Distinct Water Species Confined at the Interface of a Phospholipid Membrane

The physics of confined water has stimulated extensive research in recent years, in particular, regarding the role of hydrogen bonding as a significant factor in the observed dynamics. In this work, two-dimensional infrared spectroscopy was employed to investigate the response of the OH moiety of water in phospholipid membrane samples. The results show strong evidence for three distinct hydrogen bonding motifs (${\mathrm{H}}_2\mathrm{O}$ with zero, one, or both OH moieties hydrogen bonded), whose relative proportions at the membrane interface are estimated.

Figure 1

PLPC bilayer and phospholipid molecular structures. The large (orange), medium (blue), and small (red) spheres represent phosphorus, nitrogen, and oxygen atoms, respectively. The water molecules are shown with cyan sticks.

Figure 2

(a) Fourier-transform infrared (FTIR) spectra of water in a weakly hydrated PLPC bilayer (solid black line) and of neat liquid water (dashed red line). (b) Two-dimensional spectra in the OH stretching region, measured at zero delay time and at (c) 700 fs delay. The color bar scale is given in units of optical density $\times {10}^{-3}$ (mOD). The negative and positive spectral contributions are on the right and left sides of the isosbestic line, indicated by the arrows.

Figure 3

Individual horizontal slices of the 2D-IR spectra of Figs. 2b, 2c, taken at different pump-probe delay times. (a) Pump at $3290{\mathrm{cm}}^{-1}$, (b) pump at $3385{\mathrm{cm}}^{-1}$. In the upper panels the FTIR spectrum is given for reference.

Figure 4

Contributions of the single bonded (red dashed line) and of the double bonded plus free water molecules [gray (blue) solid line] to the overall FTIR spectrum (thick black line).

Figure 5

Voight deconvolution of nonlinear spectra (open circles) detected at zero delay time under excitations centered at 3385 (a), 3290 (b), and $3228{\mathrm{cm}}^{-1}$ (c). Solid Black lines, solid gray (blue) lines, and dashed red lines represent diagonal, off-diagonal, and excited state contributions, respectively (see text).