Fig. 5.

Conditions for measuring the association constant of a binary protein complex. To use mass spectra as a read out in measuring the association constant of a protein complex with the titration method, at least three conditions have to be fulfilled. The concentrations of the components must remain below the saturation level so that the ion intensities reflect the molecular concentration in solution. The primary droplets have to be small so that all droplets generated evaporate fast and no in-droplet complexes are formed on the basis of higher concentrations. Finally, the desolvation of molecules in the interface region of mass spectrometers has to be gentle so that correctly formed complexes do not dissociate.