--- trunk/mattDisertation/collections/initalSimulations.tex 2003/09/08 20:58:12 752 +++ trunk/mattDisertation/collections/initalSimulations.tex 2003/09/09 16:35:31 753 @@ -10,31 +10,33 @@ side spacing of $\approx\mbox{8\AA}$ (Fig.~\ref{fig:25 The 25 lipid system (scheme I) was initially started from an ordered configuration. The lipids were aligned in a 5x5 square with a side by -side spacing of $\approx\mbox{8\AA}$ (Fig.~\ref{fig:25lipidInit}) The +side spacing of $\sim \mbox{8\AA}$ (Fig.~\ref{fig:25lipidInit}) The box was then filled with water in an FCC lattice. The temperature was -then equilibrated to 300~K by resampling the atomic velocitiues from a +then equilibrated to 300~K by resampling the atomic velocities from a Maxwell-Boltzmann distribution. Once the temperature had been set, the -system was allowed to progress for $\approx 20$~ns. +system was allowed to progress for $\sim 20$~ns. \begin{figure} -\parbox{\linewidth}{Here is the initial 25 lipid configuration} -\caption[The initial configuration of scheme I]{The starting configuration of scheme I.} -\label{fig:25lipidinit} +\centering +\includegraphics[width=\linewidth]{5x5-initial_pre.eps} +\caption[The initial configuration of scheme I]{The starting configuration of scheme I after thermalization.} +\label{fig:25lipidInit} \end{figure} The 50 lipid system (scheme II) was initially started from a randomly -generarted configuration. A system box was initially filled with water +generated configuration. A system box was initially filled with water in an FCC lattice. The 50 lipid molecules were then sequentially -placed in the box with a random orientaion and position. If the lipid +placed in the box with a random orientation and position. If the lipid did not overlap with a previously placed one, its position was accepted, and the next lipid was placed. Once all 50 lipids were -positioned, any waters overlaping with the lipids were removed +positioned, any waters overlapping with the lipids were removed (Fig.~\ref{fig:r50Init}. The system was then equilibrated to 300~K in the same manner as scheme I. \begin{figure} -\parbox{\linewidth}{Here is the initial random 50 lipid configuration} -\caption[The initial configuration of scheme II]{The starting configuration of scheme II.} +\centering +\includegraphics[width=\linewidth]{r50-initial_pre.eps} +\caption[The initial configuration of scheme II]{The starting configuration of scheme II after thermalization} \label{fig:r50Init} \end{figure} @@ -42,13 +44,112 @@ Scheme I was found to split into two leaves within the \label{subSec:initSimsResults} Scheme I was found to split into two leaves within the first -5~ns. However, as the simulation progressed, it became appaarent that +5~ns. However, as the simulation progressed, it became apparent that the system was frustrated in its confined box. After 15~ns, the two -leaveswere still unable to form a bilayer. Instead, thew were only +leaves were still unable to form a bilayer. Instead, thew were only able to form two skewed layers (Fig.~\ref{fig:25lipidFinal}). -\begin{figure} -\parbox{\linewidth}{Here is the final 25 lipid configuration} -\caption[The final configuration of scheme I]{The final configuration of scheme I.} +\begin{figure} +\centering +\includegraphics[width=\linewidth]{5x5-montage_pre.eps} +\caption[The time evolution of scheme I]{The time evolution of scheme I.} \label{fig:25lipidFinal} \end{figure} + +Scheme II behaved similarly. Within the first 2~ns the system +aggregated into lipid and water regions. After $\sim 5$~ns the lipid +regions had become micelles. Over the course of the next 10~ns the +micelles underwent little change. At 15~ns simulation time, we switched +the ensemble to isobaric-isothermal, NPT. The new integrator had just +been written, and allowed form isometric scaling of the simulation +box. The new integrator allowed the system to relax more, and over the +next 10~ns several of the micelles started to merge. However, no +bilayer was formed. Fig.~\ref{fig:r50Final} shows the progression of +the simulation. + +\begin{figure} +\centering +\includegraphics[width=\linewidth]{r50-montage_pre.eps} +\caption[The time evolution of scheme II]{The time evolution of scheme II.} +\label{fig:25lipidFinal} +\end{figure} + +\begin{figure} +\centering + \subfigure[The self correlation of the phospholipid head groups. $g_{\text{Head,Head}}(r)$ is on the top, the bottom chart is the $\langle \cos \omega \rangle_{\text{Head,Head}}(r)$.]{% + \label{fig:5x5HHCorr}% + \includegraphics[angle=-90,width=0.49\linewidth]{all5x5-HEAD-HEAD.epsi}% + } + \subfigure[The $g_{\text{CH}_2\text{,CH}_2}(r)$ for the tail chains]{% + \label{fig:5x5CCg}% + \includegraphics[angle=-90,width=0.49\linewidth]{all5x5-CH2-CH2.epsi}} + \subfigure% + [The pair correlations between the head groups and the water]{% + \label{fig:5x5HXCorr}% + \includegraphics[angle=-90,width=0.49\linewidth]{all5x5-HEAD-X.epsi}} +\caption[Scheme I pair correlations]{The pair correlation functions for scheme I.} +\label{fig:5x5PairCorrs} +\end{figure} + +Structural information about the simulations were calculated via the +equations in Sec.~\ref{sec:analysis}. Fig.~\ref{fig:5x5HHCorr} shows +$g_{\text{Head,Head}}(r)$ and $\langle \cos \omega +\rangle_{\text{Head,Head}}(r)$ for scheme II. The +first peak in the $g(r)$ at 4.03~$\mbox{\AA}$ is the nearest neighbor +separation of the heads of two lipids. This corresponds to a maximum +in the $\langle \cos \omega \rangle(r)$ which means that the two +neighbors on the same leaf have their dipoles aligned. The broad peak +at 6.5~$\mbox{\AA}$ is the inter-surface spacing. Here, there is a +corresponding anti-alignment in the angular correlation. This means +that although the dipoles are aligned on the same monolayer, the +dipoles will orient themselves to be anti-aligned on a opposite facing +monolayer. With this information, the two peaks at 7.0~$\mbox{\AA}$ +and 7.4~$\mbox{\AA}$ are head groups on the same monolayer, and they +are the second nearest neighbors to the head group. The peak is likely +a split peak because of the small statistics of this system. Finally, +the peak at 8.0~$\mbox{\AA}$ is likely the second nearest neighbor on +the opposite monolayer because of the anti-alignment evident in the +angular correlation. + +Fig.~\ref{fig:5x5CCg} shows the radial distribution function for the +$\text{CH}_2$ unified atoms. The spacing of the atoms along the tail +chains accounts for the regularly spaced sharp peaks, but the broad +underlying peak with its maximum at 4.6~$\mbox{\AA}$ is the +distribution of chain-chain distances between two lipids. The final +figure, Fig.~\ref{fig:5x5HXCorr}, includes the correlation functions +between the Head group and the SSD atoms. The peak in $g(r)$ at +3.6~$\mbox{\AA}$ is the distance of closest approach between the two, +and $\langle \cos \omega \rangle(r)$ shows that the SSD atoms will +align their dipoles with the head groups at close distance. However, +as one increases the distance, the SSD atoms are no longer aligned. + +\begin{figure} +\centering + \subfigure[The self correlation of the phospholipid head groups.]{% + \label{fig:r50HHCorr}% + \includegraphics[angle=-90,width=0.49\linewidth]{r50-HEAD-HEAD.epsi}% + } + \subfigure% + [The pair correlations between the head groups and the water]{% + \label{fig:r50HXCorr}% + \includegraphics[angle=-90,width=0.49\linewidth]{r50-HEAD-X.epsi}% + } + \subfigure[The $g_{\text{CH}_2\text{,CH}_2}(r)$ for the tail chains]{% + \label{fig:r50CCg}% + \includegraphics[angle=-90,width=0.49\linewidth]{r50-CH2-CH2.epsi}} + +\caption[Scheme II pair correlations]{The pair correlation functions for scheme II} +\label{fig:r50PairCorrs} +\end{figure} + +Fig.~ \ref{fig:r50HHCorr}, \ref{fig:r50HXCorr}, and \ref{fig:r50CCg} +are the same correlation functions for scheme II as for scheme I. What +is most interesting to note, is the high degree of similarity between +the correlation functions between systems. Even though scheme I formed +a skewed bilayer and scheme II is still in the micelle stage, both +have an inter-surface spacing of 6.5~$\mbox{\AA}$ with the same +characteristic anti-alignment between surfaces. Not as surprising, is +the consistency of the closest packing statistics between +systems. Namely, a head-head closest approach distance of +4~$\mbox{\AA}$, and similar findings for the chain-chain and +head-water distributions as in scheme I.