Studying noncovalent protein complexes in aqueous solution with laser desorption mass spectrometry

The study of noncovalent aggregation with mass spectrometry has been largel y the domain of electrospray ionization mass spectrometry (ESI-MS). In cont rast, matrix-assisted laser desorption ionization mass spectrometry (MALDI- MS) has been applied to this field to a much lesser extent. The main drawba ck 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 gro up, promises to circumvent this shortcoming. It is called laser induced liq uid beam ionization/desorption mass spectrometry (LILBID-MS). The major adv antage 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 spec trometer, using the solvent (mostly water) as the natural matrix substance, thus allowing for a softer probe preparation and desorption. In this artic le we present examples for the application of this new desorption method fo r detecting noncovalent aggregates of proteins in aqueous solutions. Using ribonuclease S, calmodulin/melittin, and bovine pancreatic trypsin inhibito r as model systems, evidence is given that LILBID-MS is capable of desorbin g intact noncovalent complexes into the gas phase. Even water bound into ca vities of a protein structure can be detected. In addition, it will be show n that solution parameters (e.g. pH, temperature) have a decisive influence on the mass spectra obtained, thus confirming earlier observations that th e ions detected by LILBID-MS are formed in the solution phase and are not g as phase artifacts produced by the detection process. (C) 2000 Elsevier Sci ence B.V.