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NMR of a Mass Spec Species

I have to say, I didn’t even know that this could be done. This paper from Angewandte Chemie describes a mass spec/NMR combination analysis that had never occurred to me as possible. The authors (from Ohio U. and Purdue) are looking at a common peptide ion seen in proteomic mass spec studies. And what they do is collect enough of the ions to run an NMR. That just seems bizarre, somehow, because I think that most of us picture the ionic fragments in a mass spec as these ghostly, esoteric things that live only in the outer-space-like vacuum of the instrument (and are present in vanishingly small amounts, at that). The idea of piling them up and running their NMR spectrum seems like someone taking a reflectance IR spectrum of an angel’s wing.
That’s because NMR, for most organic chemists, is a much more home-style, hands-on technique. We take measurable amounts of compounds, stick them into glass tubes, and use pipets to dissolve them up in solvent before taking them over to the NMR instrument. You get your hands on these things – and if you need to, you can go get the tube after it comes back out of the magnet, evaporate the solvent, and get all your sample back. Mass spec, on the other hand, uses ridiculously tiny amounts of material. It’s really, really hard as an organic chemist to underload an LC/MS – it seems like you’ll always get something, if there’s something to get. You take a spec of sample, dissolve it in solvent, and then the machine takes a sip of it that a mosquito wouldn’t bother with, because it doesn’t need the rest.
The structures of these “b ions”, as they’re known in protein mass spec, has been the subject of a lot of work. Some N-terminal sequences give you oxazolones, some give you diketopiperazines, and others are still undetermined. This study used atmospheric pressure thermal dissociation (APTD), which gets around that vacuum-chamber problem, and they were able to condense material on the inside of the thermal dissociation tube. Bradykinin’s b2 ion species was isolated, and NMR showed that it was a trifluoroacetate salt of the diketopiperazine structure. A model peptide, Gly-His-Gly, gave similar results.
So how many other mass spec species can this be applied to? And here’s a weird thought: could this be a small-scale preparative method for some unusual syntheses? The thermal dissociation method is similar to pyrolysis, but I wonder if there are some sorts of structures that could be made this way that would be difficult to access by other routes. . .

13 comments on “NMR of a Mass Spec Species”

  1. DCRogers says:

    Re: “taking a reflectance IR spectrum of an angel’s wing
    Here’s a elegant bit of writing you can’t imagine reading anywhere else but here. Bravo!

  2. Tom Womack says:

    The people at Y-12 at Oak Ridge managed to make 50kg a year of U235 by preparative mass spec, so it’s not obviously absurd …

  3. anon the II says:

    Seems like I remember that when these mild electrospray techniques first showed up, someone prepped out a virus by MS and showed that it was still infective. I’m too lazy to look it up.

  4. Luigi says:

    Might be useful for ADME someday.

  5. Morten G says:

    Well, with some custom machines it might be interesting for reaction exploration chemistry, trying to find new reactions by running 1’000-10’000 reactions in parallel. You need to characterise the product there after all.

  6. Anonymous says:

    I can imagine some poor grad student or postdoc spending years collecting ions from the mass spec, one ion at a time, and putting each ion in an NMR tube with tweezers.

  7. Algirdas says:

    @2, wrt Y-12:
    there is this superb file with presentation slides at http://science.energy.gov/~/media/np/pdf/research/idpra/Enriched%20Stable%20Isotopes%20and%20Technical%20Services%20at%20ORNL.pdf
    with an introduction to Oak Ridge National Lab, Y-12, and calutrons.

  8. HordeumVulgare says:

    Derek, many thanks for getting Ohio U right, and not mistaking us for OSU. Bobcats hate that.
    In other news, the newly minted PhD who is the first name on that paper is a friend of mine. I let him know about you picking up on this, and he was pretty thrilled. Just thought you’d like to know

  9. Anonymous says:

    @8: Can you ask your friend if he/she used tweezers as per #6.

  10. Anonymous says:

    Flow cytometric cell sorting on a molecular level. Very interesting!

  11. Anonymous says:

    Actually, they produce ions at ambient pressure that can go to the mass spec detector (vacuum) or be collected in a tube also at ambient pressure. So, compounds, after being ionised, can be detected by means of NMR or MassSpec but no both.

  12. Anon says:

    No “tweezers.” They did NOT collect mass-selected ions. As noted, fragments were produced by “atmospheric pressure thermal dissociation” (APTD). A MIXTURE of fragments was collected at atmospheric pressure, then the fragment of interest was purified by HPLC. They ultimately obtained over 1 mg of this product by spraying a rather high concentration of bradykinin (5 mg/mL) and by pooling multiple (they don’t say how many) runs. Pretty clever, but also a lot of brute force.

  13. Anonymous says:

    This would be extremely useful if they could manage to get it to work for say glycomics or Metabolomics analysis to confirm MS Id. Currently one of the only ways to ID exact glycan structures on proteins is something ridiculous like MS^4. Although sensitivity is definitely an issue, NMR coupled to MS would help get rid of so many bias errors do and in mass spec data which is a big problem in glycomics.

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