Probing Local Electrostatics of Glycine in Aqueous Solution by THz Spectroscopy

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain

Hydration free energies are dictated by a subtle balance of hydrophobic and hydrophilic interactions. We present here a spectroscopic approach, which gives direct access to the two main contributions: Using THz-spectroscopy to probe the frequency range of the intermolecular stretch (150-200 cm^-1 ) and the hindered rotations (450-600 cm^-1 ), the local contributions due to cavity formation and hydrophilic interactions can be traced back. We show that via THz calorimetry these fingerprints can be correlated 1 : 1 with the group specific solvation entropy and enthalpy. This allows to deduce separately the hydrophobic (i.e. cavity formation) and hydrophilic contributions to thermodynamics, as shown for hydrated alcohols as a case study. Accompanying molecular dynamics simulations quantitatively support our experimental results. In the future our approach will allow to dissect hydration contributions in inhomogeneous mixtures and under non-equilibrium conditions.

The inner hydration in surfactant/cholesterol vesicles differs from the outer one: a spectroscopic investigation

Vesicles contain two aqueous regions: inner core and outer-to-bulk . It has remained an open question whether hydration behaviour in the inner core differs from the outer-to-bulk region, mostly owning to the inability of the conventional spectroscopic techniques to deconvolute the contribution from these two regions. We, using THz-FTIR spectroscopy (1.5-13.5 THz) experimentally probe the inner hydration of three differently charged surfactant/cholesterol vesicles composed of SDS, CTAB and Brij 30. Both dynamic light scattering (DLS) and atomic force microscopy (AFM) measurements affirm the transition from micelles to vesicles as cholesterol is added into surfactant solutions. FTIR measurements show that hydration behaviour changes significantly as micelles are converted into vesicles, the change been exclusively caused due to the formation of an inner core . Our measurements on the hydrogen bond stretch and librational motion of the inner hydration show distinct features compared to the overall hydration, which in turn is found to be surfactant type and cholesterol concentration dependent.