The effect of salt on the conformations of three model proteins is revealed by variable temperature ion mobility mass spectrometry

Abstract

Three ‘model’ proteins of varying molecular mass: lysozyme, cytochrome C, and BPTI are incubated with sodium iodide and following nano-spray ionisation, their gas-phase conformations are determined using drift tube ion mobility mass spectrometry (DT IM-MS). Ion mobility measurements were carried out on these proteins with helium as the buffer gas at three different drift cell temperatures – ‘ambient’ (300K), ‘cold’ (260K) and ‘hot’ (360K). Significant levels of adducted iodide are observed on all three proteins with the number of iodides correlating to the number of available basic sites. The conformational space occupied by each protein is altered in the presence of salt, this is exhibited by a reduction in the intensities of adducted species for high and low charge states for each protein. This ‘salting in’ reduces the CCS spread for lysozyme and BTPI to converge on the values calculated from the crystal structure data. A change in conformation of all proteins is observed as a function of charge state, which is attributed to coulombically driven unfolding. Thermally induced unfolding (which is observed at both cold and hot temperatures) is minimised in the presence of adducted iodide. This is the first report of ‘cold denaturisation’ for proteins in the gas phase and suggests that this effect maybe intrinsic to the protein fold.