Terahertz spectroscopy of l-proline in reverse aqueous micelles

Terahertz Spectroscopic Analysis in Protein Dynamics: Current Status

Proteins play a key role in living organisms. The study of proteins and their dynamics provides information about their functionality, catalysis and potential alterations towards pathological diseases. Several techniques are used for studying protein dynamics, e.g., magnetic resonance, fluorescence imaging techniques, mid-infrared spectroscopy and biochemical assays. Spectroscopic analysis, based on the use of terahertz (THz) radiation with frequencies between 0.1 and 15 THz (3–500 cm−1), was underestimated by the biochemical community. In recent years, however, the potential of THz spectroscopy in the analysis of both simple structures, such as polypeptide molecules, and complex structures, such as protein complexes, has been demonstrated. The THz absorption spectrum provides some information on proteins: for small molecules the THz spectrum is dominated by individual modes related to the presence of hydrogen bonds. For peptides, the spectral information concerns their secondary structure, while for complex proteins such as globular proteins and viral glycoproteins, spectra also provide information on collective modes. In this short review, we discuss the results obtained by THz spectroscopy in the protein dynamics investigations. In particular, we will illustrate advantages and applications of THz spectroscopy, pointing out the complementary information it may provide.

Water Librations in the Hydration Shell of Phospholipids

The hydrophilic phosphate moiety in the headgroup of phospholipids forms strong hydrogen bonds with water molecules in the first hydration layer. Time-domain terahertz spectroscopy in a range from 100 to 1000 cm^-1 reveals the influence of such interactions on rotations of water molecules. We determine librational absorption spectra of water nanopools in phospholipid reverse micelles for a range from w₀ = 2 to 16 waters per phospholipid molecule. A pronounced absorption feature with maximum at 830 cm^-1 is superimposed on a broad absorption band between 300 and 1000 cm^-1. Molecular dynamics simulations of water in the reverse micelles suggest that the feature at 830 cm^-1 arises from water molecules forming one or two strong hydrogen bonds with phosphate groups, while the broad component comes from bulk-like environments. This behavior is markedly different from water interacting with less polar surfaces.