Disordered matter: From Ionic Glasses to Proteins

Sammanfattning: Understanding the glassy state is regarded as one of the ten most important questions in physics in the 21^st century [1]. This thesis deals with structure and dynamics of a few model disordered systems ranging from an ionic glass to proteins.

The structure of an ionic liquid, Ca_0.4K_0.6(NO_0.3)_1.4, has been investigated by neutron diffraction and computer modeling at temperatures around the glass transition. The aim of the study was to quantify structural changes of the model system connected to the transition from a liquid to a solid, and to discuss the results in terms of models for the liquid-glass transition. The results show that thermal expansion is not enough to explain the changes in structure over the transition, other atomic rearrangements also take place.

The vibrational spectra of two glasses, Ca_0.4K_0.6(NO_0.3)_1.4 and Li₂O-2B₂O₃ have been recorded using inelastic neutron scattering. The experiements reveal an intimate relation between the character of the disordered structure (network glass vs. ionic glass) and the nature of the excess vibrational excitations in glasses.

Polymers of varying chain-lengths have been studied by quasi-elastic neutron scattering in an attempt to detect changes in the dynamics as the system changes from being a molecular liquid (short chains) to a complex fluid. The dynamics of the full polymer has furthermore been investigated by quasi-elastic neutron scattering in a wide temperature range. Analysis shows that contributions from segmental dynamics and side group rotations are essentially independent and in quantitative agreement with molecular dynamics simulations.

Proteins are suggested to share certain common thermodynamic characteristics with glasses. A temperature-dependent study of diffusion-like dynamics in slightly hydrated human hemoglobin show a liquid-glass like transition around 200 K. In addition differences in the dynamic structure factor depending on thermal history were detected.

Time resolved resonance Raman spectroscopy has been shown to be a valuable tool for studying dynamics linked to conformational changes in biological molecules. A new laser system suitable for such a task is presented, together with measured excitation profiles for human hemoglobin and a few aromatic acids in solution.