Using solution phase hydrogen/deuterium (H/D) exchange to determine the origin of non-covalent complexes observed by electrospray ionization mass spectrometry: in solution or in vacuo?

Abstract

Electrospray ionization (ESI) is a soft ionization technique that is able to transfer intact ions, as well as solution phase non-covalent complexes into the gas phase. With small molecules that have a high tendency to form hydrogen bonds, the observation of non-covalent complexes by ESI-MS can be the result of a non-specific interaction, due to the nature of the electrospray process. Special precautions and additional steps should be performed to identify the origin of the complexes observed with ESI-MS, and we have utilized solution phase hydrogen/deuterium (H/D) exchange as a method to determine the specificity of the complexes. By comparing the average number of exchanges for the monomer subunits to the average number of exchanges for the complex, one can distinguish if a specific complex is formed in solution. In this paper we have investigated non-covalent complexes of some common chemotherapy agents: paclitaxel, doxorubicin, and etoposide by ESI-MS. By using the solution phase H/D exchange, we were able to identify several specific drug-drug complexes. Thus, solution phase H/D exchange combined with ESI-MS provides for a convenient method in ascertaining the specificity of non-covalent complexes as being formed in solution or in vacuo.

Electron capture dissociation distinguishes a single D-amino acid in a protein and probes the tertiary structure

“…Figure 7 demonstrates the shifts in the charge states of c-and z-fragments in the ECD spectra of native 3ϩ (d-Tyr data were almost identical), which occurred after Gln 5 and Arg 16 . All fragments except z 16 appear in just one charge state, which means that two protons are located on side chains of Gln 5 and Arg 16 and there is no location distribution in 3ϩ ions for these two protons. The surprising absence of protonation at the lysine residue, no shift in the charge state of residues before or after, is presumably because its side chain cannot effectively solvate the proton on a backbone carbonyl.…”

“…The carbonyl of Lys 8 , not involved in the H-network, becomes more accessible for charge solvation because of the general loss of rigidity of the Trp-cage structure, thus providing the higher abundance of z 12 . The protons on the Nterminus and Gln 5 side chain can now be solvated on a broader range of carbonyls, which reduces the relative abundances of z 19 and z 16 ions.…”

“…Additionally, when the protonated Gln side chain is solvated on Ile 4 carbonyl the solvation energy is reduced because of the tension in the bent chain and the coulombic repulsion with the nearby third charge (the coulombic energy at such distance exceeds 1 eV). Thus the recombination energy for neutralization of this configuration is increased compared to Gln side-chain solvation elsewhere, and neutralization occurring with an increased probability leads to a z 16 ϩ⅐ ion and the complementary c 4 ϩЈ fragment. The latter acquires high kinetic energy because of the mass difference with z 16 and energetic recombination, and its ion cloud loses coherence thus being poorly detected in FTMS.…”

“…As FTICR MS has the advantages of high mass resolution and the ability to trap ions for extended time periods (minutes), it is a convenient platform for a multitude of gas-phase ion reactions, including electron capture dissociation (ECD). To date, some of the techniques used to probe gas-phase structures of biomolecules include gas-and solution-phase hydrogen-deuterium exchange [16,17], gas-phase ion mobility [18], proton transfer reactivity experiments [19], and the secondary mass spectrometry (MS/MS) techniques of collisionally activated dissociation (CAD) [20,21], infrared multiphoton dissociation (IRMPD) [22] , ECD [23,24], as well as kinetic energy release [25].…”

Comparison of local anesthetic-cyclodextrin non-covalent complexes using capillary electrophoresis and electrospray ionization mass spectrometry

“…Second, hydrogen/deuterium (H/D) exchange has been used by comparing the average number of exchanges for the individual units comprising the complex with the average number of exchanges for the complex itself. If a complex is formed during the ionization process, the average number of H/D atoms exchanged in a complex should equal the sum of H/D atoms exchanged in each of the components [12]. Third, an on-line separation technique like gel permeation was applied to remove the free guest and the free complexing agent from the specific complex in solution.…”