Probing the surface accessibility of proteins with noncovalent receptors and MALDI mass spectrometry

Matrix-assisted Laser Desorption/Ionization Mass Spectrometry is employed to analyze noncovalent complexes between arginine residues of proteins and sulfonate receptors such as naphthalene disulfonate, anthraquinone disulfonate, pyrene tri- and tetrasulfonate, and the reactive dye sulfonazo III. Using a nonacidic matrix and a layer sample preparation, the desorption/ionization becomes sufficiently gentle to transfer the intact noncovalent assemblies into the gas phase. The MALDI mass spectra show that the number of receptor adducts can be used to derive structural information, since only accessible residues on the exposed surface of the biomolecule are complexed and hence detected. We here present a correlation of the mean number of detected receptors with surface accessibility parameters obtained from computational methods. In a study on lysozyme, this correlation is explored, and successfully applied to other proteins, e.g. ribonuclease-A, myoglobin, and adenylate kinase. It is also applied to the much larger proteins aldolase and albumin; however, the correlation is less obvious in these two examples.