Electrospray ionization mass spectrometry for the study of non-covalent complexes: an emerging technology

The detection of non‐covalent complexes in the mass range 19000–34000 Da, using electrospray ionization mass spectrometry (ESI‐MS), is reviewed. The examples discussed include (1) a protein–ligand interaction (ras–GDP), (2) an inhibitor–protein–ligand interaction (SCH 54292/SCH 54341‐ras–GDP), (3) a protein–protein interaction (γ‐IFN homodimer) and (4) a protein–metal complex [HCV (1–181)–Zn]. In each case, the ESI‐MS method is capable of releasing the intact non‐covalent complex from its native solution state into the gas phase in the form of multiply‐charge ions. The molecular masses of these complexes were determined with a mass accuracy of better than 0.01%, which is far superior to the traditional methods of sodium dodecyl sulfate polyacrylamide gel electrophoresis and gel permeation chromatography. The method provides the researcher with a quick, reliable and reproducible method for probing difficult biological problems. The key to success in the study of non‐covalent complexes depends on careful understanding and manipulation of ESI source parameters and sample solution conditions; special care must be taken with the source orifice potential and the solution pH and organic co‐solvents must be avoided. This paper also illustrates the usefulness of ESI‐MS for addressing biological problems leading to the discovery of new therapeutics; the approach involves the rapid screening of potential drug candidates, such as weakly bound inhibitors. © 1998 John Wiley & Sons, Ltd.