[gmx-users] Melting temperature for the lipid bilayer

[Christopher Neale]

I presume you're heating your bilayer up slowly to assess the melting temperature? This method will give you an estimated melting temperature that is equal to the true melting temperature of that Hamiltonian in simulation only if you go infinitely slowly. At finite heating speeds, your observed melting temperature will be greater than or equal to the true simulation melting temperature.

You have to go slower than you might expect. For instance, even 0.15 K/ns overestimates the melting temperature (on average, of course).

On the other hand, if you're evaluating the Tm by running simulations at various constant temperatures and you call the Tm the temperature at which you see 50% gel and 50% liquid dynamically interconverting many times then your estimate should be correct for the Hamiltonian you selected (though I expect this approach to be way beyond current computational abilities).

________________________________________
From: gromacs.org_gmx-users-bounces at maillist.sys.kth.se <gromacs.org_gmx-users-bounces at maillist.sys.kth.se> on behalf of Mohsen Ramezanpour <ramezanpour.mohsen at gmail.com>
Sent: 22 November 2016 16:40:08
To: Discussion list for GROMACS users
Subject: Re: [gmx-users] Melting temperature for the lipid bilayer

It is worth mentioning that with Gromos53a7, the model was made by merging
head group and tails, I got T= 353 K for the right phase.

On Tue, Nov 22, 2016 at 2:37 PM, Mohsen Ramezanpour <
ramezanpour.mohsen at gmail.com> wrote:

> Hi Justin,
>
> Comments interspersed.
>
> On Tue, Nov 22, 2016 at 1:18 PM, Justin Lemkul <jalemkul at vt.edu> wrote:
>
>>
>>
>> On 11/21/16 3:28 PM, Mohsen Ramezanpour wrote:
>>
>>> Dear gromacs users,
>>>
>>> Running simulation on a lipid bilayer made by Charmm-GUI, there is about
>>> 24
>>> degrees difference between the reported temperature in Avanti and what I
>>> see in my simulations for getting a liquid disordered (L_disorder)
>>> bilayer.
>>>
>>>
>> How are you quantifying the transition in the simulation?
>
> Mainly by diffusion constant and diffusion graph. However, diffusion
> constant, bilayer thickness and area per lipid all have a shift.
> Order parameters are also higher for lower temperatures.
>
>>
>>
>> I am using all-atom Charmm36 FF for simulations.
>>>
>>
>> What is the lipid?
>>
> DSPS
>
>>
>> I know that it is not possible to get an exact match between experimental
>>> and simulation T values for L_alpha phase of bilayers, but I am not sure
>>> if
>>> these gap seems reasonable or there is something wrong in my
>>> simulations. I
>>> have checked the bonds in tails and apparently, everything looks fine
>>> with
>>> topology.
>>>
>>>
>> A gap of 24 degrees is quite substantial.  I would not consider that
>> outcome to be sufficiently accurate.
>>
>
> The melting temperature for DSPS is 68 centigrade (341 K). The
> liquid_disordered one is about 365 K (at least one with reasonable
> characteristics of liquid_disorder phase).
>
>>
>> However, based on this, doing simulation in L_alpha phase requires hight T
>>> values (close to 85 or 90 degrees of centigrade) which makes me worry
>>> about
>>> the water molecules in the system. It is a high temperature for water
>>> molecules.
>>>
>>>
>> Agreed, but TIP3P has a ton of other problems, so this is probably the
>> least of our concerns at the moment.
>>
>
>> -Justin
>>
>> Its worth mentioning that bilayer structural properties like lipid order
>>> parameters match available experimental data for these high T values.
>>>
>>> Please let me know your opinion.
>>>
>>> Cheers
>>> Mohsen
>>>
>>>
>> --
>> ==================================================
>>
>> Justin A. Lemkul, Ph.D.
>> Ruth L. Kirschstein NRSA Postdoctoral Fellow
>>
>> Department of Pharmaceutical Sciences
>> School of Pharmacy
>> Health Sciences Facility II, Room 629
>> University of Maryland, Baltimore
>> 20 Penn St.
>> Baltimore, MD 21201
>>
>> jalemkul at outerbanks.umaryland.edu | (410) 706-7441
>> http://mackerell.umaryland.edu/~jalemkul
>>
>> ==================================================
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>
>
>
> --
> *Rewards work better than punishment ...*
>



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