Selective Capture of Phosphopeptides on Modified MALDI Plates Allows Isolation of Phosphopeptide Signals
The figure below shows MALDI mass spectra of a myoglobin digest that was spiked with phosphoangiotensin (pA) and angiotensin (A). The top spectrum was taken with a conventional MALDI plate and obtained after mixing the spiked mixture with matrix (2',4',6'-trihydroxyacetophenone mixed with diammonium hydrogen citrate) and spotting this solution on the plate. Peaks due to phosphoangiotensin and angiotensin are present, but signals from myoglobin fragments dominate the spectrum. In contrast, the use of a MALDI plate modified with poly(acrylic acid)-nitrilotriacetate-Fe(III) films yielded a spectrum in which the signal for phosphoangiotensin was more than 3-fold higher than any other signal. When using the modified plate, the sample was spotted on the plate and allowed to dry, and then rinsed to remove unbound species. Finally, 2',4',6'-trihydroxyacetophenone mixed with diammonium hydrogen citrate was added and the MALDI mass spectrum was obtained. The ability of this system to selectively provide phosphopeptide signals may be of great value in identifiying phosphorylation patterns in important biological processes such as cell signalling.