A New, Modular Mass Calibrant for High-Mass MALDI-MS

The application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the analysis of high-mass proteins requires suitable calibration standards at high m/z ratios. Several possible candidates were investigated, and concatenated polyproteins based on recombinantly expressed maltodextrin-binding protein (MBP) are shown here to be well-suited for this purpose. Introduction of two specific recognition sites into the primary sequence of the polyprotein allows for the selective cleavage of MBP3 into MBP and MBP2. Moreover, these MBP2 and MBP3 oligomers can be dimerized specifically, such that generation of MPB4 and MBP6 is possible as well. With the set of calibrants presented here, the m/z range of 40-400 kDa is covered. Since all calibrants consist of the same species and differ only in mass, the ionization efficiency is expected to be similar. However, equimolar mixtures of these proteins did not yield equal signal intensities on a detector specifically designed for detecting high-mass molecules.