Probing Local Electrostatics of Glycine in Aqueous Solution by THz Spectroscopy

Abstract

Based upon precise terahertz (THz) measurements of the solvated amino acid glycine and accompanying ab‐initio molecular‐dynamics simulations, we show that the N‐C‐C‐O open/close mode at 315 cm−1 serves as a sensitive, label‐free probe for the local protonation of the amide group. Experimentally, we can show that this holds not only for glycine but also for diglycine and valine. The approach is more general, since the changes due to protonation result in intensity changes which can be probed by THz time domain (0–50 cm−1) as well as by precise THz‐FT spectroscopy (50–400 cm−1). A detailed analysis allows us to directly correlate the titration spectra with pKa values. This demonstrates the potential of THz spectroscopy to probe the charge state of a natural amino acid in water in a label‐free manner.

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain

“…We want to mention that a similar absorption increase in the THz spectra at frequencies > 400 cm À 1 has been previously reported for hydration water molecules as a result of strong interactions with ions or with polar groups of biomolecules. [43][44][45] In order to quantify this effect, we take the slope of the absorption increase in the 450-600 cm À 1 region and we compare it for different alcohols in Figure 3C. Remarkably, a similar slope is measured for EtOH, PrOH and BuOH, which are all primary alcohols (i.e.…”

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain

“…We want to mention that a similar absorption increase in the THz spectra at frequencies > 400 cm À 1 has been previously reported for hydration water molecules as a result of strong interactions with ions or with polar groups of biomolecules. [43][44][45] In order to quantify this effect, we take the slope of the absorption increase in 450-600 cm À 1 region and we compare it for different alcohols in Figure 3C. Remarkably, a similar slope is measured for EtOH, PrOH and BuOH, which are all primary alcohols (i.e.…”