Low-frequency dynamics in complex liquid systems

Abstract

Intermolecular interactions are the forces between molecules that define the physical states of matter. Molecules attract at long distances, which is why solids and liquids exist and repel each other at small separations, which is why their densities are finite [1]. Intermolecular interactions are important because they influence the physical, chemical, and biological properties of molecules, and in their absence, all matter would exist only in the gaseous phase, and life as we know it would cease to exist. The study of intermolecular interaction is vital in understanding the condensed phase dynamics, like properties of liquids and liquid mixtures, solvation, electrochemistry, structure-function correlation of bio-molecules, aggregation of molecules and many more. Ultrafast spectroscopy allows intermolecular interactions and dynamics of complex aqueous systems to be observed directly in real-time at femtosecond-picosecond time scales. This study utilizes optical Kerr effect spectroscopy (OKE) and THz time-domain spectroscopy (THz-TDS) to investigate the benzene-methanol azeotropic system, urea-water dynamics, and denaturation mechanism of different proteins using urea. THz-TDS and OKE spectroscopy are both powerful techniques to probe the rotational and vibrational dynamics in the GHz-THz regime. In the case of THz-TDS, the signal obtained is a response of the dipole moment operator, and in the case of OKE, it is that of the polarizability operator [2]. Application of these two complementary spectroscopy techniques thus provides the complete information on dynamics of the system. One of the interesting ways in which intermolecular interactions manifests, is the formation of an azeotrope. While the physical interpretation of why azeotropes occur can be explained in terms of non-ideality, azeotropes remain a mystery at a molecular level. Ultrafast spectroscopic studies on the azeotrope and other mixtures of benzene and methanol throughout the composition range not only gives us information about the relaxation and librational dynamics at ultrafast timescales but provides experimental proof that the formation of an azeotrope is a temperature-driven process. Water, especially interfacial water, has a significant effect on the protein’s internal structure and dynamics. OKE spectral density can resolve protein water dynamics from 100GHz to up to 10THz, including relaxation and librational dynamics of bulk water and water-protein interactions. Urea is a chaotropic agent and a well-known denaturant for proteins. The molecular picture of the interaction of urea with the water hydrogen bond network and the chemical denaturation of the proteins is still ambiguous. Analysis of the spectral densities of urea solutions obtained from OKE spectroscopy shows the presence of two types of environments depending on the molarity of the solution. Using OKE spectroscopy, we have investigated the effect urea has on the water hydrogen bond network and mapped out the structural changes occurring in three different proteins on the addition of urea.

References: 1) Buckingham, A.D. and Utting, B.D., Intermolecular Forces. Annual Review of Physical Chemistry, 1970, 21, 287-316. 2) Giraud, G. and Wynne, K., A comparison of the low-frequency vibrational spectra of liquids obtained through infrared and Raman spectroscopes. Journal of Chemical Physics. 2003, 119, 11753-11764.