In order to better understand the interfacial peptide–water interaction, molecular dynamics simulations were made for both water, and an amphipathic peptide, LKα14, adsorbed at hy- drophobic and hydrophilic surfaces. Structural and orientational analyses were performed on both systems. Vibrational mode frequency and oscillator coupling were analyzed for the interfacial water. When looking at the peptide, DFT (density functional theory) ab initio calculations were performed to obtain the non linear vibrational information of the different side chains conformers. Non linear vibrational spectra derived from these results were simulated for both interfacial water and adsorbed peptide. The sum frequency vibrational spectra obtained were correlated to the orientation analysis results. Comparison with literature results were made for both spectral and orientational analysis. The results obtained of water at hydrophilic surfaces lead us to conclude that the absence of signal in the 3700 cm−1 region is due to a cancellation of strongly opposite oriented water layers rather than the absence of O–H oscillators at this vibrational frequency region. The hydrophobic and water-air simulation resulted in surprisingly strong similarity but with difference in the depth of those features. When analyzing the structure of LKα14, results showed that it retained an α helix conformation preference in bulk and adsorbed on surfaces. The hydrophobic surface results lead us to conclude a strong orientation of the leucine side chain towards the interface. Results from the adsorption of LKα14 at the hydrophilic surface proved that the adsorption process takes longer than for the hydrophobic surface. Due to results of water and peptide adsorption, we propose that the time scale of the adsorption process for peptide interaction with hydrophilic surface is partially due to the multiple, strongly orientated, water layers. / Graduate

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Report (M.S.)--Virginia Polytechnic Institute and State University. M.S. 1992. / Vita. Abstract. Includes bibliographical references (leaf 48). Also available via the Internet.

Th.--Sci. phys.--Grenoble 1, 1978. N°: DE 47.

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A Twyman-Green interferometer was used to study the selective transport of ethanol-water mixtures of various concentration across a nonporous homogeneous silicone rubber membrane at 25°C. The instrument developed enabled the measurement of the transient concentration profiles within the boundary layers bathing the membrane. Measurements as close as 5um from the membrane surface were possible. The majority of the reported interferometric studies of liquid/membrane transport have been limited to the observation of the fringes and stop short of a full theoretical analysis. Such analysis is complicated by the optical effects of light deflection and the computational burden involved in the transient solution of the mathematical models required to describe membrane transport. A rigorous treatment of light deflection was developed on the basis of Fermat's principle of least time. The transient numerical solution of the model equations was accomplished by the application of the method of lines. To decouple the equilibrium and kinetic phenomena in membrane transport requires the independent measurement of the sorption isotherm. Traditional techniques for measuring the extent and composition of the imbibed phase involve removing the membrane from the liquid and are therefore limited by the inherent difficulties of obtaining a 'clean' separation. This was circumvented by measuring the excess (relative) sorption isotherm without removing the membrane from the liquid. The data was analysed in terms of Flory-Huggins thermodynamics which was fitted to the measured excess isotherm across the entire concentration range. For a binary mixture, transport across a homogeneous membrane involves two simultaneous fluxes which can be coupled through kinetic and/or equilibrium interactions. A measure of the extent of coupling was obtained by comparing the results from a simplified 'decoupled' flux model with those based on a 'coupled' flux model allowing for equilibrium interactions. Such interactions were found to have little effect on the flux of ethanol but strongly influenced the flux of water across silicone rubber. In particular, coupling through equilibrium interaction was found to be responsible for as much as 75% of the total flux of water. The diffusion coefficients of both ethanol and water in silicone rubber were shown to decrease strongly with alcohol concentration.