Long-Range Hydration Effect of Lipid Membrane Studied by Terahertz Time-Domain Spectroscopy

The hydration state of biomolecules is believed to affect their self-assembly. The hydration state of phospholipid bilayers is studied precisely by terahertz spectroscopy, by which water perturbed by a lipid membrane is detected sensitively from the observation of the relaxation dynamics of water molecules in the subpicosecond time scale. Combined with x-ray observation of the lamellar structure of the lipid, a long-range hydration effect on up to 4–5 layers of water is confirmed. Most water molecules in the lamellae fall into the hydration water, and condensation of them is also indicated.

Figure 1

Lamellar repeat distance for different concentrations of lipid ($R=[{\mathrm{H}}_2\mathrm{O}]/[\mathrm{DMPC}]$), obtained by SAXS measurements. The inset shows a magnified plot at around $d=62\AA$. Dashed lines indicate a constant repeat distance, at 62 Å.

Figure 2

Complex dielectric constant of pure water, DMPC solutions ($R=190,113,78,60,35,19$), and DMPC film ($R=4$), obtained by THz TDS measurements (0.4–2.7 THz). The left figure shows the real part of the dielectric constant, while the right one is the imaginary part. The solid lines show the fitting result by using Eq. (1) with fixing $r_h=0.944$.

Figure 3

Number of hydration water molecules per DMPC molecule, $n_h$, indicated in the ${10}^{-13}\mathrm{s}$ scale. The results are obtained by using Eq. (1) with fixing $r_h=0.944$ (${\sigma }_w=1.19$) and ${\alpha }_l=86{\AA }^3$. The dashed line is at $n_h=28.1$, i.e., the averaged value for the fully hydrated condition $R>35$.