Effects of aprotic solvents on the stability of metal-free superoxide dismutase probed by native electrospray ionization-ion mobility-mass spectrometry.

Considering aprotic solvents are often used as cosolvents in investigating the interactions between small molecules and proteins, we assessed the effects of five aprotic solvents represented by dimethyl-formamide (DMF) on the structure stabilities of metal-free SOD1 (apo-SOD1) by native electrospray ionization-ion mobility-mass spectrometry (ESI-IM-MS). These aprotic solvents include DMF, 1,3-dimethyl-2-imidazolidinone (DMI), dimethyl-sulfoxide (DMSO), acetonitrile (ACN) and tetrahydrofuran (THF). Results indicated that DMI, DMSO and DMF at low percentage concentration could reduce the average charge and the dimer dissociation of apo-SOD1. By contrast, ACN and THF at low concentration have no similar effect. DMF was selected as a representative solvent to further investigate the detailed effects on the structure stability of apo-SOD1 by using collision induced dissociation and unfolding. The results reveal that the addition of minimal DMF to an aqueous protein solution can protect against the unfolding and dissociation of dimer, even under destabilizing conditions (such as low pH or high cone voltage). When the different percentage concentrations of DMF were added, the average collision cross-section of apo-SOD1 showed that apo-SOD1 became compacted when the DMF concentration increased from 0% to 1% and eventually started extending when increased from 1% to 20%. The results indicated that DMF has similar effects to DMSO in native MS and it can also be used as a cosolvent besides DMSO in investigating the stabilities of proteins and the interactions between small molecules and proteins.