1 |
tim |
2685 |
\chapter{\label{chapt:lipid}LIPID MODELING} |
2 |
|
|
|
3 |
|
|
\section{\label{lipidSection:introduction}Introduction} |
4 |
|
|
|
5 |
tim |
2731 |
Under biologically relevant conditions, phospholipids are solvated |
6 |
|
|
in aqueous solutions at a roughly 25:1 ratio. Solvation can have a |
7 |
|
|
tremendous impact on transport phenomena in biological membranes |
8 |
|
|
since it can affect the dynamics of ions and molecules that are |
9 |
|
|
transferred across membranes. Studies suggest that because of the |
10 |
|
|
directional hydrogen bonding ability of the lipid headgroups, a |
11 |
|
|
small number of water molecules are strongly held around the |
12 |
|
|
different parts of the headgroup and are oriented by them with |
13 |
|
|
residence times for the first hydration shell being around 0.5 - 1 |
14 |
|
|
ns.[14] In the second solvation shell, some water molecules are |
15 |
|
|
weakly bound, but are still essential for determining the properties |
16 |
|
|
of the system. Transport of various molecular species into living |
17 |
|
|
cells is one of the major functions of membranes. A thorough |
18 |
|
|
understanding of the underlying molecular mechanism for solute |
19 |
|
|
diffusion is crucial to the further studies of other related |
20 |
|
|
biological processes. All transport across cell membranes takes |
21 |
|
|
place by one of two fundamental processes: Passive transport is |
22 |
|
|
driven by bulk or inter-diffusion of the molecules being transported |
23 |
|
|
or by membrane pores which facilitate crossing. Active transport |
24 |
|
|
depends upon the expenditure of cellular energy in the form of ATP |
25 |
|
|
hydrolysis. As the central processes of membrane assembly, |
26 |
|
|
translocation of phospholipids across membrane bilayers requires the |
27 |
|
|
hydrophilic head of the phospholipid to pass through the highly |
28 |
|
|
hydrophobic interior of the membrane, and for the hydrophobic tails |
29 |
|
|
to be exposed to the aqueous environment.[18] A number of studies |
30 |
|
|
indicate that the flipping of phospholipids occurs rapidly in the |
31 |
|
|
eukaryotic ER and the bacterial cytoplasmic membrane via a |
32 |
|
|
bi-directional, facilitated diffusion process requiring no metabolic |
33 |
|
|
energy input. Another system of interest would be the distribution |
34 |
|
|
of sites occupied by inhaled anesthetics in membrane. Although the |
35 |
|
|
physiological effects of anesthetics have been extensively studied, |
36 |
|
|
the controversy over their effects on lipid bilayers still |
37 |
|
|
continues. Recent deuterium NMR measurements on halothane in POPC |
38 |
|
|
lipid bilayers suggest the anesthetics are primarily located at the |
39 |
|
|
hydrocarbon chain region.[16] Infrared spectroscopy experiments |
40 |
|
|
suggest that halothane in DMPC lipid bilayers lives near the |
41 |
|
|
membrane/water interface. [17] |
42 |
|
|
|
43 |
|
|
|
44 |
tim |
2685 |
\section{\label{lipidSection:model}Model} |
45 |
|
|
|
46 |
|
|
\section{\label{lipidSection:methods}Methods} |
47 |
|
|
|
48 |
|
|
\section{\label{lipidSection:resultDiscussion}Results and Discussion} |