[gmx-users] undesired aggregation

[Ester Chiessi]

Hi.

I was trying to simulate the behaviour of N-isopropyl propionamide, CH3-CH2-CO-NH-CH-(CH3)2 (NIPPA) in aqueous solution.
This molecule has an inverted solubility behaviour with temperature and it displays a miscibility gap in water for temperatures higher than about 30°C.
Experiments show that at 293 K and 1 atm the water/NIPPA omogeneous solution phase is stable at any composition.

I performed a few NPT MD simulations of NIPPA in water SPC using the 45A3 force field, at different NIPPA concentrations, 293 K (Nose Hoover) and 1 atm (Parrinello Rahman), PME for electrostatic. Two typical systems contained [288 NIPPA molecules and 1680 water molecules] or [166 NIPPA and 4140 water molecules].
In the starting configuration the NIPPA molecules were randomly placed and solvated.

I was very surprised to see that after 2 ns ( time step 2 fs) the NIPPA molecules aggregated, in several ways (layer, cylindrical tube, spherical object) depending on the NIPPA concentration, whereas I was expecting a complete solubility in water.
In a further run at 273 K  ( a temperature about 30 degrees lower than the critical temperature) I found the same aggregation effect.

What do you think? Am I beyond the possibilties of the simulation?
Any comment is welcome. Thanks in advance

Ester

P.S. I found in the literature that the DPPC self-assembly was modeled in MD simulations ( de Vries et al JACS 2004) at 323 K, a temperature higher than the transition temperature of DPPC. Why did the Authors use a T where the organized phase is unstable? I was wondering since also for my system the temperature is a crucial parameter.





Ester Chiessi
Dipartimento di Scienze e Tecnologie Chimiche
Università di Roma Tor Vergata
Via della Ricerca Scientifica
00133 Roma
http://www.stc.uniroma2.it/cfmacro/cfmacroindex.htm
tel: *39*6*72594462
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