For the RESP server you can do a single conformation as you suggest, and
hope that the point charges are reasonable for both. I believe you can
also give multiple structures, in which case something like the average is
used for the forcefield, but I haven't done that, so can't comment with
certainty on how it works.
In terms of the aging. The trans-to-cis kinetics for something like that
should be really fast (sub-second timescale for sure), as evidenced by the
fact that you are able to see it at all on your presumably ns long
timescale MD. So, if the trans conformation is dominant when you make a
fresh solution and measure it even a minute later, that really strongly
suggests to me that the trans-conformation is the most stable one for the
unmodified molecule, and some kind of modification is happening to cause
the cis-conformation to later become more stable. This is pretty common
with organic molecules, whether from oxidation of UV light induced
changes. I'd be extraordinarily shocked if the aging you mention was
actually just because of a large kinetic barrier between the two
conformers. I guess that's just my opinion.
~Aron
On Thu, Oct 18, 2012 at 8:05 PM, bio lab <biolabcs.gmail.com> wrote:
> > the Guassian methods employed by RESP will allow for accurate
> > partial charge distribution as a result of the two rings being nearby
>
> Ok, I will do it.
>
> Please notice that I am interpreting your quoted text as "will give
> accurate charges as a result of the two rings being nearby in any of the
> two conformers", not "being nearby in the cis form".
>
> In other words, I am modeling the molecule in the trans form, and then
> assigning the charges to this conformation. And I am assuming that I would
> obtain equal (or very similar) charges in the case I use the cis
> conformation as the structure to assign the charges to.
>
> Because if the two situations lead to quite different charges, then I would
> be in trouble, as I cannot obviously modify the charges during the MD
> simulation. ...Anyway, I will test it too.
>
> > what this tells me is that the trans-form of the molecule is more stable
> > as your MD suggests, and it is some kind of modification to the molecule
> > (likely oxidation) that changes the structure
>
> No, there is no chemical modification of the molecule reported in
> literature, AFAIK. The cis and trans form are conformational rotamers, just
> like in the Wikipedia figure in my original post.
>
> Thanks a lot for your help.
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--
Aron Broom M.Sc
PhD Student
Department of Chemistry
University of Waterloo
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Received on Thu Oct 18 2012 - 19:00:05 PDT
