Knowledge about the structural and biophysical properties of proteins when they are free in solution and/or in complexes with other molecules is essential for understanding the biological processes that proteins regulate. Such knowledge is also important to drug discovery efforts, particularly those focused on the development of therapeutic agents with protein targets. In the last decade a variety of different covalent labeling techniques have been used in combination with mass spectrometry to probe the solution-phase structures and biophysical properties of proteins and protein-ligand complexes. Highlighted here are five different mass spectrometry-based covalent labeling strategies including: continuous hydrogen/deuterium (H/D) exchange labeling, hydroxyl radical-mediated footprinting, SUPREX (stability of unpurified proteins from rates of H/D exchange), PLIMSTEX (protein-ligand interaction by mass spectrometry, titration, and H/D exchange), and SPROX (stability of proteins from rates of oxidation). The basic experimental protocols used in each of the above-cited methods are summarized along with the kind of biophysical information they generate. Also discussed are the relative strengths and weaknesses of the different methods for probing the wide range of conformational states that proteins and proteinligand complexes can adopt when they are in solution. (J Am Soc Mass Spectrom 2009, 20, 1193-1206) © 2009 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry P roteins fold into elaborate three-dimensional structures and, under native solution conditions, they spend a large fraction of time in highly compact structures. However, the solution-phase structures of proteins are not static. Proteins sample a wide range of conformational states in solution. The conformational changes that proteins undergo in solution can be as dramatic as a global unfolding event in which all higher-order structure is lost or as subtle as a breathing motion in which a specific element of secondary structure is partially unfolded in a more local unfolding event (see Figure 1). Knowledge about the structures, kinetics, and thermodynamics involved in the conformational changes that proteins undergo in solution and as they interact with ligands is crucial for understanding the fundamental biological processes in which proteins participate. It is also important to drug discovery efforts, particularly those focused on the development of therapeutic agents with protein targets.Mass spectrometry (MS) has become an increasingly useful analytical tool for acquiring biophysical information about the conformational properties of proteins in solution. A common strategy that has emerged for acquiring such information is to use matrix-assisted laser desorption/ionization (MALDI) or electrospray ionization (ESI) MS to read out the results of solutionphase reactions that introduce covalent modifications into proteins. Two general types of MS-based covalent labeling strategies have proven useful for probing the biophysical p...