The study of noncovalent interactions by mass spectrometry has become an active field of research in recent years. The role of the different noncovalent intermolecular forces is not yet fully understood since they tend to be modulated upon transfer into the gas phase. The hydrophobic effect, which plays a major role in protein folding, adhesion of lipid bilayers, etc., is absent in the gas phase. Here, noncovalent complexes with different types of interaction forces were investigated by mass spectrometry and compared with the complex present in solution. Creatine kinase (CK), glutathione S-transferase (GST), ribonuclease S (RNase S), and leucine zipper (LZ), which have dissociation constants in the nM range, were studied by native nanoelectrospray mass spectrometry (nanoESI-MS) and matrix-assisted laser desorption/ ionization mass spectrometry (MALDI-MS) combined with chemical cross-linking (XL). Complexes interacting with hydrogen bonds survived the transfer into gas phase intact and were observed by nanoESI-MS. Complexes that are bound largely by the hydrophobic effect in solution were not detected or only at very low intensity. Complexes with mixed polar and hydrophobic interactions were detected by nanoESI-MS, most likely due to the contribution from polar interactions. All noncovalent complexes could easily be studied by XL MALDI-MS, which demonstrates that the noncovalently bound complexes are conserved, and a real "snap-shot" of the situation in solution can be obtained. (J Am Soc Mass Spectrom 2010, 21, 286 -289) © 2010 American Society for Mass Spectrometry E lectrospray is an exceptionally soft ionization technique, which has allowed the observation of numerous noncovalent complexes, including protein-protein, protein-small molecule, and protein-DNA complexes [1,2]. A major question is still whether the information obtained from gas-phase results is representative of the species present in solution. The term "noncovalent bonding" in biochemistry generally summarizes three types of intermolecular forces: electrostatic interactions (e.g., salt bridges), dipolar interactions (e.g., hydrogen bonds), and van der Waals interactions (e.g., hydrophobic interactions) [3]. In addition, protein-water interactions often play a major role in complex stabilization. In solution, hydrophobic interactions are very weak compared with the hydrogen bonds between water molecules. Hence, it is not the hydrophobic forces that are the actual reason for the interaction of nonpolar binding partners, but rather the surrounding water molecules that "force" hydrophobic binding partners to aggregate. This solvent-driven process is called the hydrophobic effect, and plays a major role in protein folding, adhesion of lipid bilayers, partitioning effects of drugs and metabolites, and many other aggregation phenomena in chemistry and biology [4][5][6]. The hydrophobic effect is defined by the thermodynamics of mixing hydrophobic compounds with water, and is dominated by large, entropic factors [4,7]. Obviously, in the absence of a hydr...