--- trunk/xDissertation/md.tex 2008/03/24 21:32:11 3376 +++ trunk/xDissertation/md.tex 2008/04/16 21:56:34 3383 @@ -101,7 +101,8 @@ the ripple formation can be found in section \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdLipidModels.pdf} -\caption{Three different representations of DPPC lipid molecules, +\caption[Three different representations of DPPC lipid +molecules]{Three different representations of DPPC lipid molecules, including the chemical structure, an atomistic model, and the head-body ellipsoidal coarse-grained model used in this work.\label{mdfig:lipidModels}} @@ -243,13 +244,14 @@ actual parameters used in our simulations are given in \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/md2LipidModel.pdf} -\caption{The parameters defining the behavior of the lipid -models. $\sigma_h / d$ is the ratio of the head group to body diameter. -Molecular bodies had a fixed aspect ratio of 3.0. The solvent model -was a simplified 4-water bead ($\sigma_w \approx d$) that has been -used in other coarse-grained simulations. The dipolar strength -(and the temperature and pressure) were the only other parameters that -were varied systematically.\label{mdfig:lipidModel}} +\caption[The parameters defining the behavior of the lipid +models]{The parameters defining the behavior of the lipid +models. $\sigma_h / d$ is the ratio of the head group to body +diameter. Molecular bodies had a fixed aspect ratio of 3.0. The +solvent model was a simplified 4-water bead ($\sigma_w \approx d$) +that has been used in other coarse-grained simulations. The dipolar +strength (and the temperature and pressure) were the only other +parameters that were varied systematically.\label{mdfig:lipidModel}} \end{figure} To take into account the permanent dipolar interactions of the @@ -321,8 +323,8 @@ water). \begin{table*} \begin{minipage}{\linewidth} \begin{center} -\caption{Potential parameters used for molecular-scale coarse-grained -lipid simulations} +\caption{POTENTIAL PARAMETERS USED FOR MOLECULAR SCALE COARSE-GRAINED +LIPID SIMULATIONS} \begin{tabular}{llccc} \hline & & Head & Chain & Solvent \\ @@ -343,8 +345,8 @@ $\mu$ (Debye) & & varied & 0 & 0 \\ \end{minipage} \end{table*} -\section{Experimental Methodology} -\label{mdsec:experiment} +\section{Simulation Methodology} +\label{mdsec:simulation} The parameters that were systematically varied in this study were the size of the head group ($\sigma_h$), the strength of the dipole moment @@ -439,18 +441,18 @@ phase observed when $\sigma_h = 1.35 d$. \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdPhaseCartoon.pdf} -\caption{The role of the ratio between the head group size and the -width of the molecular bodies is to increase the local membrane -curvature. With strong attractive interactions between the head -groups, this local curvature can be maintained in bilayer structures -through surface corrugation. Shown above are three phases observed in -these simulations. With $\sigma_h = 1.20 d$, the bilayer maintains a -flat topology. For larger heads ($\sigma_h = 1.35 d$) the local -curvature resolves into a symmetrically rippled phase with little or -no interdigitation between the upper and lower leaves of the membrane. -The largest heads studied ($\sigma_h = 1.41 d$) resolve into an -asymmetric rippled phases with interdigitation between the two -leaves.\label{mdfig:phaseCartoon}} +\caption[ three phases observed in the simulations]{The role of the +ratio between the head group size and the width of the molecular +bodies is to increase the local membrane curvature. With strong +attractive interactions between the head groups, this local curvature +can be maintained in bilayer structures through surface corrugation. +Shown above are three phases observed in these simulations. With +$\sigma_h = 1.20 d$, the bilayer maintains a flat topology. For +larger heads ($\sigma_h = 1.35 d$) the local curvature resolves into a +symmetrically rippled phase with little or no interdigitation between +the upper and lower leaves of the membrane. The largest heads studied +($\sigma_h = 1.41 d$) resolve into an asymmetric rippled phases with +interdigitation between the two leaves.\label{mdfig:phaseCartoon}} \end{figure} Sample structures for the flat ($\sigma_h = 1.20 d$), symmetric @@ -476,12 +478,10 @@ for PE head groups. for PE head groups. \begin{table*} -\begin{minipage}{\linewidth} \begin{center} -\caption{Phase, bilayer spacing, area per lipid, ripple wavelength -and amplitude observed as a function of the ratio between the head -beads and the diameters of the tails. Ripple wavelengths and -amplitudes are normalized to the diameter of the tail ellipsoids.} +\caption{PHASE, BILAYER SPACING, AREA PER LIPID, RIPPLE WAVELENGTH AND +AMPLITUDE OBSERVED AS A FUNCTION OF THE RATIO BETWEEN THE HEAD BEADS +AND THE DIAMETERS OF THE TAILS} \begin{tabular}{lccccc} \hline $\sigma_h / d$ & type of phase & bilayer spacing (\AA) & area per @@ -492,9 +492,14 @@ lipid (\AA$^2$) & $\lambda / d$ & $A / d$\\ 1.35 & symmetric ripple & 42.9 & 51.7 & 17.2 & 2.2 \\ 1.41 & asymmetric ripple & 37.1 & 63.1 & 15.4 & 1.5 \\ \end{tabular} +\begin{minipage}{\linewidth} +%\centering +\vspace{2mm} + Ripple wavelengths and amplitudes are normalized to the diameter of + the tail ellipsoids. \label{mdtab:property} -\end{center} \end{minipage} +\end{center} \end{table*} The membrane structures and the reduced wavelength $\lambda / d$, @@ -510,14 +515,16 @@ likely to underestimate of the true amplitudes. \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdTopDown.pdf} -\caption{Top views of the flat (upper), symmetric ripple (middle), -and asymmetric ripple (lower) phases. Note that the head-group -dipoles have formed head-to-tail chains in all three of these phases, -but in the two rippled phases, the dipolar chains are all aligned {\it -perpendicular} to the direction of the ripple. Note that the flat -membrane has multiple vortex defects in the dipolar ordering, and the -ordering on the lower leaf of the bilayer can be in an entirely -different direction from the upper leaf.\label{mdfig:topView}} +\caption[Top views of the flat, symmetric ripple, and asymmetric +ripple phases]{Top views of the flat (upper), symmetric ripple +(middle), and asymmetric ripple (lower) phases. Note that the +head-group dipoles have formed head-to-tail chains in all three of +these phases, but in the two rippled phases, the dipolar chains are +all aligned {\it perpendicular} to the direction of the ripple. Note +that the flat membrane has multiple vortex defects in the dipolar +ordering, and the ordering on the lower leaf of the bilayer can be in +an entirely different direction from the upper +leaf.\label{mdfig:topView}} \end{figure} The orientational ordering in the system is observed by $P_2$ order @@ -582,9 +589,11 @@ rapidly decreasing $P_2$ ordering for the molecular bo \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdRP2.pdf} -\caption{The $P_2$ order parameters for head groups (circles) and -molecular bodies (squares) as a function of the ratio of head group -size ($\sigma_h$) to the width of the molecular bodies ($d$). \label{mdfig:rP2}} +\caption[The $P_2$ order parameters as a function of the ratio of head group +size to the width of the molecular bodies]{The $P_2$ order parameters +for head groups (circles) and molecular bodies (squares) as a function +of the ratio of head group size ($\sigma_h$) to the width of the +molecular bodies ($d$). \label{mdfig:rP2}} \end{figure} In addition to varying the size of the head groups, we studied the @@ -633,8 +642,9 @@ vector. \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdSP2.pdf} -\caption{The $P_2$ order parameters for head group dipoles (a) and -molecular bodies (b) as a function of the strength of the dipoles. +\caption[The $P_2$ order parameters as a function of the strength of +the dipoles.]{The $P_2$ order parameters for head group dipoles (a) +and molecular bodies (b) as a function of the strength of the dipoles. These order parameters are shown for four values of the head group / molecular width ratio ($\sigma_h / d$). \label{mdfig:sP2}} \end{figure} @@ -665,10 +675,11 @@ the ripple to gel ($L_{\beta'}$) phase transition. \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdTP2.pdf} -\caption{The $P_2$ order parameters for head group dipoles (a) and -molecular bodies (b) as a function of temperature. -These order parameters are shown for four values of the head group / -molecular width ratio ($\sigma_h / d$).\label{mdfig:tP2}} +\caption[The $P_2$ order parameters as a function of temperature]{The +$P_2$ order parameters for head group dipoles (a) and molecular bodies +(b) as a function of temperature. These order parameters are shown +for four values of the head group / molecular width ratio ($\sigma_h / +d$).\label{mdfig:tP2}} \end{figure} Fig. \ref{mdfig:phaseDiagram} shows a phase diagram for the model as a @@ -687,10 +698,10 @@ dipole region of this diagram. \begin{figure} \centering \includegraphics[width=\linewidth]{./figures/mdPhaseDiagram.pdf} -\caption{Phase diagram for the simple molecular model as a function -of the head group / molecular width ratio ($\sigma_h / d$) and the -strength of the head group dipole moment -($\mu$).\label{mdfig:phaseDiagram}} +\caption[Phase diagram for the simple molecular model]{Phase diagram +for the simple molecular model as a function of the head group / +molecular width ratio ($\sigma_h / d$) and the strength of the head +group dipole moment ($\mu$).\label{mdfig:phaseDiagram}} \end{figure} We have computed translational diffusion constants for lipid molecules @@ -739,20 +750,11 @@ times that are too fast when compared with experimenta times that are too fast when compared with experimental measurements. \begin{table*} -\begin{minipage}{\linewidth} \begin{center} -\caption{Fit values for the rotational correlation times for the head -groups ($\tau^h$) and molecular bodies ($\tau^b$) as well as the -translational diffusion constants for the molecule as a function of -the head-to-body width ratio. All correlation functions and transport -coefficients were computed from microcanonical simulations with an -average temperture of 300 K. In all of the phases, the head group -correlation functions decay with an fast librational contribution ($12 -\pm 1$ ps). There are additional moderate ($\tau^h_{\rm mid}$) and -slow $\tau^h_{\rm slow}$ contributions to orientational decay that -depend strongly on the phase exhibited by the lipids. The symmetric -ripple phase ($\sigma_h / d = 1.35$) appears to exhibit the slowest -molecular reorientation.} +\caption{FIT VALUES FOR THE ROTATIONAL CORRELATION TIMES FOR THE HEAD +GROUPS ($\tau^h$) AND MOLECULAR BODIES ($\tau^b$) AS WELL AS THE +TRANSLATIONAL DIFFUSION CONSTANTS FOR THE MOL\-E\-CULE AS A FUNCTION +OF THE HEAD-TO-BODY WIDTH RATIO} \begin{tabular}{lcccc} \hline $\sigma_h / d$ & $\tau^h_{\rm mid} (ns)$ & $\tau^h_{\rm @@ -763,9 +765,20 @@ slow} (\mu s)$ & $\tau^b (\mu s)$ & $D (\times 10^{-11 1.35 & $3.2$ & $4.0$ & $0.9$ & $3.42(1)$ \\ 1.41 & $0.3$ & $23.8$ & $6.9$ & $7.16(1)$ \\ \end{tabular} +\begin{minipage}{\linewidth} +%\centering +\vspace{2mm} +All correlation functions and transport coefficients were computed +from microcanonical simulations with an average temperture of 300 K. +In all of the phases, the head group correlation functions decay with +an fast librational contribution ($12 \pm 1$ ps). There are +additional moderate ($\tau^h_{\rm mid}$) and slow $\tau^h_{\rm slow}$ +contributions to orientational decay that depend strongly on the phase +exhibited by the lipids. The symmetric ripple phase ($\sigma_h / d = +1.35$) appears to exhibit the slowest molecular reorientation. \label{mdtab:relaxation} -\end{center} \end{minipage} +\end{center} \end{table*} \section{Discussion}