Maxim Fedorov wrote:
Thank you for your message, but ...
It doesn't seem to answer for my particular question -
probably I should go in more details.
I am investigating the charge-driven unfolding of protonated polypetides
like poly-L-Lysine and other compbinations of
charged/neutral residials.
The poly-L-Lysine with ambient conditions (pH~7) is protonated,
therefore, it is quickly unfolds from an initial helical structure
(which it has with pH >10) due to repelling of the side-chain positive
charges
I am intersting in the final conformation and unfolding dynamics of such
system.
But ...
If I add some counterions into the box and allow them to approach
my polypeptide they screen the charges and it stabilises the initial
conformation. This effect of ion stabilisation is well known and it is
an intersting topic, but in this particular case I don't need this - I
want to unfold my structure as it happens in experiment (our
experimentators have some unpublished data and there several classical
papers of Sheraga and others about the pH driven unfolding of
poly-L-Lysine, which were published in 70ths).
Charge separation is expensive energetically. Even though the unfolding
may be charge-driven, it simply isn't going to be true that there are no
counter-ions around in an experiment. Thus to have a realistic
simulation you need to have some counter-ions (preferably at a realistic
ionic strength)... but you say that "some counter-ions" stabilize the
initial conformation. If you were adding charges to neutralize the
system, you could try adding fewer counter-ions as a compromise.
So, I want to get rid of this screening effect by placing the ions
somethere far from the solute. But I would like to take it in some smart
way, to reduce some possible artefacts (see the points 1) and 2) in my
previous letter.
Whatever you do to change the system is going to be a non-physical
system. You say the physically realistic simulation doesn't follow
experiment. Perhaps you should have a look at your simulation protocol,
particularly the electrostatics treatment, to see if the fault might be
there, rather than assume it's only the charge-screening effect. MM
force fields are a model of reality, often applied in MD using further
approximations to real physics, so there are multiple sources of problems.
The system seems to be well (say, more or less :-)) equilibrated - I
checked several geometrical and energetical properties they are fine.
And it doesn't want to unfold even for 50 ns - due to the charge
screening by ions. And it is not because the simulation time is still
too short - I made a good sampling of the phase space
with some Replica - Exchange run - in case of ions
the system has a global minumum in folded conformation.
In case of cut-off and absence of ions it doesn't have even local
minimum in the folded conformation - which correspond to the
experimental reslults and #common sense'.
If I don't use the ions and PME (simply using the cut-off) - the results
are more close to experiment - it quickly unfolds as it should be.
But I have to use the PME because for more comples systems the cut-off
doesn't suit our tasks.
How long does the experimental system take to unfold?
Mark
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