Matrix-assisted laser desorption/ionization analysis of non-covalent complexes: Fundamentals and applications

Mass spectrometry is now a well established and powerful method for analysing the structure of biomolecules and more recently, their non-covalent interactions. It is based on two sort ionization techniques, electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), which allow to produce biomolecule ions in gas phase with a very high efficiency for a subsequent analysis in the mass analyser. The high sensitivity and the high precision of modern mass spectrometry were first applied to elucidate the primary structure of proteins in the so-called proteomic area. It is widely used in biological and medical sciences. The new challenge for the mass spectrometrist is to decipher the quaternary structure of biomolecules by preserving into the gas phase the weak interactions that exist between molecules in solution. In this review, we will focus on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for studying the non-covalent associations. Keeping in mind that the weak interactions between partners most be preserved from the solution into the gas phase, we will describe the critical parameters involved in both the desorption/ionization processes and the target preparation including the incorporation of the non-covalent complex into the matrix crystals. Various examples will be presented showing that weak interactions can survive the entire MALDI process allowing the direct detection of intact complexes. However, all these weak interactions cannot be always preserved from the cell to the mass spectrometer. Specific methodologies have been developed to get insights of these interactions. Most of them will be presented in this review.