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\usepackage{floatflt} |
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\usepackage{amsmath} |
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\usepackage{amssymb} |
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\usepackage{subfigure} |
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\usepackage{palatino} |
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\usepackage[ref]{overcite} |
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Simulations of phospholipid bilayers are, by necessity, quite |
40 |
|
complex. The lipid molecules are large molecules containing many |
41 |
< |
atoms,and the head group of the lipid will typically contain charge |
41 |
> |
atoms, and the head group of the lipid will typically contain charge |
42 |
|
separated ions which set up a large dipole within the molecule. Adding |
43 |
|
to the complexity are the number of water molecules needed to properly |
44 |
< |
solvate the lipid bilayer. Because of these factors, many current |
45 |
< |
simulations are limited in both length and time scale due to to the |
46 |
< |
sheer number of calculations performed at every time step and the |
47 |
< |
lifetime of the researcher. A typical |
44 |
> |
solvate the lipid bilayer, typically 25 water molecules for every |
45 |
> |
lipid molecule. Because of these factors, many current simulations are |
46 |
> |
limited in both length and time scale due to to the sheer number of |
47 |
> |
calculations performed at every time step and the lifetime of the |
48 |
> |
researcher. A typical |
49 |
|
simulation\cite{saiz02,lindahl00,venable00,Marrink01} will have around |
50 |
|
64 phospholipids forming a bilayer approximately 40~$\mbox{\AA}$ by |
51 |
|
50~$\mbox{\AA}$ with roughly 25 waters for every lipid. This means |
52 |
|
there are on the order of 8,000 atoms needed to simulate these systems |
53 |
< |
and the trajectories in turn are integrated for times up to 10 ns. |
53 |
> |
and the trajectories are integrated for times up to 10 ns. |
54 |
|
|
55 |
|
These limitations make it difficult to study certain biologically |
56 |
|
interesting phenomena that don't fit within the short time and length |
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scale requirements. One such phenomena is the existence of the ripple |
57 |
> |
scale requirements. One such phenomenon is the existence of the ripple |
58 |
|
phase ($P_{\beta'}$) of the bilayer between the gel phase |
59 |
|
($L_{\beta'}$) and the fluid phase ($L_{\alpha}$). The $P_{\beta'}$ |
60 |
|
phase has been shown to have a ripple period of |
63 |
|
roughly 25 waters for every lipid to fully solvate the bilayer. With |
64 |
|
the large number of atoms involved in a simulation of this magnitude, |
65 |
|
steps \emph{must} be taken to simplify the system to the point where |
66 |
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these numbers are reasonable. |
66 |
> |
the numbers of atoms becomes reasonable. |
67 |
|
|
68 |
|
Another system of interest would be drug molecule diffusion through |
69 |
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the membrane. Due to the fluid like properties of a lipid membrane, |
69 |
> |
the membrane. Due to the fluid-like properties of a lipid membrane, |
70 |
|
not all diffusion takes place at membrane channels. It is of interest |
71 |
|
to study certain molecules that may incorporate themselves directly |
72 |
|
into the membrane. These molecules may then have an appreciable |
73 |
|
waiting time (on the order of nanoseconds) within the |
74 |
|
bilayer. Simulation of such a long time scale again requires |
75 |
|
simplification of the system in order to lower the number of |
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calculations needed at each time step. |
76 |
> |
calculations needed at each time step or to increase the length of |
77 |
> |
each time step. |
78 |
|
|
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|
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|
\section{Methodology} |
83 |
|
|
84 |
|
The length scale simplifications are aimed at increasing the number of |
85 |
|
molecules that can be simulated without drastically increasing the |
86 |
< |
computational cost of the system. This is done by a combination of |
87 |
< |
substituting less expensive interactions for expensive ones and |
88 |
< |
decreasing the number of interaction sites per molecule. Namely, |
89 |
< |
point charge distributions are replaced with dipoles, and unified atoms are |
90 |
< |
used in place of water, phospholipid head groups, and alkyl groups. |
86 |
> |
computational cost of the simulation. This is done through a |
87 |
> |
combination of substituting less expensive interactions for expensive |
88 |
> |
ones and decreasing the number of interaction sites per |
89 |
> |
molecule. Namely, groups of point charges are replaced with single |
90 |
> |
point-dipoles, and unified atoms are used in place of water, |
91 |
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phospholipid head groups, and alkyl groups. |
92 |
|
|
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The replacement of charge distributions with dipoles allows us to |
94 |
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replace an interaction that has a relatively long range, |
95 |
< |
$(\frac{1}{r})$, for the coulomb potential, with that of a relatively |
96 |
< |
short range, $(\frac{1}{r^{3}})$, for dipole - dipole |
97 |
< |
potentials. Combined with a computational simplification algorithm |
98 |
< |
such as a Verlet neighbor list,\cite{allen87:csl} this should give |
99 |
< |
computational scaling by $N$. This is in comparison to the Ewald |
100 |
< |
sum\cite{leach01:mm} needed to compute the coulomb interactions, which |
101 |
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scales at best by $N \ln N$. |
93 |
> |
The replacement of charges with dipoles allows us to replace an |
94 |
> |
interaction that has a relatively long range ($\frac{1}{r}$ for the |
95 |
> |
coulomb potential) with that of a relatively short range |
96 |
> |
($\frac{1}{r^{3}}$ for dipole - dipole potentials). Combined with |
97 |
> |
Verlet neighbor lists,\cite{allen87:csl} this should result in an |
98 |
> |
algorithm which scales linearly with increasing system size. This is |
99 |
> |
in comparison to the Ewald sum\cite{leach01:mm} needed to compute |
100 |
> |
periodic replicas of the coulomb interactions, which scales at |
101 |
> |
best\cite{darden93:pme} by $N \ln N$. |
102 |
|
|
103 |
|
The second step taken to simplify the number of calculations is to |
104 |
|
incorporate unified models for groups of atoms. In the case of water, |
105 |
|
we use the soft sticky dipole (SSD) model developed by |
106 |
< |
Ichiye\cite{Liu96} (Section~\ref{sec:ssdModel}). For the phospholipids, a |
107 |
< |
unified head atom with a dipole will replace the atoms in the head |
108 |
< |
group, while unified $\text{CH}_2$ and $\text{CH}_3$ atoms will |
109 |
< |
replace the alkyl units in the tails (Section~\ref{sec:lipidModel}). |
106 |
> |
Ichiye\cite{liu96:new_model,liu96:monte_carlo,chandra99:ssd_md} |
107 |
> |
(Section~\ref{sec:ssdModel}). For the phospholipids, a unified head |
108 |
> |
atom with a dipole will replace the atoms in the head group, while |
109 |
> |
unified $\text{CH}_2$ and $\text{CH}_3$ atoms will replace the alkyl |
110 |
> |
units in the tails (Section~\ref{sec:lipidModel}). This model is |
111 |
> |
similar in practice to that of Lipowsky and Goetz\cite{goetz98}, where |
112 |
> |
the whole system is reduced to attractive and repulsive Lennard-Jones |
113 |
> |
spheres. However, our model gives a greater level of detail to each |
114 |
> |
unified molecule, namely through dipole, bend, and torsion |
115 |
> |
interactions. |
116 |
|
|
117 |
|
The time scale simplifications are introduced so that we can take |
118 |
|
longer time steps. By increasing the size of the time steps taken by |
119 |
< |
the simulation, we are able to integrate the simulation trajectory |
120 |
< |
with fewer calculations. However, care must be taken that any |
121 |
< |
simplifications used, still conserve the total energy of the |
119 |
> |
the simulation, we are able to integrate a given length of time using |
120 |
> |
fewer calculations. However, care must be taken that any |
121 |
> |
simplifications used still conserve the total energy of the |
122 |
|
simulation. In practice, this means taking steps small enough to |
123 |
|
resolve all motion in the system without accidently moving an object |
124 |
|
too far along a repulsive energy surface before it feels the effect of |
132 |
|
energy. This is in contrast to the 1 fs time step typically needed to |
133 |
|
conserve energy when bonds lengths are allowed to oscillate. |
134 |
|
|
135 |
< |
\subsection{The Soft Sticky Water Model} |
135 |
> |
\subsection{The Soft Sticky Dipole Water Model} |
136 |
|
\label{sec:ssdModel} |
137 |
|
|
138 |
< |
%\begin{floatingfigure}{55mm} |
139 |
< |
%\includegraphics[width=45mm]{ssd.epsi} |
140 |
< |
%\caption{The SSD model with the oxygen and hydrogen atoms drawn in for reference. \vspace{5mm}} |
141 |
< |
%\label{fig:ssdModel} |
142 |
< |
%\end{floatingfigure} |
138 |
> |
\begin{figure} |
139 |
> |
\centering |
140 |
> |
\includegraphics[width=50mm]{ssd.epsi} |
141 |
> |
\caption{The SSD model with the oxygen and hydrogen atoms drawn in for reference.} |
142 |
> |
\label{fig:ssdModel} |
143 |
> |
\end{figure} |
144 |
|
|
145 |
|
The water model used in our simulations is a modified soft |
146 |
|
Stockmayer-sphere model.\cite{stevens95} Like the Stockmayer-sphere, the SSD |
147 |
< |
model\cite{Liu96} consists of a Lennard-Jones interaction site and a |
147 |
> |
model consists of a Lennard-Jones interaction site and a |
148 |
|
dipole both located at the water's center of mass (Figure |
149 |
|
\ref{fig:ssdModel}). However, the SSD model extends this by adding a |
150 |
|
tetrahedral potential to correct for hydrogen bonding. |
151 |
|
|
152 |
< |
The SSD water potential is then given by the following equation: |
152 |
> |
The SSD water potential for a pair of water molecules is then given by |
153 |
> |
the following equation: |
154 |
|
\begin{equation} |
155 |
|
V_{\text{SSD}} = V_{\text{LJ}}(r_{i\!j}) + V_{\text{dp}}(\mathbf{r}_{i\!j}, |
156 |
|
\boldsymbol{\Omega}_{i},\boldsymbol{\Omega}_{j}) |
158 |
|
\boldsymbol{\Omega}_{j}) |
159 |
|
\label{eq:ssdTotPot} |
160 |
|
\end{equation} |
161 |
< |
$V_{\text{LJ}}$ is the Lennard-Jones potential: |
161 |
> |
where $\mathbf{r}_{ij}$ is the vector between molecules $i$ and $j$, |
162 |
> |
and $\boldsymbol{\Omega}_i$ and $\boldsymbol{\Omega}_j$ are the |
163 |
> |
Euler angles of molecule $i$ or $j$ respectively. $V_{\text{LJ}}$ is |
164 |
> |
the Lennard-Jones potential: |
165 |
|
\begin{equation} |
166 |
|
V_{\text{LJ}} = |
167 |
|
4\epsilon_{ij} \biggl[ |
170 |
|
\biggr] |
171 |
|
\label{eq:lennardJonesPot} |
172 |
|
\end{equation} |
173 |
< |
where $r_{ij}$ is the distance between two $ij$ pairs, $\sigma_{ij}$ |
174 |
< |
scales the length of the iteraction, and $\epsilon_{ij}$ scales the |
175 |
< |
energy of the potential. For SSD, $\sigma_{\text{SSD}} = 3.051 \mbox{ |
173 |
> |
where $\sigma_{ij}$ scales the length of the interaction, and |
174 |
> |
$\epsilon_{ij}$ scales the energy of the potential. For SSD, |
175 |
> |
$\sigma_{\text{SSD}} = 3.051 \mbox{ |
176 |
|
\AA}$ and $\epsilon_{\text{SSD}} = 0.152\text{ kcal/mol}$. |
177 |
|
$V_{\text{dp}}$ is the dipole potential: |
178 |
|
\begin{equation} |
185 |
|
{r^{5}_{ij}} \biggr] |
186 |
|
\label{eq:dipolePot} |
187 |
|
\end{equation} |
188 |
< |
where $\mathbf{r}_{ij}$ is the vector between $i$ and $j$, |
189 |
< |
$\boldsymbol{\Omega}$ is the orientation of the species, and |
190 |
< |
$\boldsymbol{\mu}$ is the dipole vector. The SSD model specifies a dipole |
191 |
< |
magnitude of 2.35~D for water. |
188 |
> |
where $\boldsymbol{\mu}_i$ is the dipole vector of molecule $i$, |
189 |
> |
$\boldsymbol{\mu}_i$ points along the z-axis in the body fixed |
190 |
> |
frame. This frame is then oriented in space by the Euler angles, |
191 |
> |
$\boldsymbol{\Omega}_i$. The SSD model specifies a dipole magnitude of |
192 |
> |
2.35~D for water. |
193 |
|
|
194 |
< |
The hydrogen bonding of the model is governed by the $V_{\text{sp}}$ |
194 |
> |
The hydrogen bonding is modeled by the $V_{\text{sp}}$ |
195 |
|
term of the potential. Its form is as follows: |
196 |
|
\begin{equation} |
197 |
|
V_{\text{sp}}(\mathbf{r}_{i\!j},\boldsymbol{\Omega}_{i}, |
225 |
|
\label{eq:spCorrection} |
226 |
|
\end{equation} |
227 |
|
The angles $\theta_{ij}$ and $\phi_{ij}$ are defined by the spherical |
228 |
< |
polar coordinates of the position of sphere $j$ in the reference frame |
229 |
< |
fixed on sphere $i$ with the z-axis aligned with the dipole moment. |
228 |
> |
coordinates of the position of molecule $j$ in the reference frame |
229 |
> |
fixed on molecule $i$ with the z-axis aligned with the dipole moment. |
230 |
|
The correction |
231 |
|
$w^{x}_{ij}(\mathbf{r}_{ij},\boldsymbol{\Omega}_{i},\boldsymbol{\Omega}_{j})$ |
232 |
|
is needed because |
233 |
|
$w_{ij}(\mathbf{r}_{ij},\boldsymbol{\Omega}_{i},\boldsymbol{\Omega}_{j})$ |
234 |
|
vanishes when $\theta_{ij}$ is $0^\circ$ or $180^\circ$. |
235 |
|
|
236 |
< |
Finally, the sticky potential is scaled by a cutoff function, |
237 |
< |
$s(r_{ij})$ that scales smoothly between 0 and 1. It is represented |
236 |
> |
Finally, the sticky potential is scaled by a switching function, |
237 |
> |
$s(r_{ij})$, that scales smoothly between 0 and 1. It is represented |
238 |
|
by: |
239 |
|
\begin{equation} |
240 |
|
s(r_{ij}) = |
249 |
|
\end{equation} |
250 |
|
|
251 |
|
Despite the apparent complexity of Equation \ref{eq:spPot}, the SSD |
252 |
< |
model is still computationaly inexpensive. This is due to Equation |
253 |
< |
\ref{eq:spCutoff}. With $r_{L}$ being 2.75~$\mbox\AA}$ and $r_{U}$ |
252 |
> |
model is still computationally inexpensive. This is due to Equation |
253 |
> |
\ref{eq:spCutoff}. With $r_{L}$ being 2.75~$\mbox{\AA}$ and $r_{U}$ |
254 |
|
being equal to either 3.35~$\mbox{\AA}$ for $s(r_{ij})$ or |
255 |
|
4.0~$\mbox{\AA}$ for $s'(r_{ij})$, the sticky potential is only active |
256 |
< |
over an extremly short range, and then only with other SSD |
257 |
< |
molecules. Therefore, it's predominant interaction is through it's |
258 |
< |
point dipole and Lennard-Jones sphere. |
256 |
> |
over an extremely short range, and then only with other SSD |
257 |
> |
molecules. Therefore, it's predominant interaction is through the |
258 |
> |
point dipole and the Lennard-Jones sphere. Of these, only the dipole |
259 |
> |
interaction can be considered ``long-range''. |
260 |
|
|
261 |
|
\subsection{The Phospholipid Model} |
262 |
|
\label{sec:lipidModel} |
263 |
|
|
264 |
< |
\begin{floatingfigure}{90mm} |
264 |
> |
\begin{figure} |
265 |
> |
\centering |
266 |
|
\includegraphics[angle=-90,width=80mm]{lipidModel.epsi} |
267 |
< |
\caption{A representation of the lipid model. $\phi$ is the torsion angle, $\theta$ is the bend angle, $\mu$ is the dipole moment of the head group, and n is the chain length. \vspace{5mm}} |
267 |
> |
\caption{A representation of the lipid model. $\phi$ is the torsion angle, $\theta$ is the bend angle, $\mu$ is the dipole moment of the head group, and n is the chain length.} |
268 |
|
\label{fig:lipidModel} |
269 |
< |
\end{floatingfigure} |
269 |
> |
\end{figure} |
270 |
|
|
271 |
|
The lipid molecules in our simulations are unified atom models. Figure |
272 |
< |
\ref{fig:lipidModel} shows a template drawing for one of our |
273 |
< |
lipids. The Head group of the phospholipid is replaced by a single |
272 |
> |
\ref{fig:lipidModel} shows a schematic for one of our |
273 |
> |
lipids. The head group of the phospholipid is replaced by a single |
274 |
|
Lennard-Jones sphere with a freely oriented dipole placed at it's |
275 |
< |
center. The magnitude of it's dipole moment is 20.6 D. The tail atoms |
276 |
< |
are unified $\text{CH}_2$ and $\text{CH}_3$ atoms and are also modeled |
277 |
< |
as Lennard-Jones spheres. The total potential for the lipid is |
278 |
< |
represented by Equation \ref{eq:lipidModelPot}. |
275 |
> |
center. The magnitude of the dipole moment is 20.6 D, chosen to match |
276 |
> |
that of DPPC\cite{Cevc87}. The tail atoms are unified $\text{CH}_2$ |
277 |
> |
and $\text{CH}_3$ atoms and are also modeled as Lennard-Jones |
278 |
> |
spheres. The total potential for the lipid is represented by Equation |
279 |
> |
\ref{eq:lipidModelPot}. |
280 |
|
|
281 |
|
\begin{equation} |
282 |
< |
V_{\mbox{lipid}} = \overbrace{% |
283 |
< |
V_{\text{bend}}(\theta_{ijk}) +% |
284 |
< |
V_{\text{tors.}}(\phi_{ijkl})}^{bonded} |
285 |
< |
+ \overbrace{% |
286 |
< |
V_{\text{LJ}}(r_{i\!j}) + |
287 |
< |
V_{\text{dp}}(r_{i\!j},\Omega_{i},\Omega_{j})% |
268 |
< |
}^{non-bonded} |
282 |
> |
V_{\text{lipid}} = |
283 |
> |
\sum_{i}V_{i}^{\text{internal}} |
284 |
> |
+ \sum_i \sum_{j>i} \sum_{\alpha_i} |
285 |
> |
\sum_{\beta_j} |
286 |
> |
V_{\text{LJ}}(r_{\alpha_{i}\beta_{j}}) |
287 |
> |
+\sum_i\sum_{j>i}V_{\text{dp}}(r_{1_i,1_j},\Omega_{1_i},\Omega_{1_j}) |
288 |
|
\label{eq:lipidModelPot} |
289 |
|
\end{equation} |
290 |
+ |
where, |
291 |
+ |
\begin{equation} |
292 |
+ |
V_{i}^{\text{internal}} = |
293 |
+ |
\sum_{\text{bends}}V_{\text{bend}}(\theta_{\alpha\beta\gamma}) |
294 |
+ |
+ \sum_{\text{torsions}}V_{\text{tors.}}(\phi_{\alpha\beta\gamma\zeta}) |
295 |
+ |
+ \sum_{\alpha_i} \sum_{\beta_i > \alpha_i}V_{\text{LJ}} |
296 |
+ |
(r_{\alpha_i \beta_i}) |
297 |
+ |
\label{eq:lipidModelPotInternal} |
298 |
+ |
\end{equation} |
299 |
|
|
300 |
|
The non-bonded interactions, $V_{\text{LJ}}$ and $V_{\text{dp}}$, are |
301 |
|
the Lennard-Jones and dipole-dipole interactions respectively. For the |
302 |
< |
non-bonded potentials, only the bend and the torsional potentials are |
302 |
> |
bonded potentials, only the bend and the torsional potentials are |
303 |
|
calculated. The bond potential is not calculated, and the bond lengths |
304 |
|
are constrained via RATTLE.\cite{leach01:mm} The bend potential is of |
305 |
|
the form: |
306 |
|
\begin{equation} |
307 |
< |
V_{\text{bend}}(\theta_{ijk}) = k_{\theta}\frac{(\theta_{ijk} - \theta_0)^2}{2} |
307 |
> |
V_{\text{bend}}(\theta_{\alpha\beta\gamma}) |
308 |
> |
= k_{\theta}\frac{(\theta_{\alpha\beta\gamma} - \theta_0)^2}{2} |
309 |
|
\label{eq:bendPot} |
310 |
|
\end{equation} |
311 |
|
Where $k_{\theta}$ sets the stiffness of the bend potential, and $\theta_0$ |
312 |
|
sets the equilibrium bend angle. The torsion potential was given by: |
313 |
|
\begin{equation} |
314 |
< |
V_{\text{tors.}}(\phi_{ijkl})= c_1[1+\cos\phi_{ijkl}] |
315 |
< |
+ c_2 [1 - \cos(2\phi_{ijkl})] + c_3[1 + \cos(3\phi_{ijkl})] |
314 |
> |
V_{\text{tors.}}(\phi_{\alpha\beta\gamma\zeta}) |
315 |
> |
= c_1 [1+\cos\phi_{\alpha\beta\gamma\zeta}] |
316 |
> |
+ c_2 [1 - \cos(2\phi_{\alpha\beta\gamma\zeta})] |
317 |
> |
+ c_3 [1 + \cos(3\phi_{\alpha\beta\gamma\zeta})] |
318 |
|
\label{eq:torsPot} |
319 |
|
\end{equation} |
320 |
|
All parameters for bonded and non-bonded potentials in the tail atoms |
331 |
|
\subsection{Starting Configuration and Parameters} |
332 |
|
\label{sec:5x5Start} |
333 |
|
|
334 |
< |
Our first simulation was an array of 25 single chained lipids in a sea |
334 |
> |
\begin{figure} |
335 |
> |
\centering |
336 |
> |
\mbox{ |
337 |
> |
\subfigure[The starting configuration of the 25 lipid system.]{% |
338 |
> |
\label{fig:5x5Start}% |
339 |
> |
\includegraphics[width=70mm]{5x5-initial.eps}}\quad |
340 |
> |
\subfigure[The 25 lipid system at 6.27~ns.]{% |
341 |
> |
\label{fig:5x5Final}% |
342 |
> |
\includegraphics[width=70mm]{5x5-6.27ns.eps}} |
343 |
> |
} |
344 |
> |
\caption{Snapshots of the 25 lipid system. Carbon tail atoms are drawn in gray, the phospholipid head groups are colored blue, and the waters are scaled down for visibility. A box has been drawn around the periodic image.} |
345 |
> |
\end{figure} |
346 |
> |
|
347 |
> |
Our first simulation is an array of 25 single chain lipids in a sea |
348 |
|
of water (Figure \ref{fig:5x5Start}). The total number of water |
349 |
< |
molecules was 1386, giving a final of water concentration of 70\% |
350 |
< |
wt. The simulation box measured 34.5~$\mbox{\AA}$ x 39.4~$\mbox{\AA}$ |
351 |
< |
x 39.4~$\mbox{\AA}$ with periodic boundary conditions invoked. The |
352 |
< |
system was simulated in the micro-canonical (NVE) ensemble with an |
349 |
> |
molecules is 1386, giving a final of water concentration of 70\% |
350 |
> |
wt. The simulation box measures 34.5~$\mbox{\AA}$ x 39.4~$\mbox{\AA}$ |
351 |
> |
x 39.4~$\mbox{\AA}$ with periodic boundary conditions imposed. The |
352 |
> |
system is simulated in the micro-canonical (NVE) ensemble with an |
353 |
|
average temperature of 300~K. |
354 |
|
|
355 |
|
\subsection{Results} |
356 |
|
\label{sec:5x5Results} |
357 |
|
|
358 |
+ |
\begin{figure} |
359 |
+ |
\centering |
360 |
+ |
\subfigure[The self correlation of the phospholipid head groups. $g(r)$ is on the top, the bottom chart is the $g_\gamma(r)$.]{% |
361 |
+ |
\label{fig:5x5HHCorr}% |
362 |
+ |
\includegraphics[angle=-90,width=80mm]{all5x5-HEAD-HEAD.epsi}% |
363 |
+ |
} |
364 |
+ |
\subfigure[The $g(r)$ for the $\text{CH}_2$ molecules in the chain tails]{% |
365 |
+ |
\label{fig:5x5CCg}% |
366 |
+ |
\includegraphics[angle=-90,width=80mm]{all5x5-CH2-CH2.epsi}} |
367 |
+ |
\subfigure% |
368 |
+ |
[The pair correlations between the head groups and the water]{% |
369 |
+ |
\label{fig:5x5HXCorr}% |
370 |
+ |
\includegraphics[angle=-90,width=80mm]{all5x5-HEAD-X.epsi}} |
371 |
+ |
\caption{The pair correlation functions for the 25 lipid system} |
372 |
+ |
\end{figure} |
373 |
+ |
|
374 |
+ |
|
375 |
|
Figure \ref{fig:5x5Final} shows a snapshot of the system at |
376 |
< |
3.6~ns. Here it is exciting to note that the system has spontaneously |
377 |
< |
self assembled into a bilayer. Discussion of the length scales of the |
378 |
< |
bilayer will follow in this section. However, it is interesting to |
379 |
< |
note several qualitative properties of the system revealed by this |
380 |
< |
snapshot. First, is how the tail chains are tilted to the bilayer |
381 |
< |
normal. This is usually indicative of the gel ($L_{\beta'}$) |
382 |
< |
phase. Likely, the box size is too small for the bilayer to relax to |
383 |
< |
the fluid ($P_{\alpha}$ phase. Showing the need for a constant |
384 |
< |
pressure simulation versus the constant volume of the current |
324 |
< |
ensemble. |
376 |
> |
6.27~ns. Note that the system has spontaneously self assembled into a |
377 |
> |
bilayer. Discussion of the length scales of the bilayer will follow in |
378 |
> |
this section. However, it is interesting to note a key qualitative |
379 |
> |
property of the system revealed by this snapshot, the tail chains are |
380 |
> |
tilted to the bilayer normal. This is usually indicative of the gel |
381 |
> |
($L_{\beta'}$) phase. In this system, the box size is probably too |
382 |
> |
small for the bilayer to relax to the fluid ($P_{\alpha}$) phase. This |
383 |
> |
demonstrates a need for an isobaric-isothermal ensemble where the box |
384 |
> |
size may relax or expand to keep the system at 1~atm. |
385 |
|
|
386 |
< |
The trajectory of the simulation was analyzed using the pair-wise |
387 |
< |
radial distribution function, $g(r)$, which has the form: |
386 |
> |
The simulation was analyzed using the radial distribution function, |
387 |
> |
$g(r)$, which has the form: |
388 |
|
\begin{equation} |
389 |
< |
g(r) = \frac{V}{N_{\text{pairs}}}\langle \sum_{i} \sum_{j \neq i} |
389 |
> |
g(r) = \frac{V}{N_{\text{pairs}}}\langle \sum_{i} \sum_{j > i} |
390 |
|
\delta(|\mathbf{r} - \mathbf{r}_{ij}|) \rangle |
391 |
|
\label{eq:gofr} |
392 |
|
\end{equation} |
399 |
|
fluid contains no long range structure to contribute peaks in the |
400 |
|
number density. |
401 |
|
|
402 |
< |
For the species containing dipoles, a second pair wise distribution |
402 |
> |
For the species containing dipoles, a second pair-wise distribution |
403 |
|
function was used, $g_{\gamma}(r)$. It is of the form: |
404 |
|
\begin{equation} |
405 |
< |
g_{\gamma}(r) = foobar |
405 |
> |
g_{\gamma}(r) = \langle \sum_i \sum_{j>i} |
406 |
> |
(\cos \gamma_{ij}) \delta(| \mathbf{r} - \mathbf{r}_{ij}|) \rangle |
407 |
|
\label{eq:gammaofr} |
408 |
|
\end{equation} |
409 |
|
Where $\gamma_{ij}$ is the angle between the dipole of atom $j$ with |
414 |
|
distance. |
415 |
|
|
416 |
|
Figure \ref{fig:5x5HHCorr} shows the two self correlation functions |
417 |
< |
for the Head groups of the lipids. The first peak at 4.03~$\mbox{\AA}$ is the |
418 |
< |
nearest neighbor separation of the heads of two lipids. |
417 |
> |
for the Head groups of the lipids. The first peak in the $g(r)$ at |
418 |
> |
4.03~$\mbox{\AA}$ is the nearest neighbor separation of the heads of |
419 |
> |
two lipids. This corresponds to a maximum in the $g_{\gamma}(r)$ which |
420 |
> |
means that the two neighbors on the same leaf have their dipoles |
421 |
> |
aligned. The broad peak at 6.5~$\mbox{\AA}$ is the inter-surface |
422 |
> |
spacing. Here, there is a corresponding anti-alignment in the angular |
423 |
> |
correlation. This means that although the dipoles are aligned on the |
424 |
> |
same monolayer, the dipoles will orient themselves to be anti-aligned |
425 |
> |
on a opposite facing monolayer. With this information, the two peaks |
426 |
> |
at 7.0~$\mbox{\AA}$ and 7.4~$\mbox{\AA}$ are head groups on the same |
427 |
> |
monolayer, and they are the second nearest neighbors to the head |
428 |
> |
group. The peak is likely a split peak because of the small statistics |
429 |
> |
of this system. Finally, the peak at 8.0~$\mbox{\AA}$ is likely the |
430 |
> |
second nearest neighbor on the opposite monolayer because of the |
431 |
> |
anti-alignment evident in the angular correlation. |
432 |
|
|
433 |
+ |
Figure \ref{fig:5x5CCg} shows the radial distribution function for the |
434 |
+ |
$\text{CH}_2$ unified atoms. The spacing of the atoms along the tail |
435 |
+ |
chains accounts for the regularly spaced sharp peaks, but the broad |
436 |
+ |
underlying peak with its maximum at 4.6~$\mbox{\AA}$ is the |
437 |
+ |
distribution of chain-chain distances between two lipids. The final |
438 |
+ |
Figure, Figure \ref{fig:5x5HXCorr}, includes the correlation functions |
439 |
+ |
between the Head group and the SSD atoms. The peak in $g(r)$ at |
440 |
+ |
3.6~$\mbox{\AA}$ is the distance of closest approach between the two, |
441 |
+ |
and $g_{\gamma}(r)$ shows that the SSD atoms will align their dipoles |
442 |
+ |
with the head groups at close distance. However, as one increases the |
443 |
+ |
distance, the SSD atoms are no longer aligned. |
444 |
|
|
445 |
+ |
\section{Second Simulation: 50 randomly oriented lipids in water} |
446 |
+ |
\label{sec:r50} |
447 |
|
|
448 |
+ |
\subsection{Starting Configuration and Parameters} |
449 |
+ |
\label{sec:r50Start} |
450 |
|
|
451 |
< |
\section{Second Simulation: 50 randomly oriented lipids in water} |
451 |
> |
\begin{figure} |
452 |
> |
\centering |
453 |
> |
\mbox{ |
454 |
> |
\subfigure[The starting configuration of the 50 lipid system.]{% |
455 |
> |
\label{fig:r50Start}% |
456 |
> |
\includegraphics[width=70mm]{r50-initial.eps}}\quad |
457 |
> |
\subfigure[The 50 lipid system at 2.21~ns]{% |
458 |
> |
\label{fig:r50Final}% |
459 |
> |
\includegraphics[width=70mm]{r50-2.21ns.eps}} |
460 |
> |
} |
461 |
> |
\caption{Snapshots of the 50 lipid system} |
462 |
> |
\end{figure} |
463 |
|
|
464 |
< |
the second simulation |
464 |
> |
The second simulation consists of 50 single chained lipid molecules |
465 |
> |
embedded in a sea of 1384 SSD waters (54\% wt.). The lipids in this |
466 |
> |
simulation were started with random orientation and location (Figure |
467 |
> |
\ref{fig:r50Start} ) The simulation box measured 26.6~$\mbox{\AA}$ x |
468 |
> |
26.6~$\mbox{\AA}$ x 108.4~$\mbox{\AA}$ with periodic boundary conditions |
469 |
> |
imposed. The simulation was run in the NVE ensemble with an average |
470 |
> |
temperature of 300~K. |
471 |
|
|
472 |
+ |
\subsection{Results} |
473 |
+ |
\label{sec:r50Results} |
474 |
+ |
|
475 |
+ |
\begin{figure} |
476 |
+ |
\centering |
477 |
+ |
\subfigure[The self correlation of the phospholipid head groups.]{% |
478 |
+ |
\label{fig:r50HHCorr}% |
479 |
+ |
\includegraphics[angle=-90,width=80mm]{r50-HEAD-HEAD.epsi}% |
480 |
+ |
} |
481 |
+ |
\subfigure% |
482 |
+ |
[The pair correlations between the head groups and the water]{% |
483 |
+ |
\label{fig:r50HXCorr}% |
484 |
+ |
\includegraphics[angle=-90,width=80mm]{r50-HEAD-X.epsi}% |
485 |
+ |
} |
486 |
+ |
\subfigure[The $g(r)$ for the $\text{CH}_2$ molecules in the chain tails]{% |
487 |
+ |
\label{fig:r50CCg}% |
488 |
+ |
\includegraphics[angle=-90,width=80mm]{r50-CH2-CH2.epsi}} |
489 |
+ |
|
490 |
+ |
\caption{The pair correlation functions for the 50 lipid system} |
491 |
+ |
\end{figure} |
492 |
+ |
|
493 |
+ |
Figure \ref{fig:r50Final} is a snapshot of the system at 2.21~ns. Here |
494 |
+ |
we see that the system has already aggregated into several micelles |
495 |
+ |
and two are even starting to merge. It will be interesting to watch as |
496 |
+ |
this simulation continues what the total time scale for the micelle |
497 |
+ |
aggregation and bilayer formation will be, in Marrink's\cite{Marrink01} |
498 |
+ |
simulation, bilayer aggregation is predicted to occur around 10~ns. |
499 |
+ |
|
500 |
+ |
Figures \ref{fig:r50HHCorr}, \ref{fig:r50HXCorr}, and \ref{fig:r50CCg} are |
501 |
+ |
the same correlation functions for the random 50 simulation as for the |
502 |
+ |
previous simulation of 25 lipids. What is most interesting to note, is |
503 |
+ |
the high degree of similarity between the correlation functions |
504 |
+ |
between systems. Even though the 25 lipid simulation formed a bilayer |
505 |
+ |
and the random 50 simulation is still in the micelle stage, both have |
506 |
+ |
an inter-surface spacing of 6.5~$\mbox{\AA}$ with the same |
507 |
+ |
characteristic anti-alignment between surfaces. Not as surprising, is |
508 |
+ |
the consistency of the closest packing statistics between |
509 |
+ |
systems. Namely, a head-head closest approach distance of |
510 |
+ |
4~$\mbox{\AA}$, and similar findings for the chain-chain and |
511 |
+ |
head-water distributions as in the 25 lipid system. |
512 |
+ |
|
513 |
|
\section{Future Directions} |
514 |
|
|
515 |
+ |
Current simulations indicate that our model is a feasible one, however |
516 |
+ |
improvements will need to be made to allow the system to be simulated |
517 |
+ |
in the isobaric-isothermal ensemble. This will relax the system to an |
518 |
+ |
equilibrium configuration at room temperature and pressure allowing us |
519 |
+ |
to compare our model to experimental results. Also, we are in the |
520 |
+ |
process of parallelizing the code for an even greater speedup. This |
521 |
+ |
will allow us to simulate large enough systems to examine phenomena |
522 |
+ |
such as the ripple phase and drug molecule diffusion |
523 |
|
|
524 |
< |
\pagebreak |
525 |
< |
\bibliographystyle{achemso} |
526 |
< |
\bibliography{canidacy_paper} \end{document} |
524 |
> |
Once the work has been completed on the simulation engine, we will |
525 |
> |
then use it to explore the phase diagram for our model. By |
526 |
> |
characterizing how our model parameters affect the bilayer properties, |
527 |
> |
we will tailor our model to more closely match real biological |
528 |
> |
molecules. With this information, we will then incorporate |
529 |
> |
biologically relevant molecules into the system and observe their |
530 |
> |
transport properties across the membrane. |
531 |
> |
|
532 |
> |
\section{Acknowledgments} |
533 |
> |
|
534 |
> |
I would like to thank Dr.~J.~Daniel Gezelter for his guidance on this |
535 |
> |
project. I would also like to acknowledge the following for their help |
536 |
> |
and discussions during this project: Christopher Fennell, Charles |
537 |
> |
Vardeman, Teng Lin, Megan Sprague, Patrick Conforti, and Dan |
538 |
> |
Combest. Funding for this project came from the National Science |
539 |
> |
Foundation. |
540 |
> |
|
541 |
> |
\pagebreak |
542 |
> |
\bibliographystyle{achemso} |
543 |
> |
\bibliography{canidacy_paper} |
544 |
> |
\end{document} |