PRESERVATION OF NONCOVALENT ASSOCIATIONS IN ELECTROSPRAY IONIZATION MASS-SPECTROMETRY - MULTIPLY CHARGED POLYPEPTIDE AND PROTEIN DIMERS

The 'softness' of the electrospray ionization (ESI) method provides a direct link between solution chemistry and the inherent gas-phase environment of mass spectrometry. Available results related to the preservation of non-covalent associations into the gas phase after ESI are reviewed. These associations include the possible retention of elements of higher order protein structure, non-covalent polypeptide-heme associations and enzyme complexes. Experimental results are presented showing that non-covalently bound polypeptide and protein dimer ions are relatively common as low level contributions to ESI mass spectra. It is argued that these dimers are reflective of multimeric species in solution since Coulombic barriers preclude dimerization after ESI, although uncertainty remains regarding whether they exist prior to the formation of highly charged droplets. The dissociation of dimers is facile, and for proteins can yield monomers having a broad distribution of charge states. The detection of non-covalently associated dimers requires gentle ESI mass spectrometer interface conditions, yielding relatively low levels of internal excitation. Under such conditions, incomplete molecular ion desolvation can result in experimental artifacts for tandem mass spectrometric experiments. ESI mass spectrometry may have broad potential for the study of noncovalent liquid phase associations.