The addition of salts into protein aqueous solutions causes changes in protein solubility and stability, whose ability is known to be ordered in the Hofmeister series. We investigated the effects of Hofmeister salts on the picosecond dynamics of water around a lysozyme molecule using terahertz time-domain spectroscopy. The change in the absorption coefficient for 200 mg mL(-1) lysozyme aqueous solution by the addition of salts was found to depend on the salts used, whereas that for pure water was almost independent of salts. From the difference in the salt concentration dependence for various salts, it has been found that chaotropic anions make the dynamics of water around the lysozyme molecule slower, whereas kosmotropic anions make the dynamics faster. The ability of an anion to slow down the water dynamics was found to have the following order: SCN(-) > Cl(-) > H2PO4(-) > NO3(-) ≈ SO4(2-). This result indicates that the effects of anions on the dynamics of water around the lysozyme molecule are the opposite of those for bulk water. This finding agrees with a prediction from a molecular model proposed by Collins [K. D. Collins, Methods, 2004, 34, 300]. The results presented here are compared with the results from preferential interaction studies and the results from sum frequency generation spectroscopy. These discussions have led to the conclusion that the picosecond dynamics of protein hydration water strongly contributes to protein stability, whereas electrostatic interactions between protein molecules contribute to protein solubility.
We demonstrate a broadband (0.3–7 THz) terahertz pulse generation using GaP generation and detection crystals with an 80-MHz-repetition-rate Ti:Sapphire laser. We also use simulations of cascaded terahertz generation to model the observed terahertz gain profiles. The efficient generation of the broadband terahertz pulses is based on optimizing the phase matching between the optical and terahertz pulses, which is achieved by tuning the wavelength of the laser. We observed further improvement in the generation efficiency due to a mismatching of the imaginary part of the complex wavevectors. Together, these effects allow the use of thicker crystals and thus lead to increased terahertz emission. Our results show that optical rectification by GaP can be used to generate broadband terahertz pulses with MHz-repetition-rate lasers up to a dynamic range that is comparable to commonly used sources like ZnTe and photo-conductive antennas.
Terahertz time-domain spectroscopy was used to investigate the salt effect of ammonium sulfate on the dynamics of hydration water of lysozyme in aqueous solution. The absorption coefficient of lysozyme aqueous solutions containing salt was subtracted by that of the water and ammonium sulfate contained in the lysozyme solution. The results revealed that ammonium sulfate increases the absorption coefficient of the hydration water, which indicates that the dynamics of the hydration water becomes faster and/or the number of hydration water molecules decreases with increasing ammonium sulfate concentration.
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