Probing electrostatic interactions and structural changes in highly charged protein polyanions by conformer-selective photoelectron spectroscopy.

We have recorded the first conformer-selective photoelectron spectra of a protein polyanion in the gas-phase. Bovine cytochrome c protein was studied in 8 different negative charge states ranging from 5- to 12-. Electron binding energies were extracted for all charge states and used as a direct probe of intramolecular Coulomb repulsion. Comparison of experimental results with simulations shows that the experimental outcome can be reproduced with a simple electrostatic model. Energetics are consistent with a structural transition from a folded to an unfolded conformational state of the protein as the number of charges increases. Furthermore, the additional ion-mobility data show that the onset of unfolding can be assigned to charge state 6- where three conformers can be distinguished.

Heating a bowl of single-molecule-soup: structure and desorption energetics of water-encapsulated open-cage [60] fullerenoid anions in the gas-phase.

The gas-phase unimolecular decay kinetics of an anionic, open-cage [60] fullerene derivative encapsulating one water molecule is studied by means of black-body IR radiation induced dissociation (BIRD) in the temperature programmable ion trap of a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The primary reaction channel observed is escape of the water molecule from the fullerenoid bowl. The rate constants for this water loss as a function of temperature are evaluated using the Arrhenius equation to yield an activation energy of 104 ± 4 kJ mol(-1). A complementary ion mobility spectrometry study contrasting the water-encapsulated and the empty fullerene cages finds identical collision cross sections to within experimental error-supporting the structural assignment of this gas-phase anion as an endohedral (i.e. encapsulated) species. Both experiments were compared with quantum-chemical computations which well-describe the transition state for water desorption and the concomitant binding and activation energies.

Probing the electronic stability of multiply charged anions: sulfonated pyrene tri- and tetraanions.

The strong intramolecular Coulomb repulsion in multiply charged anions (MCAs) creates a potential barrier that provides dynamic stability to MCAs and allows electronically metastable species to be observed. The 1-hydroxy-3,6,8-pyrene-trisulfonate {[Py(OH)(SO(3))(3)](3-) or HPTS(3-)} was recently observed as a long-lived metastable MCA with a large negative electron binding energy of -0.66 eV. Here we use Penning trap mass spectrometry to monitor the spontaneous decay of HPTS(3-) --> HPTS(*2-) + e(-) and have determined the half-life of HPTS(3-) to be 0.1 s. To explore the limit of electronic metastability, we tried to make the related quadruply charged pyrene-1,3,6,8-tetrasulfonate {[Py(SO(3))(4)](4-)}. However, only its decay product, the triply charged radical anion [Py(SO(3))(4)](*3-), as well as the triply charged ion-pairs [Py(SO(3))(4)H](3-) and [Py(SO(3))(4)Na](3-), was observed, suggesting that the tremendous intramolecular Coulomb repulsion makes the [Py(SO(3))(4)](4-) anion extremely short-lived. Photoelectron spectroscopy data showed that [Py(SO(3))(4)](*3-) is an electronically stable species with electron binding energies of +0.5 eV, whereas [Py(SO(3))(4)H](3-) and [Py(SO(3))(4)Na](3-) possess electron binding energies of 0.0 and -0.1 eV, respectively. Ab initio calculations confirmed the stability of these triply charged species and further predicted a large negative electron binding energy (-2.78 eV) for [Py(SO(3))(4)](4-), consistent with its short lifetime.