Oxidative Protein Labeling with Analysis by Mass Spectrometry for the Study of Structure, Folding, and Dynamics

Significance: Analytical approaches that can provide insights into the mechanistic processes underlying protein folding and dynamics are few since the target analytes-high-energy structural intermediates-are short lived and often difficult to distinguish from coexisting structures. Folding "intermediates'' can be populated at equilibrium using weakly denaturing solvents, but it is not clear that these species are identical to those that are transiently populated during folding under "native'' conditions. Oxidative labeling with mass spectrometric analysis is a powerful alternative for structural characterization of proteins and transient protein species based on solvent exposure at specific sites. Recent Advances: Oxidative labeling is increasingly used with exceedingly short (mu s) labeling pulses, both to minimize the occurrence of artifactual structural changes due to the incorporation of label and to detect short-lived species. The recent introduction of facile photolytic approaches for producing reactive oxygen species is an important technological advance that will enable more widespread adoption of the technique. Critical Issues: The most common critique of oxidative labeling data is that even with brief labeling pulses, covalent modification of the protein may cause significant artifactual structural changes. Future Directions: While the oxidative labeling with the analysis by mass spectrometry is mature enough that most basic methodological issues have been addressed, a complete systematic understanding of side chain reactivity in the context of intact proteins is an avenue for future work. Specifically, there remain issues around the impact of primary sequence and side chain interactions on the reactivity of "solvent-exposed'' residues. Due to its analytical power, wide range of applications, and relative ease of implementation, oxidative labeling is an increasingly important technique in the bioanalytical toolbox.