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[Mighty Mouse]

Quick question regarding the addition of TCA to a solution of trypsinized proteins. I have two samples that I have exposed to trypsin for digestion prior to MS. The MS folks suggested adding TCA to about 1% prior to sending them the samples to stabilize the proteins. In one of my samples (basically just some cell lysate), once I added the TCA I saw a very clear precipitate form, the protein. In the other sample (trypsinized via the same method as the first, but these proteins were attached to some streptavidin coated magnetic beads) I saw no precipitate form upon the addition of TCA. I'm trying to figure out if this is indicative of a lack of trypsinization in these bound samples or if I simply shouldn't necessarily expect to see a protein precipitate following the addition of 1% TCA in a sample of trypsinized peptides.

As a corollary, does anyone know a simple test for determining whether or not the trypsinization worked in these samples, besides running MS on them?

MM