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\section{\label{lipidSection:introduction}Introduction} |
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Under biologically relevant conditions, phospholipids are solvated |
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in aqueous solutions at a roughly 25:1 ratio. Solvation can have a |
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tremendous impact on transport phenomena in biological membranes |
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since it can affect the dynamics of ions and molecules that are |
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transferred across membranes. Studies suggest that because of the |
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directional hydrogen bonding ability of the lipid headgroups, a |
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small number of water molecules are strongly held around the |
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different parts of the headgroup and are oriented by them with |
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residence times for the first hydration shell being around 0.5 - 1 |
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ns.[14] In the second solvation shell, some water molecules are |
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weakly bound, but are still essential for determining the properties |
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of the system. Transport of various molecular species into living |
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cells is one of the major functions of membranes. A thorough |
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understanding of the underlying molecular mechanism for solute |
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diffusion is crucial to the further studies of other related |
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biological processes. All transport across cell membranes takes |
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place by one of two fundamental processes: Passive transport is |
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driven by bulk or inter-diffusion of the molecules being transported |
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or by membrane pores which facilitate crossing. Active transport |
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depends upon the expenditure of cellular energy in the form of ATP |
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hydrolysis. As the central processes of membrane assembly, |
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translocation of phospholipids across membrane bilayers requires the |
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hydrophilic head of the phospholipid to pass through the highly |
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hydrophobic interior of the membrane, and for the hydrophobic tails |
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to be exposed to the aqueous environment.[18] A number of studies |
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indicate that the flipping of phospholipids occurs rapidly in the |
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eukaryotic ER and the bacterial cytoplasmic membrane via a |
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bi-directional, facilitated diffusion process requiring no metabolic |
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energy input. Another system of interest would be the distribution |
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of sites occupied by inhaled anesthetics in membrane. Although the |
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physiological effects of anesthetics have been extensively studied, |
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the controversy over their effects on lipid bilayers still |
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continues. Recent deuterium NMR measurements on halothane in POPC |
| 38 |
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lipid bilayers suggest the anesthetics are primarily located at the |
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hydrocarbon chain region.[16] Infrared spectroscopy experiments |
| 40 |
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suggest that halothane in DMPC lipid bilayers lives near the |
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membrane/water interface. [17] |
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|
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|
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\section{\label{lipidSection:model}Model} |
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|
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\section{\label{lipidSection:methods}Methods} |
