RESUMEN
Broadband SFG spectroscopy is shown to offer considerable advantages over scanning systems in terms of signal-to-noise ratios when probing well-formed single-component supported lipid bilayers formed from zwitterionic lipids with PC headgroups. The SFG spectra obtained from bilayers formed from DOPC, POPC, DLPC, DMPC, DPPC and DSPC show a common peak at â¼2980 cm-1, which is subject to interference between the C-H and the O-H stretches from the aqueous phase, while membranes having transition temperatures above the laboratory temperature produce SFG spectra with at least two additional peaks, one at â¼2920 cm-1 and another at â¼2880 cm-1. The results validate spectroscopic and structural data from SFG experiments utilizing asymmetric bilayers in which one leaflet differs from the other in the extent of deuteration. Differences in H2O-D2O exchange experiments reveal that the lineshapes of the broadband SFG spectra are significantly influenced by interference from OH oscillators in the aqueous phase, even when those oscillators are not probed by the incident infrared light in our broadband setup. In the absence of spectral interference from the OH stretches of the solvent, the alkyl chain terminal methyl group of the bilayer is found to be tilted at an angle of 15° to 35° from the surface normal.
RESUMEN
Mechanistic insight into how polycations disrupt and cross cell membranes is needed for understanding and controlling polycation-membrane interactions, yet such information is surprisingly difficult to obtain at the molecular level. We use second harmonic and vibrational sum frequency generation spectroscopies along with quartz crystal microbalance with dissipation monitoring and computer simulations to quantify the interaction of poly(allylamine) hydrochloride (PAH) and its monomeric precursor allylamine hydrochloride (AH) with lipid bilayers. We find PAH adsorption to be reversible and nondisruptive to the bilayer under the conditions of our experiments. With an observed free adsorption energy of -52.7 ± 0.6 kJ/mol, PAH adsorption was found to be surprisingly less favorable relative to AH (-14.6 ± 0.4 kJ/mol) when considering a simple additive model. By experimentally quantifying the number of adsorbates and the average amount of charge carried by each adsorbate, we find that the PAH is associated with only 70% of the positive charges it could hold while the AH remains mostly charged while attached to the membrane. Simulations indicate that PAH pulls in condensed counterions from solution to avoid charge-repulsion along its backbone and with other PAH molecules to attach to, and completely cover, the bilayer surface. In addition, computations indicate that the amine groups shift their pKa values due to the confined environment upon adsorption to the surface. Our results provide experimental constraints for theoretical calculations, which yield atomistic views of the structures that are formed when polycations interact with lipid membranes that will be important for predicting polycation-membrane interactions.
RESUMEN
By combining dynamic light scattering (DLS) measurements with the interface and bond specificity of vibrational sum frequency generation scattering (SFS) spectroscopy, we probe several structural aspects of how zwitterionic DMPC lipids adsorbed to oil droplets suspended in water (D2O) respond to the presence of the common polycation poly(allylamine hydrochloride) (PAH) in the presence of low and high salt concentration. We show that the polycation interactions with the lipids generally result in two distinct outcomes that depend upon salt and PAH concentration, identified here as Scheme 1 (observed under conditions of high salt concentration) and Scheme 2 (observed under conditions of low salt concentration). The schemes differ in the extent of changes to droplet size and droplet coalescence coinciding with PAH addition. Our combined DLS and SFS results illustrate that cationic polymers do not always interact in the same fashion with lipid membranes and demonstrate the feasibility of second-order spectroscopic methods to probe those interactions with chemical bond specificity, not only for the alkyl tails (C-H stretches) but also for the choline headgroup (P-O stretches).
RESUMEN
With production of carbon nanotubes surpassing billions of tons per annum, concern about their potential interactions with biological systems is growing. Herein, we utilize second harmonic generation spectroscopy, sum frequency generation spectroscopy, and quartz crystal microbalance with dissipation monitoring to probe the interactions between oxidized multiwalled carbon nanotubes (O-MWCNTs) and supported lipid bilayers composed of phospholipids with phosphatidylcholine head groups as the dominant component. We quantify O-MWCNT attachment to supported lipid bilayers under biogeochemically relevant conditions and discern that the interactions occur without disrupting the structural integrity of the lipid bilayers for the systems probed. The extent of O-MWCNT sorption was far below a monolayer even at 100 mM NaCl and was independent of the chemical composition of the supported lipid bilayer.