[mwarner89]

Hi,
I have this question to do as part of a masters course in chemistry.
I can interpret it for a single protein using formulas but do not know how to interpret it for 2 proteins in a mixture. For the formulas i have you have to assume the peaks are adjacent, separated by a single charge and are of the same molecule.

Help would be very much appreciated
Many thanks

An ESI mass spectrum was obtained for a mixture of two polypeptides. The following apparent m/z ratios were observed: 541.7, 569.85, 577.74, 617.26, 644.05 and 666.47. Unfortunately not all of the expected peaks were seen. Determine (a) the charges on all the peaks seen, (b) the molecular masses of the two components (c) the charges and apparent m/z for any missing peaks between the extreme values seen, that is in the range [541.7, 666.47].

[MOTOBALL]

Since the m/z values are very close, there must be a high number of charges on each m/z species.

Let your MW values be Ma and Mb.

Suppose m/z 666.47 has n +ve charges

then Ma = 666.47 x n - nH = 665.47 x n

Suppose m/z 541.7 has n+1 +ve charges

then Mb = 541.7 (n +1) - (n +1) = (n + 1) x 540.7

does Ma = Mb

i.e. does 665.47 x n = 540.7 x n  + 540.7 ??
i.e. does 124.77 x n = 540.7 ??
i.e. does n = 4.33  No, but it might = 4.33 x 3 = 13
then Ma = 8651.11, and successive ions are at m/z (8651.11 + 14)/14 = 618.9
                                                                m/z (8651.11 + 15)/15 = 577.74
                                                                m/z (8651.11 + 16)/16 = 541.7

You should be able to solve from here.

[mwarner89]

Thanks, however still confused 

Can anybody please explain this further. Im not sure how to identify between the two molecules given the data, along with the fact that some of the peaks are meant to be missing.

Cheers

[mwarner89]

Ok so this is what i have so far!

As i don't know what peaks belong with what molecule or if they are adjacent i systematically worked out the possible adjacent peaks and their respective charges using, z1 = A2-1.0079 / A2 - A1
I then focused on the results that gave an integer as the positive charge. From this i induced that the peak at 541.7 and 577.74 were adjacent and the charge was 16 and 15 respectively. I found the Mr for this molecule was 8652.46 using:
Mr = {(A1-1.0079)(A2-1.0079)} / A2-A1
I found that the peak at 666.47 was part of the same molecule with a charge of 13 and that the peak with a charge at 14 was missing.

Using the same method i found the peaks at 569.85 and 617.26 were adjacent with charges of 13 and 12. The Mr then worked out to be 7394.01.

I cannot find an explanation for the peak at 644.05 since it does not conform to my results. I was testing the m/z values with the formula A1 = (Mr +1.0079) / z

Question: Is what I have done correct, and how do i account for the peak at 644.05? I have a feeling i have gone wrong somewhere but can't figure out where!

Many Thanks

[MOTOBALL]

I think that you are making this problem unnecessarily complicated.  Instead of doing multiple calculations with the formula, take the second set of data (m/z 644.05, 617.26 and 569.85) and apply the same method that I used for the first protein.

Assume that 644.05 has n charges and 569.85 has n+1---does n = an integer ?? (clearly not, since there is at least one other m/z value between them; so multiply by smallest whole number to make n = integer).

The numbers do work out correctly (I have gone through it), and there is also a missing number in this series. 

You cannot explain m/z 644.05 because your calculated charge states for 569.85 and 617.26 are not correct.

Your value of 8652.46 for first protein is also, I believe, incorrect.  8651.11 gives spot-on values for the observed ions, whereas 8652.46 gives values that are slightly off.
Please let us know your result for second protein.

Good Luck

[mwarner89]

Thanks for your help, Its really appreciated

[mwarner89]

Hey, Ive worked it through and come up with charges of 23,24,25,26 for the second protein at peaks 643.05, 617.26, 592.61 and 569.85 and an Mr of 14790.15.
Thanks for your help, was trying to use formulas from notes which over complicated things at the start!

[MOTOBALL]

OK, you've got it.

I'm not sure that I would want to do this longhand for a mixture of more than 2-3 proteins.

Hope that you enjoy chemistry and stay with it.