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978-3-8439-2032-2, Reihe Thermodynamik

Sascha Hempel
Force Field Development for Activity Coefficient Calculations in Aqueous Amino Acid Solutions (Band 14)

137 Seiten, Dissertation Technische Universität Dortmund (2014), Softcover, A5

Zusammenfassung / Abstract

In this work methods from atomistic molecular dynamics simulation were applied to calculate activity coefficients. The overlapping distribution method was verified using simple mixtures and afterwards extended to systems of biological importance, in particular aqueous amino acid solutions. Alcohols of different chain length were studied with respect to their thermodynamical values. Since the conceptually simple overlapping distribution method is easy to parallelize an efficient application to large systems like long chained polymers was possible. The results for polymers as well as alcohols are in good agreement with experimental values. The main part of this work deals with amino acids and the calculation of activity coefficients of aqueous amino acid solutions. Using quantum chemical calculations a new force field was developed specially designed for simulation of aqueous solutions of amino acids. The new force field was verified and the calculated activity coefficients compared to results using available force fields. The optimized force field was used for calculation of 9 different amino acids. The calculated concentration dependent activity coefficients are in very good agreement with correlations from PC-SAFT. For amino acids a trend with respect to the side chains is observed. With higher hydrophobicity of the side chain, the water activity coefficient becomes smaller, whereas for alcohols the results are reversed. For an indepth study of these results the radial distribution functions were calculated. From these a direct connection between the structure of the water molecules surrounding the amino acid and the activity coefficient can be observed. Structure building as well as structure breaking effects can be found. The spatial configuration of the amino acids has a significant impact on the structure of the surrounding solvent molecules. If there is a large distance between the amino- and carboxyterminus of the amino acid or if hydrophobic side chains are present, which can work as a barrier between the two termini, a decrease of the activity coefficient can be expected. Otherwise defects and void spaces can be found in the hydrogen bond network. These defects lead to a weakened water structure and therefore to an increased activity coefficient.