Mass spectrometric noncovalent probing of amino acids in peptides and proteins

Mass spectrometric methods are becoming available that probe structural or topological aspects of peptides and proteins. However, most of them require extensive chemical modification or covalent crosslinking of the biomolecules. We present a novel strategy based on the formation of noncovalent complexes between basic acidic amino acid residues and selective ligands. Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) are employed to analyze the noncovalent assemblies. Different basic sites are probed with sulfonic acid derivatives. Naphthalene-disulfonic acid selectively binds to arginine residues and free amino termini; here we provide a chemical explanation for this behavior. Acidic sites are probed with molecules carrying guanidinium groups. However, only cysteic acid residues can so far be detected in this fashion; identification of glutamic and aspartic acid still awaits further investigation.