Differences in Hydration Structure Around Hydrophobic and Hydrophilic Model Peptides Probed by THz Spectroscopy
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We have recorded the THz spectra of the peptides NALA and NAGA as well as the amino acid leucine as model systems for hydrophobic and hydrophilic hydration. The spectra were recorded as a function of temperature and concentration and were analyzed in terms of a principal component analysis approach. NAGA shows positive absorptions with an increasing effective absorption coefficient for increasing concentrations. We conclude that NAGA due to its polar and hydrophilic structure does not have a significant influence on the surrounding water network, but is instead integrated into the water network forming a supramolecular complex. In contrast, for NALA, one hydrogen atom is substituted by a hydrophobic iso-butyl chain. We observe for NALA a decrease in absorption below 1.5 THz and a nonlinearity with a turning point around 0.75 M. Our measurements indicate that the first hydration shell of NALA is still intact at 0.75 M (corresponding to 65 water molecules per NALA). However, for larger concentrations the hydration shells can overlap, which explains the nonlinearity. For leucine, the changes in the spectrum occur at smaller concentrations. This might indicate that leucine exhibits a long-range effect on the solvating water network.
KeywordsHydrophobic Hydration Hydrophilic Hydration Model Peptides Solvation science
We thank J. Savolainen for helpful discussions when setting up the THz—time-domain laser system and G. Schwaab for his support when analysing the data.
This work is part of the Cluster of Excellence Ruhr Explores Solvation (RESOLV) (EXC 1069) funded by the Deutsche Forschungsgemeinschaft.
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