/MS spectrum.
Ioannis Papayannopoulos
AstraZeneca R&D Boston
Worcester, MA
On Thu Dec 7 2000 hi-Sheng LI (chembiology00@hotmail.com) wrote:
>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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This archive was generated by hypermail 2b29 : Fri Jan 05 2001 - 08:42:46 EST