Probing the Gas-Phase Interactions of Ions with Molecules of Biological Significance.

Ionic complexes containing biological molecules have been prepared in the gas-phase using electrospray-ionisation, to allow for the investigation of their structures using collision-induced dissociation within a quadrupole ion trap. These complexes have been studied as model systems for characterising the non-covalent interactions of biological molecules, without complication from a bulk solvent or crystal lattice.
The first series of experiments investigated the hydrogen bonding interactions between the nucleobases adenine, cytosine, thymine and uracil with the platinum cyanide dianions [Pt(CN)4]2- and [Pt(CN)6]2-. The [Pt(CN)6Mn]2- clusters fragmented by the sequential dissociation of the neutral nucleobases, while the [Pt(CN)4Mn]2- clusters fragmented by either sequential dissociation of the nucleobases or by proton transfer to form [Pt(CN)4H]-. DFT calculations were performed to support the interpretation that multiple nucleobases bind independently to different sites on the dianion. Comparison of the energies required for fragmentation of the various complexes allowed for the relative binding energies of the different nucleobases to the dianions to be determined.
Next, complexes of arginine with deprotonated monocarboxylic acids, H3C(CH2)nCO2·Arg-, and dicarboxylic acids HO2C(CH2)nCO2-, were investigated to probe the nature of the interactions as a function of anion size. For the dicarboxylic acid clusters, the chain length was found to have a significant effect on the fragmentation energy of the complexes, with the n=9 and 10 systems fragmenting at significantly lower energies than the corresponding shorter chain analogues. Molecular mechanics calculations indicate that the dicarboxylic acid complexes transition to a ring-structure as the acid chain length increases.
The final series of experiments investigated subnanometre clusters using electrospray-ionisation mass spectrometry, synthesised in-house using previously published methods. Systems consisting of tryptophan and silver nitrate, histidine and chloroauric acid, glutathione and gold nanoparticles, and mercaptosuccinic acid and silver nitrate were investigated, however, no suitable system was found. Suggestions for further work in this area are also described.