Seminars Archive

Low-energy fluctuations in biological systems

Abstract
Vibrational dynamics at low energy are fundamental in a multitude of biochemical processes ranging from ligand binding to photo-isomerisation and charge transfer. While it is possible to study Raman-like modes with ultra-fast optical tools, only optically-active vibrations at TeraHertz frequencies are far more challenging. This is due to the fact that the native environment for several biologically-relevant macromolecules is water, and water strongly absorbs THz radiation. Here we show that intense single-cycle THz fields generated by titled-front optical rectification can be used to study water-based samples as thick as 0.5 mm. We demonstrate that this approach is able to measure tiny temperature fluctuations, triggered by ultra-fast laser excitations, in water solutions containing gold nanoparticles. As gold nanoparticles are extensively used in the thermal treatment of tumors, this novel experimental approach might allow to map in real time the heat diffusion in vivo thus allowing for optimal cancer therapy. We also show that high-peak THz radiation is very sensitive to the aggregation states of human lysozyme proteins in solution with water, and that a peculiar dipolar coupling is effective between different proteins only above a certain concentration. These results indicate a dielectric coupling between two macro-solutes mediated by a perturbed water network that could be of relevance in chemical recognition processes.