LILBID-mass spectrometry applied to the mass analysis of RNA polymerase II and an F1Fo-ATP synthase

Mass spectrometry of large macromolecules is still a methodological challenge. We here report on the application of the recently developed LILBID (laser induced liquid bead ion desorption) mass spectrometry by which the biomolecules dissolved in microdroplets are desorbed/ablated by a mid-IR laser into vacuum. Two modes of desorption are possible: an ultrasoft mode at low laser intensity in which a macromolecule is desorbed as integral complex into vacuum and a harsher mode at higher intensity, by which it is dissociated into its covalent subunits. With this method we studied the soluble core polymerase II and a membrane-embedded F1Fo-ATP synthase, solubilized by detergent. For both complexes the complete complex in different charge state is observed at ultrasoft conditions. At elevated laser intensities all 10 subunits could be assigned for the core Pol II. In the case of the ATP synthase under equal conditions all eight subunits appear in the mass spectrum, assigned by a correspondence of the expected theoretical masses of the subunits and the observed ones. In addition the method requires only sample volumes of microliter at micromolar concentration and is tolerant to detergents. Therefore it is a low consumptive method well adapted for the mass analysis of biomolecules of low availability such as membrane molecules.