Solution Dependence of the Collisional Activation of Ubiquitin [M + 7H]⁷⁺ Ions

Abstract

The solution dependence of gas-phase unfolding for ubiquitin [M+7H]7+ ions has been studied by ion mobility spectrometry-mass spectrometry (IMS-MS). Different acidic water:methanol solutions are used to favor the native (N), more helical (A), or unfolded (U) solution states of ubiquitin. Unfolding of gas-phase ubiquitin ions is achieved by collisional heating and newly formed structures are examined by IMS. With an activation voltage of 100 V, a selected distribution of compact structures unfolds, forming three resolvable elongated states (E1-E3). The relative populations of these elongated structures depend strongly on the solution composition. Activation of compact ions from aqueous solutions known to favor N-state ubiquitin produces mostly the E1 type elongated state, whereas, activation of compact ions from methanol containing solutions that populate A-state ubiquitin favors the E3 elongated state. Presumably, this difference arises because of differences in precursor ion structures emerging from solution. Thus, it appears that information about solution populations can be retained after ionization, selection, and activation to produce the elongated states. These data as well as others are discussed.

How Closely Related Are Conformations of Protein Ions Sampled by IM-MS to Native Solution Structures?

“…There are two potential approaches that can be used to probe folding/unfolding pathways of gas phase ions as a function of E int and or time: (1) IM-MS [35,55,56] and (2) electroncapture dissociation (ECD) combined with hydrogen/ deuterium exchange (HDX) [50,57,58]. Here, MD simulations are used to generate temperature-dependent conformer preferences [59,60].…”

“…Ubiquitin is an excellent model for investigating the effects of E int on ion mobility CCS because it has been extensively studied, both in solution and gas phase as well as by explicit solvent MD simulations [33]. Clemmer et al noted that ubq 6+ and ubq 7+ ions have CCSs that are similar to the native fold [34], and collisional heating of the ions promotes unfolding [35]. Wyttenbach and Bowers also showed that ubiquitin ions formed by ESI from solutions that stabilize the native state were tightly folded solvent-free ions that have CCS that match the sizes of the native state [36].…”

Cryogenic Ion Mobility-Mass Spectrometry: Tracking Ion Structure from Solution to the Gas Phase

“…30 Ubiquitin ions formed under native ESI conditions produced primarily the [M + 7H] 7+ charge state, which has been characterized previously as being compact in aqueous solutions over a large pH range. 31 Unfolding of this compact, native-state (N-state) conformer occurs through a series of intermediate structures, by both collisional activation as well as the presence of organic solvents. 31 While the N-state conformation is made up of α-helix and β-sheet structural elements, partially unfolded structures are elongated, typically take on more charge, and possess substantially more α-helical character.…”

“…31 Unfolding of this compact, native-state (N-state) conformer occurs through a series of intermediate structures, by both collisional activation as well as the presence of organic solvents. 31 While the N-state conformation is made up of α-helix and β-sheet structural elements, partially unfolded structures are elongated, typically take on more charge, and possess substantially more α-helical character. 32 Ubiquitin also contains 12 basic residues, many of which are located near the surface of the folded structure, 33 meaning that when these residues are protonated they should readily solvate by large numbers of water molecules.…”

From Compact to String—The Role of Secondary and Tertiary Structure in Charge-Induced Unzipping of Gas-Phase Proteins

“…For ions with ~ 11 charges and less such a comparison indicates a mostly helical/disordered structure. This agrees well with the predominantly helical structure of the A state (A) but does not support the β-sheet containing native solution structure (N), which is similar in size to the most compact ions found for ions with charges below 7+ [7,24]. A recent study suggests that it is possible to transfer native structural elements into the gas phase under native, buffered solution conditions [25].…”