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gmx-helix - Calculate basic properties of alpha helices


gmx helix [-s [<.tpr>]] [-n [<.ndx>]] [-f [<.xtc/.trr/...>]]
[-cz [<.gro/.g96/...>]] [-b <time>] [-e <time>]
[-dt <time>] [-[no]w] [-r0 <int>] [-[no]q] [-[no]F]
[-[no]db] [-[no]ev] [-ahxstart <int>] [-ahxend <int>]


gmx helix computes all kinds of helix properties. First, the peptide is checked to find
the longest helical part, as determined by hydrogen bonds and phi/psi angles. That bit is
fitted to an ideal helix around the z-axis and centered around the origin. Then the
following properties are computed:

· Helix radius (file radius.xvg). This is merely the RMS deviation in two dimensions
for all Calpha atoms. it is calculated as sqrt((sum_i (x^2(i)+y^2(i)))/N) where N is
the number of backbone atoms. For an ideal helix the radius is 0.23 nm.

· Twist (file twist.xvg). The average helical angle per residue is calculated. For an
alpha-helix it is 100 degrees, for 3-10 helices it will be smaller, and for 5-helices
it will be larger.

· Rise per residue (file rise.xvg). The helical rise per residue is plotted as the
difference in z-coordinate between Calpha atoms. For an ideal helix, this is 0.15 nm.

· Total helix length (file len-ahx.xvg). The total length of the helix in nm. This is
simply the average rise (see above) times the number of helical residues (see below).

· Helix dipole, backbone only (file dip-ahx.xvg).

· RMS deviation from ideal helix, calculated for the Calpha atoms only (file

· Average Calpha - Calpha dihedral angle (file phi-ahx.xvg).

· Average phi and psi angles (file phipsi.xvg).

· Ellipticity at 222 nm according to Hirst and Brooks.


Options to specify input files:

-s [<.tpr>] (topol.tpr)
Portable xdr run input file

-n [<.ndx>] (index.ndx)
Index file

-f [<.xtc/.trr/...>] (traj.xtc)
Trajectory: xtc trr cpt gro g96 pdb tng

Options to specify output files:

-cz [<.gro/.g96/...>] (zconf.gro)
Structure file: gro g96 pdb brk ent esp

Other options:

-b <time> (0)
First frame (ps) to read from trajectory

-e <time> (0)
Last frame (ps) to read from trajectory

-dt <time> (0)
Only use frame when t MOD dt = first time (ps)

-[no]w (no)
View output .xvg, .xpm, .eps and .pdb files

-r0 <int> (1)
The first residue number in the sequence

-[no]q (no)
Check at every step which part of the sequence is helical

-[no]F (yes)
Toggle fit to a perfect helix

-[no]db (no)
Print debug info

-[no]ev (no)
Write a new 'trajectory' file for ED

-ahxstart <int> (0)
First residue in helix

-ahxend <int> (0)
Last residue in helix

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