[gmx-users] fatty acid monolayer - surface pressure isotherm

[Pedro Alexandre Lapido Loureiro]

> Greetings all.
>  
> I have a fatty acid monolayer system consisting of 64 arachidic acid
> molecules in vacuum.  We are trying to simulate a Langmuir trough
> experiment
> in which the lateral pressure is monitored as a function of the packing
> density.  The x and y vectors of the box are initially set at values that
> provide a monolayer packing density of 18.5 A^2/molecule.  
>  
> At this packing density, I first perform a MD simulation for 1000 ps with
> position restraints on all atoms.  This is followed by another 1000 ps
> simulation in which the acid headgroups are position restrained while the
> aliphatic chains are not.  I then execute a 2000 ps simulation without
> restraints to allow equilibration followed by another 2000 ps simulation
> for
> the analysis.  Once this has been completed I take the resulting gromos
> file
> and use editconf to increase the box vectors so that a packing density of
> 19
> A^2/molecule is obtained.  Similar to the case of 18.5 A^2/molecule, a 2000
> ps equilibration MD run is completed for the 19 A^2/molecule system
> followed
> by another 2000 ps simulation for analysis.  The resulting gromos file from
> the second 2000 ps run for the 19 A^2/molecule system is then used via
> editconf to generate a box size yielding a packing density of 19.5
> A^2/molecule.  Again, a 2000 ps equilibration simulation is conducted
> followed by a 2000 ps analysis run.  This process is continued up to 25
> A^2/molecule.
>  
> As you probably expect, I am trying to generate a Pi-A (surface pressure
> vs.
> area) isotherm by manipulating the area per molecule and monitoring the
> pressure.  The problem I have encountered is that the diagonal pressure
> values (Pres-XX, Pres-YY, and Pres-ZZ) fluctuate quite drastically from one
> simulation at a given area per molecule to another.  For example when going
> from 18.5 to 19 A^2/molecule, I see an increase rather a decrease in the
> diagonal pressure elements.  When going from 19 to 19.5 A^2/molecule I
> observe a decrease in pressure followed by an increase from 19.5 to 20
> A^2/molecule.  This occurs at all packing densities up to 25 A^2/molecule.
>  
> The work described above, as noted, was performed in vacuum.  There has
> been
> some work cited in the literature in which an entire Pi-A diagram was
> obtained for a fatty acid monolayer.  Their system was in vacuum, however,
> they utilized a modified LJ potential in a plane just below the headgroups
> to simulate the interaction of the fatty acids with water (something my
> system is missing).  Alternatively, I have also looked at work by Alhstrom
> and Berendsen who simulated a lecithin monolayer (using a dual monolayer
> system) and monitored the surface pressure at a specified packing density
> which agreed quite well with experiments.  They employed a dual monolayer
> system with a layer of water in between the two monolayers (This was done
> to
> provide periodic boundary conditions in z).  I am in the process of
> conducting similar simulations, however, I first wanted to see if it is
> possible to find any success with a system in vacuum.
>  
> Finally, to the question.  Does anyone have any tips or suggestions? 
> Below,
> I have provided some of the system parameters in the event that anyone is
> interested.  For example, I only generate velocities and have a constrained
> start in the initial position restrained simulation.  The length of the box
> in the z-direction is 7.5 nm (fatty acid length = 2.6 nm) so that the
> headgroups do not interact with the chains.  The methyl and methylene
> groups
> are represented by CH3 and CH2 groups in the GROMACS topology file.
> Periodic boundary conditions are imposed in xyz.
>  
> Thank you,
> Roger McMullen
> 
Hi,

here are some tips:
-you could increase the simulation length. 
-if you are using Berendsen pressure bath, you could increase tau-p.
-you can simulate a bigger monolayer.
-what forcefield are you using?

Pedro.


--
Pedro Alexandre Lapido Loureiro
Laboratório de Física Biológica
Instituto de Biofísica
UFRJ
Brasil