Studying noncovalent protein complexes in aqueous solution with laser desorption mass spectrometry
Faculty of Sciences. Chemistry
International journal of mass spectrometry. - Amsterdam
, p. 4957-
The study of noncovalent aggregation with mass spectrometry has been largely the domain of electrospray ionization mass spectrometry (ESI-MS). In contrast, matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) has been applied to this field to a much lesser extent. The main drawback of MALDI-MS is that the sample preparation requires a crystalline matrix. This disrupts the solution environment and often leads to dissociation of noncovalent complexes. A new laser desorption method, developed in our group, promises to circumvent this shortcoming. It is called laser induced liquid beam ionization/desorption mass spectrometry (LILBID-MS). The major advantage of this new method is the use of a liquid beam in vacuum for sample preparation and as target. The beam is directly injected into the mass spectrometer, using the solvent (mostly water) as the natural matrix substance, thus allowing for a softer probe preparation and desorption. In this article we present examples for the application of this new desorption method for detecting noncovalent aggregates of proteins in aqueous solutions. Using ribonuclease S, calmodulin/melittin, and bovine pancreatic trypsin inhibitor as model systems, evidence is given that LILBID-MS is capable of desorbing intact noncovalent complexes into the gas phase. Even water bound into cavities of a protein structure can be detected. In addition, it will be shown that solution parameters (e.g. pH, temperature) have a decisive influence on the mass spectra obtained, thus confirming earlier observations that the ions detected by LILBID-MS are formed in the solution phase and are not gas phase artifacts produced by the detection process.