Terahertz spectroscopy identifies the picosecond dynamics of water and biomolecules in aqueous solutions

http://www.physics.ucsb.edu/news/event/635-011312

Speaker:

Dr. Vinh Q. Nguyen - University of California, Santa Barbara

A number of measurement techniques have been used to probe the extent to which ions and biomolecules affect the structure and dynamics of the water molecules directly surrounding them. The majority of these experiments, however, employed infrared spectroscopy, which is sensitive to femtosecond-scale intramolecular dynamics (i.e., bond vibrations). Terahertz spectroscopy, in contrast, is sensitive to the picosecond intermolecular dynamics of water (i.e, molecular rotations associated with hydrogen bond breaking). Here I use a vector network analyzer based terahertz dielectric spectrometer operating over the frequency range from 65 to 720 GHz to probe the dynamics of water and lysozyme protein in aqueous solutions. For the biomolecular solutions, I employ an effective medium approximation to extract the complex dielectric response of the protein in solution. The extracted dielectric response suggests that each protein molecule is surrounded by a tightly held layer of 164 ± 5 water molecules that behave as if they are an integral part of the protein. The size of this hydration shell, the dielectric response of the solvated protein, and the dielectric response of the hydration shell are all independent of protein concentration. For the ionic solutions, my measurements clarify the situation and confirm that the dynamics of water over this regime are best described in terms of three Debye relaxation processes with the characteristic times of 8.56, 1.1 ps and 179 fs (at 25˚C). Remarkably, while the relative strengths of these relaxation modes depend in a systematic way on solute concentration, the relaxation times themselves remain fixed. I will discuss these results in terms of relating the salt concentration dependent strength of the three processes to the dynamics and structure of hydration shells around the solvated ions. Our measurements shed light on the dynamics of hydration shells around solute molecules in a biologically relevant environment.