How good is electrospray ionization MS ( ESI - MS ) at detecting weakly bound complexes ?

How good is electrospray ionization MS (ESI-MS) at detecting weakly bound complexes? Richard Griffey and colleagues at Ibis Therapeutics report a spectrum of a complex held together by a single hydrogen bond. They also describe the binding stoichiometry for complexes of multiple ligands with RNA and an example where a ligand binds to two different sites. The key is choosing the right conditions for the ESI-MS experiment. The experiments focus on low-affinity complexes formed with a 27-nucleotide RNA model of the 16S rRNA A-site (16S). As a first step, the researchers adjust the “harshness” of ESI-MS conditions by looking for the spectrum of ammonia-adducted ions of 16S. This spectrum appears by either lowering the capillary–skimmer potential or the desolvation capillary temperature. Binding of the ligand 2-deoxystreptamine (2-DOS) was the basis of several experiments. In one, dissociation constants for ligand-to-complex ratios ranging from 1:1 to 4:1 were determined. However, the chemistry is even more complex, because 2-DOS binds to two distinct sites on 16S. To investigate the stability of these two forms, the researchers turned to collisionally activated dissociation and MS/MS. By varying the relative dissociation energy, the complex was dissociated into the 1:1 complex and the free 16S and 2-DOS ions. MS also revealed multiple different ligands concurrently binding 16S. One particularly complex experiment found the monoand bis-ligand complexes of 2-DOS and 16S, plus a complex with 2-DOS and a diaminotriazole simultaneously bound to 16S. The triazole is believed to be hydrogen bound to the RNA. Finally, this tour de force study reports the relative gas-phase activation energies for the dissociation of a series of ligand–16S complexes. (J. Am. Chem. Soc. 2000, 122, 9933–9938) n e w s