Electrospray ionization mass spectrometry of biotin binding to streptavidin.

Stepwise binding of biotin to streptavidin via several intermediates was monitored with electrospray ionization mass spectrometry (ESIMS). Protein ligand interactions that result in conformational changes could be recognized with ESIMS by a mass shift and a change of the average multiple charge state of this protein. In addition, mass spectrometry for the ions in the gas phase revealed a much greater strength of the noncovalent bonds between the streptavidin subunits in the tetrameric complex than between the streptavidin and biotin molecules and remarkable differences in stability for the different charge states of the biotin-streptavidin noncovalent complex.