[gmx-users] What's the point of adding ions to neutralize the system?

[Smith, Micholas D.]

Just to add to Justin's third point; hydrophobic interactions can also be affected by the presence of salt in the system (read: Hoffemeiser effect). A better question would be not why do we add salt, but why do we only add enough to neutralize? And why do studies typically only add NaCl to neutralize the system. 

===================
Micholas Dean Smith, PhD.
Post-doctoral Research Associate
University of Tennessee/Oak Ridge National Laboratory
Center for Molecular Biophysics

________________________________________
From: gromacs.org_gmx-users-bounces at maillist.sys.kth.se <gromacs.org_gmx-users-bounces at maillist.sys.kth.se> on behalf of Justin Lemkul <jalemkul at vt.edu>
Sent: Thursday, August 20, 2015 4:44 PM
To: gmx-users at gromacs.org
Subject: Re: [gmx-users] What's the point of adding ions to neutralize the system?

On 8/20/15 4:15 PM, Victor Rosas Garcia wrote:
> You said it yourself: to neutralize the net charge of the system.
>

Great answer: "you neutralize the system in order to neutralize the system" :)

>> Since the ion added is usually very far away from the protein (or molecule
>> of interest), I don't see how it can affect the results of the molecular
>> dynamics simulation. So why is it so important to add ions to neutralize
>> the net-charge of the system?

One effect is practical - in the absence of a net-neutral system, PME applies a
neutralizing background charge, which for a homogeneous medium, is an adequate
approximation.  But this is a very bad approximation in many other cases.  There
are even relevant inhomogeneities at protein surfaces, though the effects are
not as profound as, say, in a membrane system.

Second point - life exists in salt.  We like to model things that are
biologically relevant.

Third and final point - it is fundamentally incorrect to assume that there will
be no effect of the ions.  DNA dynamics are exquisitely sensitive to ionic
concentration, for instance (though additive models are not nearly as responsive
as polarizable ones, there are some effects).  Protein salt bridges are affected
by ionic concentration, even if the interactions are not direct, there is a
change in ionic environment within a hydration shell or two and that will affect
dynamics.  And even if ions are far away from the solute to start, they don't
stay there...

a-Justin

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--
==================================================

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
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