[gmx-users] lipid monolayer isotherm

[Hector Mtz-Seara]

Hi everyone,

I'm making an isotherm (surface pressure vs are/mol) of a monolayer of
two different compounds. These compounds are Arachidic Acid ( a fatty
acid) and an azobenzene acid. I have to parametrise both using PRODRG
and then modifying by hand to have a behavior similar to what I expect (
maybe this is not the correct way but this is the only way I know, If
someone has optimized the parameters any fatty acid I'll be glad if
files can be sent to me to compare). The force field I'm using is the
Gromacs force field (ffgmx), just because when I started PRODRG only
support this force field and also because in gromacs manual advise that
for long chain molecules is better this force field than the newer one.
I know that PRODRG now support the new force field and maybe is time to
change, but I'm not sure if anyone has some experience in fatty acid
monolayers with the new force field let me know your opinion.

My system consist in :

VACUUM
----------------
FATTY ACID
---------------
SPC water
---------------
FATTY ACID
---------------
VACUUM

I've been running several test dynamics and I'm really confused. In
general the surface tensions that gromacs gives me are much higher than
experimental ones, many times more than 3 times (I know that to have in
mN/m units I have to divide by 10 and by two for the two monolayers) and
it means that they are higher than experimental water, 72mNm( someone
knows the surface tension of the SPC water that is in gromacs??), but
what is really annoying me is that in theory the surface tension of a
fatty acid monolayer goes down when the concentration of the lipid in
the monolayer goes up but the results seem to be the opposite, and I say
"seems" because sometimes the same dynamic brings me to different
equilibrium surface tension or area/lipid ( it happens in both ensembles
I use TNAhz and TN(st)hz). If anyone can give me any clue why I been
having all this problems and if there's any way to solve them, I'll be
so pleased.

thank you in avance, Hector.

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Hector Martinez-Seara Monne

Universidad de Barcelona
Dept. Quimica Fisica

hseara at netscape.net
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