Monitoring Conformational Changes of Lysozyme–Polyelectrolyte Complexes Using Trapped Ion Mobility-Mass Spectrometry (IM-MS)

Abstract

The conformational changes of lysozyme due to the formation of its complexes with polystyrene sulfonate (PSS) oligomers were monitored at different charge states using trapped ion mobility spectrometry-time-of-flight mass spectrometer (TIMS-TOF-MS). The mixtures of disulfide-intact lysozyme and PSS oligomers were directly infused into the ESI source of the TIMS-TOF-MS instrument. At charge states from 7+ to 12+ in the gas phase, mobilograms obtained from TIMS-TOF-MS analyses were extracted for the protein and each complex ion. Changes occurring in the protein conformation as a result of the binding of PSS oligomers of different chain lengths were evaluated by comparing the obtained integrated peak areas for compact and extended conformers in the extracted ion mobilograms. The increase in the abundance of compact conformers due to the complexation of lysozyme and PSS oligomers was observed more clearly in the 8+ charge state. The number and lengths of PSS chains had little or no effect on the complex conformation in the higher charge states that did not provide compaction in the entire structure. It was observed that the molecular dynamics (MD) simulations of the protein and complexes correlate well with the results obtained by TIMS-TOF-MS. The PSS chains interact with the positively charged sites of the protein and cause compaction in the conformation by changing the locations of the excess charge on the structure in the gas phase that was monitored using TIMS regarding the length and number of the attached polyelectrolyte chain.