By combining ultrafast mixing and mass spectrometry, researchers can now monitor reactions that occur as fast as one microsecond, Evan R. Williams, a chemistry professor at the University of California, Berkeley, reported today at the American Chemical Society national meeting in San Diego. This reaction speed is significantly faster than the 8 μs that can be attained with conventional mixers.
To achieve these ultrafast reaction times, Williams and graduate student Daniel N. Mortensen use devices called theta-glass emitters: double-barreled glass capillaries with a cross-section that looks like the Greek letter theta. The devices mix two solutions, which combine during an ionization process and form nano-sized droplets that the researchers send into a mass spectrometer for analysis. During tests, one solution contained a protein with a known folding time constant, and the other contained ammonium acetate, which increased the pH of the droplets and induced protein folding when the two solutions were mixed.
From the charge-state distribution of the mass spectrum they obtained, Williams and Mortensen determined the extent of the protein folding reaction. By then combining that information with the protein’s known folding time constant, the researchers calculated the lifetime of the electrospray droplets, a value that they could then apply to monitoring other, unknown reaction kinetics.
For instance, the pair determined how fast a β-hairpin peptide structure folds. With a folding time constant of 2.2 μs, it’s the fastest reaction yet observed using a rapid-mixing technique, Williams said at the meeting during a Division of Analytical Chemistry symposium. Williams and Mortensen recently published this work in the Journal of the American Chemical Society (2016, DOI: 10.1021/jacs.5b13081).
Williams now hopes to use the method to study hydrogen-deuterium (H-D) exchange in protein side chains. Measuring the rate of protein H-D exchange has been limited to studying amide protons in protein backbones because they exchange slowly enough that they can be detected by conventional methods. The new method is fast enough that it might allow H-D exchange, which is already a popular method, to be applied to protein side chains as well as protein backbones, Williams told C&EN.
“Unrivaled insight into fast reactions of biological species will be accessible using dual-channel nanoelectrospray,” Peter Derrick, a mass spectrometrist at the University of Auckland, told C&EN. “I predict that the technique will be taken up widely.” However, he notes, the experiments are not yet “off-the-shelf,” and the technical demands are currently high for most biology labs.