Solvation dynamics of model peptides probed by terahertz spectroscopy. Observation of the onset of collective network motions.

We have studied the solvation of model peptides at low hydration levels by terahertz absorption spectroscopy. We have recorded the concentration-dependent terahertz absorption coefficients of N-acetyl-glycine-amide (NAGA), N-acetyl-glycine-methylamide (NAGMA), N-acetyl-leucine-amide (NALA), N-acetyl-leucine-methylamide (NALMA), and N-acetyl-tryptophan-amide (NATA) in aqueous solution. We find a dramatic decrease in the THz absorption, if the number of water molecules per solute is less than 18-20. This change is taken as a signature for the breakdown of peptide-water network motions, which supports the hypothesis that a minimum number of hydration waters is required to activate these motions. This is well below a monolayer coverage of the model peptides. It is interesting to note that the required hydration level corresponds to the number of water molecules which are required for biological functionality.