Structure and Orientation of Water at Charged Lipid Monolayer/Water Interfaces Probed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

Cell membrane/water interfaces provide a unique environment for many biochemical reactions, and associated interfacial water is an integral part of such reactions. A molecular level understanding of the structure and orientation of water at lipid/water interfaces is required to realize the complex chemistry at biointerfaces. Here we report the heterodyne-detected vibrational sum frequency generation (HD-VSFG) studies of lipid monolayer/water interfaces. At charged lipid/water interfaces, the orientation of interfacial water is governed by the net charge on the lipid headgroup; at an anionic lipid/water interface, water is in the hydrogen-up orientation, and at the cationic lipid/water interface, water is in the hydrogen-down orientation. At the cationic and anionic lipid/water interfaces, interfacial water has comparable hydrogen bond strength, and it is analogous to the bulk water.

Femtosecond Hydrogen Bond Dynamics of Bulk‐like and Bound Water at Positively and Negatively Charged Lipid Interfaces Revealed by 2D HD‐VSFG Spectroscopy

“…Such a bimodal temporal behavior is characteristic of the frequency correlation loss of bulk HOD in D 2 O reported by Tokmakoff and co‐workers with 2D IR, and the faster (60 and 130 fs) and slower (1.4 ps) dynamics of bulk HOD in D 2 O were attributed to hydrogen bond fluctuation and rearrangement, respectively 12. The present 2D HD‐VSFG experiment strongly suggests that the spectral diffusion of HOD at the DPTAP interface occurs in a similar manner, not only because of similar temporal behavior but also because the interfacial water at the DPTAP interface has almost the same hydrogen bond strength as bulk water 6c. The difference in the time constant of the slow component, when comparing the interface and the bulk, may be due to the existence of the charge at the interface and/or the difference in the degree of the isotopic dilution.…”

“…The VSFG technique has been applied to lipid interfaces to investigate the properties of interfacial water 6. In particular, heterodyne‐detected (HD‐) VSFG spectroscopy enables measurement of the imaginary part of χ (2) (Imχ (2) ), which provides interface‐selective vibrational spectra that can be directly compared to IR and Raman spectra 6c,6d, 7. Furthermore, the sign of the Imχ (2) signal gives information about the orientation of interfacial molecules 6c,6d, 8…”

“…Since the ratio of each water species is H 2 O:HOD:D 2 O=1:8:16, HOD is the predominant species that gives rise to the signal in the OH stretch region. Because the Fermi resonance and other couplings are suppressed by the isotopic dilution, the Imχ (2) spectra only exhibit one broad band, which is attributable to the OH stretch of hydrogen‐bonded water below the lipid head group 6c. The sign of Imχ (2) is negative for the positively charged DPTAP interface but positive for the negatively charged DPPG interface, indicating that water has hydrogen (H‐) down orientation at the DPTAP interface and H‐up orientation at the DPPG interface.…”

“…At a glance, interfacial water at both lipid interfaces would look “bulk‐like” because the peak positions of the OH stretch bands are nearly equal to that of the IR spectrum of HOD in bulk D 2 O 6c. However, 2D HD‐VSFG reveals that interfacial water at the DPTAP and DPPG interfaces is completely different from a dynamical viewpoint.…”

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces

“…These include techniques to probe surface properties such as wetting and adhesion, methods to determine structure, e.g. X-ray [31][32][33] and neutron reflectivity [34][35][36][37], several types of microscopy and various spectroscopic methods [38][39][40][41][42][43]. For the purposes of this review paper, we shall concentrate on the vibrational spectroscopy methods, whose use will be exemplified for two types of systems associated with biointerfaces.…”