Characterization of water mediated hydrophobic and ionic interactions using Raman spectroscopy
Raman spectroscopy can be combined with multivariate curve resolution (MCR), more specifically, self modeling curve resolution (SMCR) to study biologically relevant phenomena, as well as fundamental questions. SMCR can be used to decompose the mixture solution spectrum of two components into the solvent and solute-correlate (SC) spectra. The SC spectrum contains features that arise from the solute, as well as perturbed solvent molecules, and can give insight into the hydration or solvation of the solute. For example, the hydration shell of carbon dioxide was qualitatively compared with the similarly sized hydrophobic group of ethanol. The results reveal that the hydration shells of both molecules are characterized by enhanced tetrahedral order compared to bulk water. However, the tetrahedral order is disrupted by entropically stabilized weak hydrogen bonding between hydration shell water molecules and CO2. Additionally, in contrast to ethanol, the temperature dependence of the CO2 SC spectrum reveals that hydration shell water molecules undergo a structural transformation at low (physiological) temperatures, which suggests some biological importance. Raman-MCR is also used to investigate the water mediated interactions associated with protein systems. Zinc, calcium, and magnesium are essential for hundreds of enzyme pathways, DNA and RNA synthesis, and the stabilization of ATP, and yet their relative binding strength to carboxylate groups is unknown. The order of Zn2+>Ca2+>Mg 2+ was determined by probing the blue shift and intensity decrease of the carboxylate CO stretching band. Additionally, the influence on hydrophobic hydration of charged polar species is investigated. SC spectra of aqueous amphiphilic solutes containing neutral or charged groups were used to elucidate the influence of charge on hydrophobic hydration.The results suggest that neutral head groups have less influence on hydrophobic hydration shell structure than charged groups, and cationic groups stabilize neighboring clathratelike hydrophobic hydration shell structures more significantly than anionic groups, both of which have implications related to the charged patches of proteins.
Ben-Amotz, Purdue University.
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