[gmx-users] LJ values for a fatty acid

[Eric Jakobsson]

Did you use the hydrocarbon parameters in

Chiu, S. W., Clark, M. M., Jakobsson, E., Subramaniam, S., and H. L. Scott. 
1999b. Optimizations of hydrocarbon chain interaction parameters: 
Application to the simulation of fluid phase lipid bilayers.  J. Phys. 
Chem.  B 103:6323-6327

I think these are the right united atom parameters because they match the 
specific volume of the fluid hydrocarbons over all chain lengths, and that 
provides just the right number of constraints to determine the parameters 
unambiguously.

Eric

At 11:09 AM 2/2/2004 -0500, you wrote:
>Hi all.
>
>I am simulating a monolayer system of eicosanoic acid (arachidic acid) on
>SPC water.  We are trying to simulate a Langmuir trough experiment in which
>lateral pressure is monitored as a function of the molecular packing density
>(perimeter area).  I have tried to use several different united atoms for
>the hydrocarbon tails, however, the resulting Pi-A diagram is always shifted
>to higher packing densities (e.g. from 18.5 A^2/molecule to 17.5
>A^2/molecule) in each case.   The shift in the Pi-A diagram is not feasible
>as the cross-sectional area of the fatty is 18.5 A^2/molecule.  Below are
>three of the systems.
>
>System 1:  GMX forcefield with CH2 and CH3 united atoms.
>System 2:  GMX forcefield with CP2 and CP3 united atoms.
>System 3:  GMX forcefield with LP2 and LP3 united atoms.
>
>My suspicion is that the shift in the Pi-A diagram has less to do with the
>tail interactions (in this case) and more to do with the headgroup
>interactions.  Does this occur because I am using the GMX carbonyl carbon,
>GMX carbonyl oxygen, GMX carboxyl oxygen, and GMX hydrogen for the headgroup
>atoms?  My headgroup has no net charge, but the headgroup atoms have the
>same partial charges as the lauroic acid topology file provided with the
>distribution.  I have plotted the LJ potentials of the tail atoms and
>headgroup atoms for the above three systems as well as for the ffOPLS and
>ffG43a2 forcefields.  It does not seem that the headgroup atoms for ffOPLS
>and ffG43a2 are drastically different.  Do I need to parameterize headgroup
>atoms for this particular application?  If so, does anyone have any
>suggestions?
>
>Thanks,
>Roger McMullen
>
>
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---------------------------------
Eric Jakobsson, Ph.D.
Professor, Department of Molecular and Integrative Physiology, and of 
Biochemistry
Senior Research Scientist, National Center for Supercomputing Applications
Professor, Beckman Institute for Advanced Science and Technology
4021 Beckman Institute, mc251
405 N. Mathews Avenue
University of Illinois, Urbana, IL 61801
ph. 217-244-2896       fax 217-244-2909

(Currently on leave to NIH to serve as Director of Center for 
Bioinformatics and Computational Biology at the National Institute of 
General Medical Sciences and Chair of the NIH Biomedical Information 
Science and Technology Initiative Consortium.  Usual schedule is four days 
a week at NIH and three days a week at Illlinois.)


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