Correlating solvation with conformational pathways of proteins in alcohol-water mixtures: a THz spectroscopic insight

Impact of hydrophobicity on local solvation structures and its connection with the global solubilization thermodynamics of amphiphilic molecules

The relationship between the local solvation structures and global thermodynamics, specifically in the case of amphiphilic molecules, is a complex phenomenon and is not yet fully understood. With the prior knowledge that local solvation structures can impose a significant impact on the overall solvation process, we here combine THz spectroscopic analysis with MD simulations to investigate the impact of the altered hydrophobicity and polarity of amphiphilic solute molecules on the local solvation configurations. We use two water soluble alcohols: ethanol (EtOH) and its fluorinated counterpart, 2,2,2-trifluoroethanol (TFE), as model solutes. Our study is aimed to determine the relative abundance of different hydrogen bonded conformers and to establish a correlation between the spectral signatures (as obtained from THz spectroscopic measurements) and microscopic solute-solvent interactions associated with the local solvation structures (as obtained from MD simulations). Finally, we estimate the possible energetic parameters associated with the alcohol solubilization process. We found that while both the alcohols are completely water soluble, they receive a contrasting solvation energy share in terms of entropy and enthalpy. We understand that these findings are not limited to the specific system studied here but can be broadly extrapolated to other amphiphilic aqueous solutions.

Addition of cholesterol alters the hydration at the surface of model lipids: a spectroscopic investigation

Cholesterol is known to modify the phase behavior of model lipid membranes as it makes phospholipid bilayers more structured. Simulation results have shown that the addition of cholesterol allows more bulk-like water to protrude into phospholipid interfaces. However, such claims have not yet been verified experimentally. We have investigated the alteration in the hydrogen bond network structure of water at the surface of two model phospholipids DOPC and DOPG as cholesterol is added into these using ATR-FTIR spectroscopy in the FIR-THz region. Our measurements and analysis led us to probe the collective H-bond network explicitly at the lipid surface. A detailed principal component analysis of the measured data concludes that the water-water H-bond vibration dynamics gets slower at the lipid surface as compared to bulk water, the effect being more prominent in the case of the charged phospholipid, DOPG. However, as cholesterol is added and more bulk-like water protrudes into the liposome interface, the H-bond vibration gets weaker and correspondingly the dynamics gets accelerated.