Anhydrous protein ions.

The recent development of electrospray ionization (ESI)1 and matrix-assisted laser desorption ionization (MALDI)2 sources for creation of macromolecular ions has brought about a renaissance in the field of mass spectrometry (MS). It is now common to determine molecular weights of large molecules such as proteins to within a few daltons (or less)! MS-based methods for sequencing are proliferating and replacing (or complementing) traditional biochemical methods for many applications. However, analysis of covalent structure is only part of the story. A number of studies have investigated the use of MS strategies for examining noncovalent biomolecular structure. Mounting evidence suggests that some conformational properties of biomolecules in solution are preserved during the ionization process and persist over the transient time that ions exist in mass spectrometers. The persistence of solution-like structure in the gas phase provides opportunities to examine conformation and folding as well as binding interactions (e.g., enzyme-substrate, receptor-ligand interactions) in an entirely new environment. In the past 5 years, a new field involving structural studies of biomolecular ions has emerged. At first glance, the relevance of biomolecular ion structure in the gas phase to studies of functional molecules in solution may seem far removed, perhaps not unlike questions about the relevance of structural measurements of protein crystals 20-30 years ago. Studies of biomolecular ions in the gas phase provide information about the nature of conformation in the absence of solvent; thus, the roles of intramolecular interactions and solvent can be discerned by comparison of similarities between gasand solutionphase structure. Additionally, the utility of MS strategies for sampling noncovalent binding between molecules in solution hinge on a fragile balance of interactions associated with the assembled pair as the ions travel through the mass analyzer. As these strategies become widely used, the differentiation 3037 Chem. Rev. 1999, 99, 3037−3079