Theoretical vibrational sum-frequency generation spectroscopy of water near lipid and surfactant monolayer interfaces. II. Two-dimensional spectra

Abstract

The structural stability and function of biomolecules is strongly influenced by the dynamics and hydrogen bonding of interfacial water. Understanding and characterizing the dynamics of these water molecules require a surface-sensitive technique such as two-dimensional vibrational sum-frequency generation (2DSFG) spectroscopy. We have combined theoretical 2DSFG calculations with molecular dynamics simulations in order to investigate the dynamics of water near different lipid and surfactant monolayer surfaces. We show that 2DSFG can distinguish the dynamics of interfacial water as a function of the lipid charge and headgroup chemistry. The dynamics of water is slow compared to the bulk near water-zwitterionic and water-anionic interfaces due to conformational constraints on interfacial water imposed by strong phosphate-water hydrogen bonding. The dynamics of water is somewhat faster near water-cationic lipid interfaces as no such constraint is present. Using hydrogen bonding and rotational correlation functions, we characterize the dynamics of water as a function of the distance from the interface between water and zwitterionic lipids. We find that there is a transition from bulk-like to interface-like dynamics approximately 7 Å away from a zwitterionic phosphatidylcholine monolayer surface.

Enhanced chain packing achieved via putative headgroup ion-triplet formation in binary anionic lipid/cationic surfactant mixed monolayers

“…Nevertheless there is a significant condensing effect of adding an equimolar quantity of DHDAB to DPPG, which has similarly been observed with another similar cationic surfactant dipalmitoyltrimethylammonium propane (DPTAP) (Roy et al, 2014).…”

“…This led us to speculate on the molecular interactions between the anionic lipid and cationic surfactant, firstly whether neutralization of DPPG/DHDAB 1:1 vesicles resulted from true ion pairing, and secondly whether putative ion triplet formation in DPPG/DHDAB 2:1 mixtures, was responsible for their increased thermal stability. Air/liquid interface Langmuir monolayers have previously been used to study the thermodynamics of mixing of anionic phospholipids and cationic surfactants (Panda et al, 2010;Roy et al, 2014;Sung et al, 2010), and therefore constitute a useful system for comparing DPPG/DHDAB mixtures of interest, whilst circumventing the need for an excess of one charge to ensure vesicle stability. These mixed monolayers can be used as a platform for a variety of techniques designed to examine their structural and interfacial physicochemical properties.…”

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

“…44 This can be accessed using 2D SFG and TR-SFG spectroscopy. 94,95 These results help shed light on the structure of water in contact with biosurfaces containing both cationic and anionic groups, a complex system that is a major component of biological membranes.…”

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

“…Recently Skinner and co-workers calculated 2D HD-VSFG spectra using molecular dynamics simulations, and discussed the dynamics of isotopically diluted water at the DPTAP and DPPG interfaces. [15] Their calculation showed that the spectral diffusion dynamics of the OH stretch at both interfaces are bimodal, but that fast dynamics (~300 fs) are predominant at the positively charged DPTAP interface, whereas slow dynamics (~10 ps) are dominant at the negatively charged DPPG interface.T hey concluded that the dynamics of water at the DPPG interface are slow compared to the DPTAP interface,m ostly because of the conformational constraints on water imposed by the hydrogen bond with the head group of DPPG.O ur experimental results are very consistent with their predictions about the bimodal feature and the change in the amplitudes of the fast and slow components,a lthough the calculated time constants are considerably different. The2 Dl obe shape,a sw ell as the amplitude of the steady-state spectrum [16] calculated by the Skinner group,a re also in qualitative agreement with the present experimental results for both the DPTAP and DPPG interfaces.…”

“…In the case of DPPG,the interfacial water can be strongly hydrogen-bonded to the phosphatidylglycerol head group:the phosphate group forms firm hydrogen bonds with water, and glycerol hydroxy groups at the terminal end can also provide additional hydrogen bonding sites. [15] It is very likely that the hydrogen bond with the phosphatidylglycerol head group largely limits the fast fluctuation of interfacial water because of the slow motion of DPPG.Note that the phosphatidylglycerol group of DPPG can form hydrogen bonds with water, whereas the choline group of DPTAP cannot. The2Dspectra demonstrate that the dynamics of interfacial water are crucially dependent…”