Terahertz spectroscopic techniques for the study of proteins in aqueous solutions

Abstract

Liquid water is a very strong absorber in the THz frequency range. We have set-up a unique germanium laser spectrometer consisting of a Ge:Be laser, tunable from 1 to 4 THz, and a sensitive Ge photoconductor detector. The spectrometer uses a measurement scheme alternating sample and reference signal while placed in an environmentally controlled housing for high stability of temperature and humidity. The laser system leads to a very small statistical error in the absolute absorption coefficient (400-500 cm^-1) of less than 0.1% corresponding to 0.3 cm^-1 while systematic errors due to filling of the sample cells become dominant. The high accuracy allows us to systematically investigate the effects of different solvates on water dynamics. Even a single point mutation in a protein can be measured in the THz absorption coefficient in the spectral range from 2 to 3 THz. The system has been recently used to study various solvates in liquid water like sugars and prototype proteins in aqueous buffer solutions in dependence of temperature, pH values, and denaturants. These studies are now augmented by time-resolved measurements using THz time-domain spectroscopy to analyze the kinetics of protein folding. We also discuss other THz sources and detection methods including the investigation of coherent synchrotron radiation at the synchrotron ANKA in Karlsruhe.