Use of polymer-modified MALDI-MS probes to improve analyses of protein digests and DNA.
The use of sample probe surfaces patterned with 200-microm-diameter spots of hydrophilic, charged polymers significantly enhances the analysis of protein digests and DNA by MALDI-MS. Selective adsorption on these polymer-modified surfaces allows collection of specific proteolytic peptides, while subsequent rinsing of the deposited sample removes contaminants. In the case of partially digested myoglobin, the mass spectrum obtained using a sample probe modified with polyanionic functionalities permits detection of 22 proteolytic fragments, while analysis using a stainless steel MALDI sample probe gives only 11 detectable fragments. Similarly, during the analysis of bovine serum albumin digests, the use of several different surface-modified MALDI sample probes increases sequence coverage from 61.3 to 74.5%. Detection of phosphorylated peptides can be quite challenging during analyses of phosphoprotein digests by MALDI-MS because these anionic proteolytic fragments have low ionization efficiencies. However, MALDI signals from the phosphorylated proteolytic fragments sometimes increase dramatically when using a sample probe surface modified by a polycation (polyethylenimine or poly(acrylic acid) complexed with Fe(3+)). The signal enhancement apparently occurs because the positive surface selectively binds the phosphorylated peptides. The use of patterned, polycationic surfaces also shows great promise for selective adsorption and decontamination of DNA samples; a simple water rinse diminishes or eliminates the formation of multi-ion adducts, thereby improving mass resolution during subsequent analysis by MALDI-MS.