Ultrafast chemical biology in the gas phase

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Since the elucidation of the entire human genetic sequence, scientists have been inundated with a wealth of information on protein structure. However, although the availability of a protein structure is valuable, it may not provide any information on the biological function of a specific protein. In photoinduced biomolecular processes, the protein environment of the chromophore plays an essential role in determining the reaction pathway and product distribution of the chromphore. Whilst in solution based reactions the chromophore molecules move freely around within the solvent, in protein based reactions the protein environment provides both a static and dynamical constraint on the motions of the constituent atoms within the chromophore. In order to understand the role of the protein environment on photoinduced biological processes in detail it is necessary to investigate the dynamics of the chromophore in a controlled environment, i.e. in the gas-phase. One of the experimental challenges is generating a stable source of the chromophore in its controlled protein environment. Electrospray ionisation (ESI) has emerged as a very powerful soft ionisation method that has the capability of taking any molecule that exists in solution into the gas phase in its native environment or a well-controlled artificial environment, including proteins as large a several hundred kilodaltons. The dynamics take place on the timescale of nuclear rearrangement, i.e. the femtosecond timescale (1 femtosecond = 1 millionth of a billionth of a second). Therefore, femtosecond laser sources are ideal tools for observing the dynamics of these biological processes in real time. But the value of any time-resolved femtosecond experiment depends on the probe scheme and the challenge is to find a global detection method. Time-resolved photoelectron imaging spectroscopy has recently emerged as an extremely powerful technique for mapping out ultrafast dynamical processes in the gas phase. An initial pump laser pulse excites an electron in the chromophore and a delayed probe laser pulse ionises the molecule. The kinetic energies of the photoelectrons and their angular distributions provide information about the geometry of the chromophore and its electronic wave function at the time of ionisation. Ionisation is a global phenomenon so with this approach there is, in principle, no limitation to the type of system that that can be investigated. We propose to design, construct and optimise a unique instrument comprising an electrospray or nanospray source, a time-of-flight mass spectrometer and photoelectron imaging apparatus for investigating the ultrafast dynamics of real biological systems of several hundreds of kDa. To test the instrument we will investigate the dynamics of the model biological system Bacteriorhodopsin - the molecule responsible for light detection in the process of vision. Although the science will be interesting in its own right as there is still some controversy over the photochemical pathway, the most successful outcome of this project will be to demonstrate the potential of this new instrument as a generic tool for studying fundamental biological processes. In the longer term we would hope to explore the possibilities of using this type of approach to study transient protein-protein interactions, protein-ligand interactions and fundamental protein folding mechanisms. We would then be in a strong position to explore the possibility of mutating the protein environment or shaping the femtosecond light pulses to enable us to identify patterns of dynamic behaviour unique to specific biological systems.