Effects of protein-surface interactions on protein ion signals in MALDI mass spectrometry

The influence of polymer surface-protein binding affinity on protein ion si gnals in matrix-assisted laser desorption/ionization (MALDI) mass spectrome try is examined. The surfaces of poly(vinylidene fluoride) and poly(ethylen e terephthalate) polymer substrates are modified by pulsed rf plasma deposi tion of allylamine. By varying the on/off duty cycle of the pulsed rf plasm a, the polymer substrate surfaces are coated with thin films having varying densities of surface amine groups, The varying surface amine density is sh own to lead to systematic changes in the surface binding affinity for the I -125-radiolabeled peptides angiotensin I and porcine insulin. Unlabeled ang iotensin I and porcine insulin are then deposited on the pulsed rf plasma-m odified substrates and analyzed by MALDI mass spec spectrometry. The experi mental approach involves applying the peptide to the modified polymer surfa ce in an aqueous phosphate-buffered saline solution and allowing the peptid e solution to dry completely under ambient conditions. Subsequently, the MA LDI matrix alpha-cyano-4-hydroxycinnamic acid in methanol and 10% trifluoro acetic acid in water are added to the peptide-coated modified polymer surfa ces. The results of these studies demonstrate that, for the sample preparat ion method employed, increases in the surface peptide binding affinity lead to decreases in the peptide MALDI ion signal.