Friday, February 11, 2022

APCI and APPI -- for identifying peptides???


I'm going to leave this here, but while I feel like this is something that someone probably did before, maybe back when we had to go to libraries to photocopy journal articles, I can't think of who. I certainly can't think of a recent study.

For a refresher if you haven't thought of it in a long time, APCI is like this (diagram stolen from Creative Proteomics, thanks!) 

Basically, your liquid purposely flies by a corona discharge that you did on purpose! 

Ugh. I tried to find a gif of beer coming out of a corona bottle to make a joke, but all I found was famous people pulling down their masks to sneeze or cough. corona discharge jokes today....

In ESI, cranking up your voltage to a point where you can see blue light coming off your emitter with your naked eye generally means you've done something wrong and should stop doing it. In APCI it's a great thing. I've often made fun of APCI for pesticide analysis because I feel like if you can't see it with ESI you aren't trying hard enough, but for things with a lot of Chlorines or Fluorines on them it can be easier to see them with APCI, primarily because it will degrade them or force an adduct on them that will allow them to ionize. 

APPI? Literally no one on earth knows that that is. If you equip your source with APCI you can pay a nominal fee to have APPI added to it. You'll never use it and you'll never learn what it is.

Or will you? Check out that bar chart I stole from the paper at the top. There is actually a peptide that looks better with APCI than ESI!

That peptide is AWSVAF. Where does the proton go on a peptide this crappy? It looks to me like in ESI you'll only get the mobile proton, but since I forgot that F was an amino acid until just now (temporarily forgot, it's like 4am here) but I don't see too many Fs. 

Is APCI or whatever APPI is a solution for anything I can think of right this second? Nope! But I'm honestly still distracted by the NBA trade deadline. But if you have a super weird peptide (weird endogenous stuff?) and you've got these things in a box and you don't remember why, maybe it's worth giving this a shot? 

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