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TSPA (What about the biochemists?)

By moleculeoftheday on September 28, 2007.

As I noted in a recent entry on strychnine, NMR is important. Sometimes reference compounds like yesterday's molecule, TMS, will help you out.

This is all well and good for most organic chemists - TMS will go into just about any organic solvent. It won't work so well in water, however, so there's a second, water-soluble chemical shift reference: enter TSPA:

i-b5fa54ab091a07d832fe3739ef15a4f7-tspa.gif

TSPA has a chemical shift of 0, just like TMS, and it's soluble in water - good for biochemists, who have to work in water (with at least a little of its deuterated buddy).

TSPA is probably even more controversial than TMS - it won't evaporate and you're left to other methods to remove it. At the risk of attracting the ire of the organic jocks out there, biomolecules tend to be much more labor-intensive to purify, so once you've sweat out a few months of your life getting a protein or nucleic acid scrupulously clean, you're disinclined to add ANYTHING that doesn't need to be there.

The "reference problem" is a little trickier in water, too, because biochemists often must use elaborate solvent-suppression schemes that utterly obliterate your (potential reference) solvent peak. See here for a (pdf) list, including my favorite, the whimsically named WATERGATE.

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The NMR field is full of crazy acronyms. From the basic ones that every chemist has heard of (COSY, HETCOR, HMQC) to some rather more exotic ones like PASADENA, or some of the insane 3+ dimensional sequences and others whose acronyms I can't seem to remember.

NMR spectroscopists of all stripes love silly names for their pulse sequences or suppression schemes. My personal favorites are INEPT, SOGGY (yet another water-suppression sequence), and, of course, the homonuclear Hartman-Hahn transfer or HOHAHA (now more commonly called TOCSY). I also once heard an amusing anecdote from a solid-state NMR spectroscopist who was forbidden by a journal from calling a new pulse sequence FREDOM. Presumably the editor just couldn't take it anymore.

And you can't forget the FID in the Magic angle

Isn't the shift 0 *by definition*, just as TMS is also 0 *by definition*, or a mole is the number of particles in 12.0000000000000000000000 (etc) g of 12C ???

Of course, there are also the methylenes, which don't resonate at 0 as well (http://dx.doi.org/10.1021/jo971176v)