Surface Phase Behavior Of Phospholipid Monolayers At The Air Water Interface And Interactions With Lung Annexin I

Author : Jennifer Alexis Staton
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.6/5 (985 download)

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Download or read book Kinetics of Phospholipid Transport to Fluid Interfaces written by Jennifer Alexis Staton and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Despite the widespread use of phospholipid vesicles in nature and in industry, the process by which the phospholipid molecule is transferred from vesicles to an interface is not well understood. A major goal of my research performed in this dissertation was to elucidate the mechanism of adsorption for dilauroyl phosphatidylcholine (DLPC) or dimyristoyl phosphatidylcholine (DMPC) onto a clean air-water interface from a dispersion of large, unilamellar vesicles. DMPC is analogous to DLPC, but with two additional CH2 groups on each acyl chain. The adsorption kinetics were tracked using dynamic surface tension measurements at a variety of concentrations, temperatures (above the lipid phase transition temperature), vesicle diameters and stir rates. The adsorption time was always found to decrease with increasing lipid concentration and decreasing vesicle diameter for a fixed lipid concentration, indicating a clear mechanistic role for the vesicles. The adsorption time was also found to increase with increasing temperature or lipid solubility (due to a decrease in the acyl chain length), with the latter having a profound effect on the adsorption kinetics. An induction period was observed at early times for all adsorption kinetics, during which lipid adsorption occurred without significantly changing the surface tension. Convection was introduced into these experiments to probe the importance of bulk transport mechanisms on the overall kinetics. There are two potential available pathways for the lipid to move to the interface and form a monolayer: (1) molecular transport of dissolved phospholipid that is enhanced by release from nearby vesicles; and (2) lipid delivery by vesicles directly to the interface. Theoretical expressions for transport via these pathways were developed and used to predict associated transport time scales for different scenarios. Experimentally measured times, representative of both early and late adsorption times, were compared to theoretically predicted time scales in order to provide insight into the adsorption mechanism. The early- or late-time regimes for both lipids represented interfacial coverages from 0 to ~60% or ~60-90% of the maximum surface coverage, respectively. For DLPC concentrations ≥0.1mM or DMPC concentrations ≥1mM, results for both early and late adsorption times were only consistent with an adsorption mechanism controlled by molecular transport that was enhanced by vesicle-to-monomer exchange beneath the interface (i.e., the reaction-enhanced, molecular mechanism). Temperatures examined were 22°C, 30°C and 38°C for DLPC or 50°C and 58°C for DMPC. The lipid aqueous solubility was shown to have the most significant impact on the adsorption time; a decrease in the lipid solubility resulted in a dramatic decrease in molecular release rates from vesicles. At later adsorption times, a significant molecular interfacial barrier was predicted to arise for both lipids, likely due to packing constraints at the interface. This molecular interfacial barrier was found to be negligible at early times. For early times, at DLPC concentrations ≤0.25mM for 22°C and 30°C or ≤0.75mM at 38°C, a strong effect of lipid concentration was observed. This was also observed at later times for DLPC concentrations below ~1mM for all three temperatures. For DLPC ≤0.1mM at 22°C, an effect of convection was also observed. At these lower concentrations, transport controlled by the vesicle pathway reasonably captured the observed effect of lipid concentration; however, neither molecular nor vesicle pathway mechanisms captured the influence of convection. It is plausible that rate of adsorption via the molecular pathway decreased due to vesicle diffusion limitations, resulting in a stronger effect of lipid concentration on the dynamics. Future work in this area is required to fully understand kinetics for this low concentration regime. The second major aim of the research performed in this dissertation focused on contributions of water-soluble lysolauroyl phosphatidylcholine (LLPC) at low mass fractions in DLPC vesicle dispersions. Adsorption dynamics were tracked using dynamic surface tension measurements for an air-water interface at 22°C. The total amount of LLPC in the mixture was maintained at 2.5wt% or 5wt%, so that when DLPC was varied from 0.5 to 5mM, the LLPC concentration likewise increased. The effect of LLPC on the adsorption dynamics was assessed by comparing experimental dynamic tension results to theoretical predictions for lipid transport. The presence of LLPC in the mixture resulted in the elimination of an induction regime for all mixtures, and consequently faster dynamics at early times, as compared to pure DLPC dynamics. At later times, the mixture exhibited tensions below those observed for either pure LLPC or pure DLPC at comparable times. The dynamics for the mixture ultimately achieve an equilibrium tension of 23.8±0.7mN/m, in keeping with that observed for pure DLPC. The initial adsorption of LLPC appeared to influence the dynamics by inducing a shift in the interfacial phase behavior, such that DLPC could adsorb into a liquid expanded phase as opposed to a gaseous phase. This shift allowed for an increase in intermolecular interactions and a consequently deeper decline in surface tension at early times, when compared to pure DLPC dynamics. Results for a natural lecithin product, Sunlipon 90, agree qualitatively with results for the DLPC/LLPC mixture. Further consideration of the shift in interfacial phase behavior involved mixtures of Laponite nanoparticles with DLPC vesicles, as well as DMPC adsorption to a pure corn oil-water interface.