Perturbation of Hydrogen Bonding Networks over Supported Lipid Bilayers by Poly (Allylamine Hydrochloride)

Abstract

<div><div><div><p>Water is vital to many biochemical processes and is necessary for driving many fundamental interactions of cell membranes with their external environments, yet it is difficult to probe the membrane/water interface directly and without the use of external labels. Here, we employ vibrational sum frequency generation (SFG) spectroscopy to understand the role of interfacial water molecules above bilayers formed from zwitterionic (phosphatidylcholine, PC) and anionic (phosphatidylglycerol, PG, and phosphatidylserine, PS) lipids as they are exposed to the common polycation poly (allylamine hydrochloride) (PAH) in 100 mM NaCl. We show that as the concentration of PAH is increased, the interfacial water molecules are irreversibly displaced and find that it requires 10 times more PAH to displace interfacial water molecules from membranes formed from purely zwitterionic lipids when compared to membranes that contain the anionic PG and PS lipids. This outcome is likely due to difference in (1) the energy with which water molecules are bound to the lipid headgroups, (2) the number of water molecules bound to the headgroups, which is related to the headgroup area, and (3) the electrostatic interactions between the PAH molecules and the negatively charged lipids that are favored when compared to the zwitterionic lipid headgroups. The findings presented here contribute to establishing causal relationships in nanotoxicology and to understanding, controlling, and predicting the initial steps that lead to the lysis of cells exposed to membrane disrupting polycations, or to transfection.</p></div></div></div>

Electrostatics, Hydrogen Bonding, and Molecular Structure at Polycation and Peptide:Lipid Membrane Interfaces

“…As the PAH concentration is further increased to 10 µM and 0.1 mM, the resulting SFG signal intensity reduction across the hydrogen bonding network exceeds 70 percent. 62 The reversibility of the displacement of interfacial water molecules with PAH was assessed by rinsing the bilayer with PAH-free buffer ( Figure 6). After rinsing, the spectral features of the bilayer in the C-H stretching region do not revert, neither do the peaks at 3200 cm -1 or 3400 cm -1 , consistent with the possibility that water repulsion induced by PAH is irreversible under our experimental conditions.…”

Specifics about Specific Ion Adsorption from Heterodyne-Detected Second Harmonic Generation

“…Compounding the complexity of the problem is the observation from electrokinetic measurements that the interfacial charge density of certain oxides varies with ionic strength, at least for the ionic strengths accessible in those experiments. [42][43][44]117 Despite these challenges, recent years have seen some significant progress: For instance, ion identity (element and charge), electrolyte concentration, and bulk solution pH are some of the factors that are now known to control surface charge density and the structure of interfacial water molecules and the hydrogen-bond networks they establish within the electrical double layer. [12][13][14][15][16][17] Moreover, potentiometric measurements and calculations with surface complexation models have established that cations and anions influence charge densities at silica/water interfaces via their adsorption affinities.…”

Direct Measurement of Charge Reversal on Lipid Bilayers using Heterodyne-Detected Second Harmonic Generation Spectroscopy

“…3B further confirms that the change in 𝐸 ?7g is dominated by the change in the potential-dependent 𝜒 (0) term. Given that 𝜒 (-) is comprised of constructive (in-phase) and destructive (out-of-phase) contributions from the silica, the lipid bilayer, the water molecules, the ions, and PAH, any net change in 𝜒 (-) is likely to be due to a combination of a loss of water molecules (as we recently reported using vibrational sum frequency generation spectroscopy) 39 and thus a reduction in 𝜒 ›@G<r (-)…”

“…Indeed, nonresonant homodyne-detected SHG has been widely used in label-free in situ studies of charged surfaces. [32][33][34][35] Accounting for dispersion between the fundamental and second harmonic beams within the EDL, the interpretation of nonresonant SHG data from charged interfaces is based on the following model: 21,29,[36][37][38][39][40][41][42][43][44][45][46][47][48][49] 𝐸 sig 𝑒 𝑖𝜑 𝑠𝑖𝑔 ∝ 𝜒 (-) + 𝜒 (0) Φ(0) • 𝑇(𝜑 56 )𝑒 78 9: = 𝜒 <==,? @ABC< (-) D…”