This paper shows matrix for the tryptic digest of alpha casein. However, I am looking for intact molecule of alpha casein.
I've seen some spectra obtained using sinapinic acid. However, caseine was hardly detected. I think it might be difficult without a digestion. What about ESI-MS, possibly with HPLC separation? Should be much easier to get reasonable results.
You may try to check this one:
http://lait.dairy-journal.org/articles/lait/abs/2001/05/07-pierre/07-pierre.html
ESI MS spectra is complicated with many multiply charged species.
I would like to see the molecule prior to digestion.
Yes, that's true, but most of the modern ESI-MS systems have a software for the automatic determination of MW from the multiply charged ions, so you could easily see the intact molecule. Still, I would also start with MALDI.
Casein is a relatively small and water-soluble protein, so CHCA, DHB and Sinapinic Acid should all work. You probably have done the following, but just to make sure:
- have you checked instrument performance in liner mode on a standard protein of similar molecular weight? Even something a little smaller like Myoglobin should work
- have you desalted your sample properly, using e.g.C8 or C18 ZipTips, or floating membrane dialysis? Commercial qualities of casein can be quite salty
- are you looking in the right concentration range? Try 1-50 pmol on target. Commercial casein is actually several proteins, i.e. alpha-S1, alpha-S2, beta and kappa
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