The urge to develop molecular models of solute-solvent interactions has been increasing over the last few years for a number of reasons. Firstly, the development of time-resolved spectroscopic techniques made it possible to elucidate the contributions of solvent translational, reorientational, conformational and collective modes to solution state relaxation processes. Secondly, there is an increasing realisation that the solute-solvent effects play crucial roles in macromolecular and biological systems.

Instrumentation for variable pressure/temperature control (left) and Brillouin spectra showing the large effect of compression on the mode frequecncies on a liquid sample (right).

Pressure is recognised as being a particularly useful variable for studying the dynamics and interactions of molecules in condensed phases because it directly reduces intermolecular distances and exposes the molecules to increasingly anharmonic portions of the intermolecular potential at constant thermal energy. However, due to considerable instrumentation complexity, small sample volumes, low signals, contaminant scattering, high background, restrictive geometries, expense, and residual depolarisation effects, it remains a greatly under-used variable in dynamical investigations. We propose to develop and implement novel instrumentation to explore solute-solvent interactions ranging from small prototype systems up to biologically-relevant macromolecules. Experimental methodologies involve combinations of Raman, Brillouin and time-resolved fluorescence spectroscopies. The project also involves significant input from classical and ab initio computer simulation to assist the interpretation of experimental data.

Specific projects within COSMIC include:

Exotic properties of supercritical solutions: Supercritical fluids (SCFs) form when a gas or liquid is heated and compressed above a critical temperature and pressure. In this extreme regime, changing the pressure or temperature tunes the density between gas-like and liquid-like extremes. This tuneability makes SCFs attractive for a number of industrial applications. Near the critical point, SCFs show very unusual behaviour related to divergences in thermodynamic variables. The aim of this project is to explore SCFs and solutions (using combinations of time and frequency-domain optical spectroscopies as well as computer simulation) and thereby to develop molecular-level models of their properties.

Dynamics of fast solvent relaxation: The fluorescence properties of many fluorophores are profoundly influenced by the solvent environment. Solvent relaxation around the new electron distribution of the excited fluorophore leads to shifts of the fluorescence spectrum relative to the excitation (absorption) spectrum. Measurement of these shifts on the picosecond timescale will be used in this project to probe the dynamics of the solvent relaxation process.

Protein folding procceses: There is increasing interest in the use of hydrostatic pressure to study the factors that determine the structure of proteins, because the effects of pressure are more amenable to interpretation than those of temperature. Application of pressure affects internal interactions solely by changes in volume, whereas a change in temperature changes both the energy content and the volume of the system. The project is aimed at understanding developing models for the folding process under compression.

Collaborators: Centre for Science at Extreme Conditions (University of Edinburgh), University of Strathclyde, Glasgow University, Rutherford Appleton Laboratory, University College London, Edinburgh Instruments.