Figure 1.

Nano-electrospray ionization of a hypothetical trimeric heterocomplex composed of two square and one round subunits. (a) Soft ionization results in a single mass which unequivocally determines the stoichiometry of the complex. Supramolecular assembly cannot, however, be determined. (b) By increasing the electric potential in the source of the mass spectrometer, the energy of collisions between the complex and neutral residual gas causes the complex to fall apart. The identity of the subsequently measured subcomplexes reveals that the assembly is square-circle-square. (c) The assembly of complexes can be determined in real time by mixing the subcomponents and loading the reaction into the capillary needle of the mass spectrometer. Unique to mass spectrometry, the separate rates of assembly may be measured. (d) Fändrich et al. have now gone further by developing a temperature controller for the nanospray capillary which allows thermal melts to be conducted on the sample. The products of heat-induced dissociation can be measured directly. As the subcomplexes are separately detected, denaturation curves can be assigned explicitly to particular subcomponents.