The effects of cation adduction upon the conformation of three-helix bundle protein domains

The ubiquitin-binding, three-helix bundle domains of the proteins ubiquilin 1 (UQ1) and hHR23A both exhibited remarkably high, but discrete, ammonium ion adduction when electrosprayed from aqueous ammoniumacetate. The degree of adduction was highly charge state dependent with, unusually, the lowest charge states (+3 for UQ1 and +4 for hHR23A) showing almost no adducts and the highest charge states (+5 for UQ1 and +6 for hHR23A) exhibiting adduction with two ammonium cations as the most abundant form. As the charge state of protein ions produced by electrospray ionisation (ESI) is related to solvent-accessible surface area we inferred that the ammonium-carrying ions were of a more open conformation than their protonated counterparts. This was confirmed by ESI-travelling wave ion mobility spectrometry-mass spectrometry (TWIMS-MS), which showed that, although the purely protonated ions were compact, their equivalents bearing one or two ammonium adducts exhibited populations of significantly larger collisional cross section (CCS). We postulate that complexation with the ammonium cation may disrupt a key salt bridge (s) in the compact structure. A similar effect is observed with mono-sodium ion adduction, but this is diminished with each additional sodium ion in the complex to produce more compact structures.