Research ArticleBIOPHYSICS

Saturation of charge-induced water alignment at model membrane surfaces

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Science Advances  28 Mar 2018:
Vol. 4, no. 3, eaap7415
DOI: 10.1126/sciadv.aap7415
  • Fig. 1 Monolayer charge-dependent saturation of water alignment.

    (A) Chemical structures of DPTAP and DPPG; (B) SFG spectra of the D2O air interface covered with different mixtures of these two oppositely charged lipids at an area per molecule of 54 Å2; (C) spectral area calculated as a sum of the peak areas of the 2400 and 2500 cm−1 modes as a function of surface charge density. The two data points at −0.3 and +0.3 C/m2 correspond to the signals of the pure DPPG and DPTAP layers. The positive and negative spectral areas stem from the opposite orientation of the interfacial water molecules, which is indicated by the cartoon of the water molecules. The line is a sigmoidal fit to the data points. a.u., arbitrary unit.

  • Fig. 2 Spectroscopic evidence for counterion condensation.

    (A) SFG spectra of 10 μM NaSCN in D2O with varying amount of DPTAP on the surface (dots) along with fits (lines). (B) Spectral area (sum of the absolute values of An) of the SCN and O-D vibrations of the spectra in Fig. 2A plotted versus the charge density of the DPTAP monolayer. The SCN signal intensity remains constant and small up to a charge density of ~0.2 C/m2, where it starts increasing. Horizontal error bars result from the error in the amount of added lipid to the surface, whereas vertical error bars reflect the accuracy of the fits.

  • Fig. 3 MD simulations reveal the mechanism underlying water alignment saturation with increasing surface charge.

    (A) Simulated dipole moment density profile of water along the surface normal (z) for different ratios of positively charged DOTAP and negatively charged POPG lipids. The opposite sign of the profile for the mixtures containing an excess of DOTAP or POPG results from a flip in the orientation of the water molecules. (B) Integrated density of the dipole moment of water versus surface charge density for the different interfacial regions highlighted with corresponding colors in (A) and a calculation of the effective surface charge density according to Manning (black line; see the Supplementary Materials) (10).

  • Fig. 4 Charge distribution and ionic field.

    (A) Average probability of finding Cl counterions (purple lines) in the vicinity of the positively charged nitrogen atom of the TAP head group (green lines). Note that with increasing charge density (increasing line thickness), the average distance between headgroup charge and counterion diminishes. (B) The resulting (ionic) electric field along the z axis as obtained from the integrated distribution of all charged (positively and negatively charged headgroups and counterions) groups.

  • Fig. 5 Counterion distribution and surface potential as a function of nominal surface charge.

    The left panel shows the four points along the sigmoidal curve, for which schematics are depicted, and a legend of the symbols. At near-zero surface charge (A), water is randomly oriented at the interface, and the surface potential is negligible. (B) In the low-charge region, water alignment linearly increases with surface charge with contributions from the Stern layer [within the outer Helmholtz plane (OHP)] and the diffuse layer (DL). (C) With increasing charge density, first, the diffuse layer potential drop ϕDL saturates by counterion condensation at the OHP. (D) For even higher charge densities, the Stern layer potential also saturates due to ions penetrating the lipid surface and rearrangement of the lipid organization.

  • Table 1 Inverse rates (m2/C) of change of the water signal with surface charge density (proportional to the slope at the inflection point) obtained from the sigmoidal fits (see also the Supplementary Materials).

    For the MD simulations, the different interfacial regions (Fig. 3B) are distinguished; the SFG experiment corresponds to that presented in Fig. 1B. The errors represent SDs of the fits to the sigmoids.

    DiffuseSternWhole rangeSFG experiment
    61.0 ± 7.025.2 ± 3.036.1 ± 4.136.1 ± 3.4

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaap7415/DC1

    section S1. Brewster angle microscopy

    section S2. Sigmoidal fits

    section S3. SFG data analysis

    section S4. SFG experiments with different electrolytes in the subphase

    section S5. Calculation of interference between χ(2) and χ(3) terms

    section S6. SFG experiments measured with ppp and pss polarization combination

    section S7. SFG experiments of different lipid mixtures, different layer preparation, and isotopic dilution of the subphase

    section S8. SFG experiments with different lipid surface area per molecule and Brewster angle microscopy measurement

    section S9. SFG experiments of lipid mixtures on pure D2O

    section S10. SFG experiments with NaSCN in the subphase

    section S11. Supplementary information to MD simulations and additional MD simulations

    section S12. Calculation of another orientational metric

    section S13. Calculation of charge condensation

    table S1. Signals frequencies (ω), widths (2Γ), and their corresponding molecular vibrations and the nonresonant amplitudes and the amplitudes of all resonances of the SFG spectra of the D2O-lipid interface with a varying ratio of DPTAP/DPPG at the surface and 10 mM NaCl in the subphase.

    table S2. Peak position (ω), width (2Γ), and assignment to the corresponding molecular vibrations and nonresonant and resonant amplitudes for the SFG spectra of the D2O-DPTAP interface with 10-μm NaSCN in the subphase.

    table S3. Frequency (ω), area (A), phase (ϕ), and width (2Γ) of the χ(2) and χ(3) contributions for the lineshape calculation.

    fig. S1. Electrolyte-dependent saturation of water alignment.

    fig. S2. Effect of interference between χ(2) and χ(3) terms.

    fig. S3. SFG spectra of the lipid mixtures at different polarization combinations.

    fig. S4. Polarization-dependent saturation of water alignment.

    fig. S5. Systematic investigation of the water alignment saturation effect.

    fig. S6. Surface area dependence of the saturation of the water alignment.

    fig. S7. Brewster angle microscopy images of different lipid layers.

    fig. S8. Influence of salt on the water alignment saturation effect.

    fig. S9. SFG spectra of the lipid mixtures with NaSCN in the subphase.

    fig. S10. Counterion concentration–dependent MD simulations.

    fig. S11. Calculation of another orientational metric from MD simulations.

    fig. S12. Critical charge density for charge condensation.

    References (5969)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Brewster angle microscopy
    • section S2. Sigmoidal fits
    • section S3. SFG data analysis
    • section S4. SFG experiments with different electrolytes in the subphase
    • section S5. Calculation of interference between χ(2) and χ(3) terms
    • section S6. SFG experiments measured with ppp and pss polarization combination
    • section S7. SFG experiments of different lipid mixtures, different layer preparation, and isotopic dilution of the subphase
    • section S8. SFG experiments with different lipid surface area per molecule and Brewster angle microscopy measurement
    • section S9. SFG experiments of lipid mixtures on pure D2O
    • section S10. SFG experiments with NaSCN in the subphase
    • section S11. Supplementary information to MD simulations and additional MD simulations
    • section S12. Calculation of another orientational metric
    • section S13. Calculation of charge condensation
    • table S1. Signals frequencies (ω), widths (2Γ), and their corresponding molecular vibrations and the nonresonant amplitudes and the amplitudes of all resonances of the SFG spectra of the D2O-lipid interface with a varying ratio of DPTAP/DPPG at the surface and 10 mM NaCl in the subphase.
    • table S2. Peak position (ω), width (2Γ), and assignment to the corresponding molecular vibrations and nonresonant and resonant amplitudes for the SFG spectra of the D2O-DPTAP interface with 10-μm NaSCN in the subphase.
    • table S3. Frequency (ω), area (A), phase (ϕ), and width (2Γ) of the χ(2) and χ(3) contributions for the lineshape calculation.
    • fig. S1. Electrolyte-dependent saturation of water alignment.
    • fig. S2. Effect of interference between χ(2) and χ(3) terms.
    • fig. S3. SFG spectra of the lipid mixtures at different polarization combinations.
    • fig. S4. Polarization-dependent saturation of water alignment.
    • fig. S5. Systematic investigation of the water alignment saturation effect.
    • fig. S6. Surface area dependence of the saturation of the water alignment.
    • fig. S7. Brewster angle microscopy images of different lipid layers.
    • fig. S8. Influence of salt on the water alignment saturation effect.
    • fig. S9. SFG spectra of the lipid mixtures with NaSCN in the subphase.
    • fig. S10. Counterion concentration–dependent MD simulations.
    • fig. S11. Calculation of another orientational metric from MD simulations.
    • fig. S12. Critical charge density for charge condensation.
    • References (59–69)

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