Photoelectron spectroscopy in a liquid microjet: unravelling the excited state dynamics of photoactive proteins

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The extensive use of efficient light-induced processes in nature is inspiring efforts to exploit similar processes in functional synthetic systems. For example, fluorescent proteins have revolutionalised biological imaging. However, our understanding of the crucial role of the protein surrounding the chromophore in photoactive proteins is still far from complete. The aim of this research is to gain a molecular level understanding of the interactions between photoactive protein chromophores and their environment by a systematic investigation of the electronic structure and excited state dynamics of a series of chromophores in vacuo, in solution and in protein. Photoelectron spectroscopy is a particularly valuable tool for measuring the binding energies of electrons in molecules and femtosecond time-resolved photoelectron spectroscopy has emerged as a very powerful technique for probing the flow of energy in a molecule following photoexcitation. In femtosecond time-resolved photoelectron spectroscopy, a femtosecond laser pulse (pump) excites a molecule and after some delay a second femtosecond laser pulse (probe) ionises the molecule. The kinetic energy and angular distribution of the resulting photoelectrons provides information about the electronic and vibrational states of the molecule at the time of ionisation. Recording a series of photoelectron spectra at different pump-probe delays allows us to record a molecular "movie" of the flow of electronic and vibrational energy in the molecule. Time-resolved photoelectron spectroscopy has proved remarkably successful for investigating electronic structure and dynamics in the gas phase and in solids but aqueous solutions have presented more of a challenge. Recent technical advances in liquid microjet technology have enabled time-resolved photoelectron spectroscopy in solutions to become a real and exciting possibility. We will exploit these developments to create a photoelectron spectroscopy apparatus that is capable of investigating the femtosecond dynamics of chromophores in solution and in their protein environments. In a biological system, the molecular dynamics after photoexcitation are controlled by the molecular and electronic structure of the chromophore and by its interaction with the environment. For example, the environment of the GFP chromophore defines its optical properties: the chromophore is strongly fluorescent inside its barrel-shaped protein, while the fuorescence is lost when the protein is denatured but it returns upon renaturation; the isolated chromophore is non-fluorescent in aqueous solution and it is also non-fluorescent in the gas phase, yet the absorption spectrum of the isolated molecule in the gas phase is remarkably similar to that in the protein. In order to unravel the important role of the environment in defining the optical properties of fluorescent proteins, we will investigate how systematic changes to the electronic and structural properties of the chromophore and mutations to the protein influence binding energies and electronic relaxation following photoexcitation. High-level electronic structure and dynamics calculations will assist the interpretation of the experimental results. Organic chemistry and molecular biology methods will be employed to create the series of chromophores and proteins for systematic evaluation of the influence of electronic, structural, and environmental changes. The multidisciplinary team that has been assembled is ideally suited to tackle this important problem which will have an impact in many areas of science.