Our group’s main research objective is to better understand the structure and conformations of peptides and proteins and how these are affected by solvent interaction.
Towards this end, we have developed a spectroscopy interface for a Quadrupole Ion Trap (QIT) mass spectrometer. This enables the use of laser spectroscopy to probe of the conformation of trapped ions.
Gaseous ions are formed using electrospray ionization (ESI). We modified a commercial QIT (Esquire3000+ from Bruker Daltonics) by drilling 3 channels into the ring electrode: two for the entry and exit of the laser beam and one for collection of the emitted fluorescence.
With this setup, we can study the photophysics of the trapped ions. Upon absorption of a photon, several deactivation channels are available to trapped molecular ions:
- Emission of fluorescence
- Photo-detachment of an electron (for anions)
The mass spectrometer/optical spectroscopy set-up we have developed allows monitoring of each of these possible outcomes from photo-excitation of the mass-selected ions.
We have used this setup to measure the gas-phase spectroscopic properties of several well known ionic dyes, including fluorescein and various rhodamines. While these dyes are well-studied in solution, in many cases little was known about their intrinsic (gas-phase) properties. This is probably due to difficulties with production of gas-phase ions of this size. Thermal vaporization (heating) produces neutral gaseous species of small molecules, but larger molecules thermally degrade prior to vapourization. We use electrospray ionization (ESI) to produce gaseous ions. ESI is a common ion source on many commercial mass spectrometers. It is a soft ionization source which can transfer molecules and their complexes intact into gas phase. By harnessing techniques from mass spectrometry such as ESI, we are able to measure the spectroscopic properties of molecular ions in our modified instrument.
Recently, we used our setup to measure Fluorescence Resonance Energy Transfer (FRET) in the gas phase. This work is the first showing FRET efficiency measurements in the gas phase, with full spectral and temporal characterization.
One advantage of measuring FRET from ions in a trapping mass spectrometer is the ability to select the ions that we want to measure fluorescence from. The top graph below shows the electrospray mass spectrum of our initial solution. Many peaks are visible, illustrating the fact that the solution is a complex mixture, containing many different species. Built into most commercial mass spectrometers, is the ability to isolate one particular ion (mass-to-charge ratio) by ejecting all others. This feature allows us to individual probe the peptides labelled with only the Donor dye, only the Acceptor or both dyes attached (lower graph). It is then also possible to examine different charge states of a molecule if multiple charge states are produced in the electrospray.
 Q. Bian, M.W. Forbes, F.O. Talbot, and R.A. Jockusch, Phys.Chem.Chem.Phys. 12, 2590-2598 (2010).
 McQueen, P.D., Sagoo, S., Yao, H. & Jockusch, R.A. Angew. Chem. Int. Ed., DOI: 10.1002/anie.201004366 (2010).
 M.W. Forbes & R.A. Jockusch, J. Am. Soc. Mass. Spectrom., Volume 22, Number 1, 93-109, DOI: 10.1007/s13361-010-0017-4
 F.O. Talbot, A. Rullo, H. Yao, and R.A. Jockusch, J. Am. Chem. Soc., (2010) DOI: 10.1021/ja1067405.