Charge Mediated Compaction and Rearrangement of Gas-Phase Proteins: A Case Study Considering Two Proteins at Opposing Ends of the Structure-Disorder Continuum

Jacquelyn R Jhingree,Kamila J Pacholarz,Perdita E Barran,Bruno Bellina

doi:10.1007/s13361-017-1692-1

Jacquelyn R Jhingree, Kamila J Pacholarz + Show 2 more

Open Access

https://doi.org/10.1007/s13361-017-1692-1

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Abstract

Charge reduction in the gas phase provides a direct means of manipulating protein charge state, and when coupled to ion mobility mass spectrometry (IM-MS), it is possible to monitor the effect of charge on protein conformation in the absence of solution. Use of the electron transfer reagent 1,3-dicyanobenzene, coupled with IM-MS, allows us to monitor the effect of charge reduction on the conformation of two proteins deliberately chosen from opposite sides of the order to disorder continuum: bovine pancreatic trypsin inhibitor (BPTI) and beta casein. The ordered BPTI presents compact conformers for each of three charge states accompanied by narrow collision cross-section distributions (TWCCSDN2→He). Upon reduction of BPTI, irrespective of precursor charge state, the TWCCSN2→He decreases to a similar distribution as found for the nESI generated ion of identical charge. The behavior of beta casein upon charge reduction is more complex. It presents over a wide charge state range (9–28), and intermediate charge states (13–18) have broad TWCCSDN2→He with multiple conformations, where both compaction and rearrangement are seen. Further, we see that the TWCCSDN2→He of the latter charge states are even affected by the presence of radical anions. Overall, we conclude that the flexible nature of some proteins result in broad conformational distributions comprised of many families, even for single charge states, and the barrier between different states can be easily overcome by an alteration of the net charge.Graphical ᅟ

Highlights

I t has become common practice to infer the populations of protein conformations in the gas phase from the charge state distribution (CSD) it presents following electrospray ionization (ESI) [1, 2]
Our measurements done on a travelling wave instrument in nitrogen buffer gas and converted to helium cross-sections, TWCCSN2→He, using the latter measurements, are centered at 793 Å2 (4+), 800 Å2 (5+), and 900 Å2 (6+), each charge state showing a single sharp peak in the collision cross-section distribution (TWCCSDN2→He)
The narrow TWCCSDN2→He indicate that each charge state of bovine pancreatic trypsin inhibitor (BPTI) is present as a compact conformer in the gas phase

Summary

Introduction

I t has become common practice to infer the populations of protein conformations in the gas phase from the charge state distribution (CSD) it presents following electrospray ionization (ESI) [1, 2]. It is apparent that the CSD can be affected by many. Dedicated to Scott McLuckey: A NoD to his pioneering work on electron mediated D and noD reactions. Electronic supplementary material The online version of this article 1007/s13361-017-1692-1) contains supplementary material, which is available to authorized users. Jhingree et al.: Charge Mediated Conformational Rearrangement process causes proton transfer as the protein desolvates. Higher charge states are susceptible to repulsive Coulombic effects caused by proximal charges, which in turn will affect the structure of the gas-phase ion [9, 14]

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