Effect of aliphatic chain length on stability of poly(ethylene glycol)-grafted phospholipid monolayers at the air/water interface

Acylcarnitines at the Membrane Surface: Insertion Parameters for a Mitochondrial Leaflet Model

Excessive accumulation of acylcarnitines (ACs), often caused by metabolic disorders, has been associated with obesity, arrhythmias, cardiac ischemia, insulin resistance, etc. Mechanisms whereby elevated ACs might contribute to pathophysiological effects remain largely unexplored. We have aimed to gain insight into AC interactions with the mitochondrial inner membrane. To model its outer leaflet, Langmuir monolayers and cushioned supported bilayers were employed. Their interactions with ACs were monitored with epifluorescence microscopy, which revealed a local leaflet expansion upon exposure to elevated concentrations of a long-chain AC, plausibly caused by its insertion. To assess the AC insertion parameters, constant-pressure insertion assays were performed. A value of 21 ± 3 Å² was obtained for the AC insertion area, which is roughly the same as the cross-sectional area of an acyl chain. By contrast, the carnitine moiety was found to require an area of 37 ± 3 Å². The AC insertion has thus been concluded to involve solely the AC acyl chain. This mode of insertion implies that the carnitine moiety, with its nontitratable positive charge, is left dangling at the membrane surface, which is likely to alter the surface electrostatics of the outer leaflet. The extrapolation of these findings has enabled us to hypothesize that, by altering the morphology and surface electrostatics of the outer leaflet, the insertion of ACs, in particular their long-chain counterparts, may trigger a nonspecific activation of signaling pathways in the inner mitochondrial membrane, thereby modulating its function and potentially leading to pathophysiological responses.

Applications of Brewster angle microscopy from biological materials to biological systems

Brewster angle microscopy (BAM) is a powerful technique that allows for real-time visualization of Langmuir monolayers. The lateral organization of these films can be investigated, including phase separation and the formation of domains, which may be of different sizes and shapes depending on the properties of the monolayer. Different molecules or small changes within a molecule such as the molecule's length or presence of a double bond can alter the monolayer's lateral organization that is usually undetected using surface pressure-area isotherms. The effect of such changes can be clearly observed using BAM in real-time, under full hydration, which is an experimental advantage in many cases. While previous BAM reviews focused more on selected compounds or compared the impact of structural variations on the lateral domain formation, this review provided a broader overview of BAM application using biological materials and systems including the visualization of amphiphilic molecules, proteins, drugs, extracts, DNA, and nanoparticles at the air-water interface.

Microbubbles coated with disaturated lipids and DSPE-PEG2000: phase behavior, collapse transitions, and permeability

Saturated diacyl (disaturated) phosphatidylcholine (PC) mixed with the lipopolymer distearoylphosphatidylethanolamine (DSPE)-polyethyleneglycol molecular weight 2000 (PEG2000) self-assemble as a monolayer at the air-water interface of air-in-water micrometer-scale bubbles (microbubbles), similar to coatings (shells) on leading medical ultrasound contrast agents (UCAs). This system is characterized here to study the impact of the DSPE-PEG2000 species and PC chain-length on the monolayer coating phase behavior, collapse, shedding, and air transport resistance and microbubble dissolution rate and surface contour. Using fluorescence microscopy of dissolving microbubbles, we found that film microstructure and collapse behavior for all chain lengths (n = 14-20) was indicative of primarily condensed phase monolayers, unlike similar coatings containing polyethyleneglycol 40 stearate (PEG40S) that are either expanded phase or coexisting expanded-condensed phase monolayers. Additionally, we observed a new surface buckling type of behavior with all chain lengths, by bright field microscopy, where the air-water interface continuously appears rough (rather than cyclically rough and smooth), with this behavior most frequently observed for n = 16. In correlating the statistical frequency of this behavior with the monolayer microstructure, we propose that it arises from a slowed nucleation rate of collapse structures at condensed-condensed phase interfaces, not present in systems containing PEG40S. By modeling the dissolution (radius vs time) data, we obtained, for each chain length, the film air transport resistance (R(shell)) that was then fit to a chain-length-dependent energy barrier model. Importantly, the pre-exponential factor was approximately 10 x higher and the microbubbles persisted approximately 4 x longer (from 15 microm at a fixed dissolved oxygen content) in comparison to previously studied films containing PEG40S. We attribute the unique stability properties of microbubble coatings containing DSPE-PEG2000 to the propensity of this molecule to form a condensed-phase monolayer, such that the monolayer coatings approach the properties of one continuous condensed domain.

Lateral distribution of a poly(ethylene glycol)-grafted phospholipid in phosphocholine monolayers studied by epifluorescence microscopy

Mixed monolayers of distearoylphosphatidylcholine (DSPC) and a poly(ethylene glycol)-(PEG)-grafted distearoylphosphatidylethanolamine with a PEG molecular weight of 2000, DSPE-PEG2000, spread on phosphate-buffered saline (PBS) were used as models of bio-non-fouling membrane-mimetic surfaces in order to visualize the lateral distribution of PEG2000-phospholipid in the host phospholipid matrix. Epifluorescence microscopy (EFM) was used to locate DSPE-PEG2000 molecules in the DSPC matrix by detecting the fluorescence from a fluorescein fluorophore attached to the distal end of the PEG2000 chain. Comparative analysis of surface pressure-area isotherms and EFM images revealed that DSPE-PEG2000 mixes nonideally with DSPC in monolayers on a PBS subphase. A transition from a phase-separated monolayer to a homogeneous mixture was observed with increasing surface pressure and PEG content. The effect of nonideal mixing behavior of DSPE-PEG2000 on its lateral distribution in the DSPC matrix was interpreted in terms of excluded volume interactions between the PEG2000 chains and a mismatch in the tilt of aliphatic chains on DSPC and DSPE-PEG2000 molecules.