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Mass Spectrometry

Measuring protein collision cross sections without ion mobility

Collision cross section can be calculated from Orbitrap data

by Celia Henry Arnaud
April 23, 2018 | APPEARED IN VOLUME 96, ISSUE 17

Because the collision cross section (CCS) of a protein is related to its overall shape, the measurement can provide information about various conformations the molecule adopts. A protein’s CCS, which relates its shape to the probability of interacting with gas molecules, is often determined using ion mobility separation in combination with quadrupole-time-of-flight mass spectrometry. Jennifer S. Brodbelt of the University of Texas, Austin, Alexander A. Makarov of Thermo Fisher Scientific, and coworkers now show that they can calculate protein CCS using an Orbitrap mass spectrometer without the need for extra hardware such as an ion mobility cell (Anal. Chem. 2018, DOI: 10.1021/acs.analchem.8b00724). They calculate the CCS from the decay rate of a particular mass-to-charge ratio (m/z) at a particular pressure. Because ions with different CCSs aren’t physically separated, the method provides an average value for all conformations of a given m/z, which means the method is best suited to protein charge states with just a single conformation. The researchers measured the CCS of multiple charge states of three proteins. CCSs obtained with an Orbitrap differed by less than 7% from those measured via ion mobility. Although the new method doesn’t completely replace ion mobility, it might enable more structural biologists to obtain conformational information.

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Comments
Glenn E. Spangler, PhD (May 6, 2018 4:58 PM)
I am not sure about the assessment in this article. My understanding is that the collision cross section is a product of a hard-core cross section times another term that adjusts for the interaction potential between the colliding particles and the effective temperature of the ions. Pressure would enter through the effective temperature which is dependent on E/N. Since the collision cross section measured with a linear ion mobility spectrometer can be influenced by cluster activity of the ions with water, solvent, etc., it might take both techniques to resolve outstanding issues on measuring collision cross section. Under vacuum conditions of a mass spectrometer, peak broadening would be due to mutual diffusion which is related to ion mobility through the Einstein relationship. The comment concerning overlap of multiple conformations should be taken seriously. I applaud continuing discussion on this topic.

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