[gmx-users] Re: Hexamer problem/ The N and C termini of peptides

Justin A. Lemkul jalemkul at vt.edu
Thu Jul 7 20:55:27 CEST 2011

errabah fatima ezzahra wrote:
> *
> hello
> Does anybody knows why The N and C termini of peptides can be 
> neutralized before running simulation of peptides  ?? i read this some 
> where in a research paper , they dont say why but they do that using 
> acetyl amine groops. Probably to evoid the repulsive interactions 
> between the end of the peptides , please correct if i am wrong as my 
> chemistry is not good, my one trimer is  made of 3 peptides that ends 
> with GLU LEU LEU and the other trimer ends with is LEU GLU LEU , should 
> i worry about neutralizing the c and N termini

Capping groups can be added to termini using different software programs.  There 
is no utility in Gromacs to do so.  Once built, choose 'None' for the termini 
when running pdb2gmx to build a normal peptide bond between the terminal 
residue(s) and capping group(s).

Such groups are often added (1) if artificial terminal attraction or repulsion 
should be avoided or (2) if the modeled peptide is a segment of a longer 
polypeptide or protein, in which case such integral charges are an incorrect 
representation of reality.

>  Also what the important of running 20 simulations  of the same 6 
> peptides ???. is that to compare the 20 simulation results and see **** 
> which give better simulation sorry if my question are something i should 
> know. i am trying to find how to get six peptides to self assemble to a 
> hexamer  . i will really appreciate your answers.

Running multiple simulations of a given configuration (using different starting 
velocities) gives better sampling.  You can't conclude anything from a single 
trajectory.  Just as you wouldn't run a single experiment on the bench and call 
it conclusive, so too is it true of simulations - if you run just one 
simulation, how do you know you're not seeing the one outlier in the data set?



Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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