Thermoresponsive MALDI probe surfaces as a tool for protein on-probe purification

Thin film depositions of rf plasma polymerized N-isopropylacrylamide (ppNIPAM) show a phase transition temperature below which the polymer surface is hydrophilic, and protein nonadsorptive, and above which the polymer surface is hydrophobic, and protein-retentive. Results presented here demonstrate that this thermoresponsive plasma polymer can be coated on the surface of a MALDI probe and subsequently used for on-probe biomolecule cleanup. Specifically, a contaminated biomolecule can be applied to the ppNIPAM coated MALDI probe surface at a temperature above the phase transition temperature, washed using solvent also held above the phase transition temperature, and then analyzed by reducing the probe temperature to room temperature before adding the MALDI matrix. With the use of this approach, it is demonstrated that cytochrome c contaminated with 0.3% SDS, which yields only a very weak MALDI ion signal as directly deposited, can be purified on-probe using the thermoresponsive plasma polymer to improve significantly the ion signal. It is further shown that the decontamination of whole cell protein extracts from cyanobacteria is augmented through the use of the ppNIPAM coated MALDI probe.