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

Abstract

Interfacial water in the vicinity of lipids plays an important role in many biological processes, such as drug delivery, ion transportation, and lipid fusion. Hence, molecular‐level elucidation of the properties of water at lipid interfaces is of the utmost importance. We report the two‐dimensional heterodyne‐detected vibrational sum frequency generation (2D HD‐VSFG) study of the OH stretch of HOD at charged lipid interfaces, which shows that the hydrogen bond dynamics of interfacial water differ drastically, depending on the lipids. The data indicate that the spectral diffusion of the OH stretch at a positively charged lipid interface is dominated by the ultrafast (<∼100 fs) component, followed by the minor sub‐picosecond slow dynamics, while the dynamics at a negatively charged lipid interface exhibit sub‐picosecond dynamics almost exclusively, implying that fast hydrogen bond fluctuation is prohibited. These results reveal that the ultrafast hydrogen bond dynamics at the positively charged lipid–water interface are attributable to the bulk‐like property of interfacial water, whereas the slow dynamics at the negatively charged lipid interface are due to bound water, which is hydrogen‐bonded to the hydrophilic head group.

Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two‐Dimensional Heterodyne‐Detected VSFG Spectroscopy

“…These fast and slow components are assigned to hydrogen‐bond fluctuations and hydrogen‐bond rearrangements, respectively . Because the ultrafast hydrogen‐bond fluctuations (≈60 fs) cannot be directly detected with the time resolution (≈200 fs) of the present 2D HD‐VSFG experiments, this fast fluctuation is expected to result in only a small initial CLS value . Therefore, the small initial CLS value indicates a large contribution from the fast hydrogen‐bond fluctuations whereas a large initial CLS value implies its suppression.…”

“…In our previous 2D HD‐VSFG study on the cationic DPTAP (1,2‐dipalmitoyl‐3‐trimethyl ammonium propane) and anionic DPPG (1,2‐dipalmitoyl‐ sn ‐glycero‐3‐phosphorylglycerol) lipid/HOD–D 2 O interfaces, we reported that the initial CLS of the DPTAP (0.29±0.04) and DPPG (0.80±0.03) interfaces are significantly different from each other. This difference was attributed to the difference in the hydrogen bonds between the lipid head groups and interfacial water, because the choline moiety of the DPTAP cannot form any hydrogen bonds whereas the phosphoglycerol moiety of DPPG forms strong hydrogen bonds with interfacial water.…”

Structure from Dynamics: Vibrational Dynamics of Interfacial Water as a Probe of Aqueous Heterogeneity

“…The water at the negatively charged biomolecule surface is oriented with the hydrogens of the O–H modes pointing on average toward the air (up). 86 …”

Orientation independent vibrational dynamics of lipid-bound interfacial water

“…Work on DPTAP showed only a single broad peak when studied with D 2 O and HOD. 6,8 Overall, the discussion of the origin of the cross-peak is out of the scope of this paper. Instead, the goal of this work is to investigate and highlight the possible effects of interfacial water orientation on interfacial vibrational water dynamics.…”

“…For negative (positive) lipids, the water dipole points towards (away from) the lipid layer, exhibiting an ultrafast relaxation path available only for the positively charged lipid. [6][7][8] In the case of the negatively charged surfactant sodium dodecyl sulfate (SDS) water interface two types of water were reported. 9 Given the ubiquitous presence of the lipid/water interface, these kinds of differences in vibrational dynamics -that reflect differences in structure 10 -could be important for the functionality of lipid bilayers.…”