Units of SASA generated from GROMACS gmx_sasa?

Question

I'm wanting to use gmx sasa. I've done this on some residues I'm interested in analyzing. When I look at the output, I expect some value of $\pu{nm^2}$ or maybe $\pu{\unicode[.8,0]{x212B}^2}$. Instead, the .xvg file that is output has the following:

#   gmx_mpi sasa -f md_nojump.xtc -s md.tpr -n dyes.ndx -o gromacsSASA.xvg -odg gromacsSASADeltaG.xvg -or gromacsSASAAverage.xvg -tv gromacsSASAVolume.xvg -e 2010000 -pbc -rmpbc -probe 0.14 -ndots 24
# gmx sasa is part of G R O M A C S:
#
# Glycine aRginine prOline Methionine Alanine Cystine Serine
#
@    title "Solvent Accessible Surface"
@    xaxis  label "Time (ps)"
@    yaxis  label "Area (nm\S2\N)"
@TYPE xy
@ view 0.15, 0.15, 0.75, 0.85
@ legend on
@ legend box on
@ legend loctype view
@ legend 0.78, 0.8
@ legend length 2
@ s0 legend "Total"
      0.000   11.749
     10.000   11.430
     20.000   11.209
     30.000   11.114
     40.000   11.620
     50.000   11.810
     60.000   10.976
     70.000   11.592
     80.000   10.932
...

Here is a sample of what the atoms I'm interested. They are a part of a DNA-ligand type system. They are about 120 atoms total I'm looking at.

Does anyone know what that area unit is? It looks something like (nm)(seconds^-2)(Newton^-1). This would not be an area unit if that's the case, so this doesn't make sense to me.

Or maybe it is nm^2/(number of dots)?. I think that might make sense, but the GROMACS documentation doesn't say. The paper they reference that talks about the algorithm doesn't give any kind of hints for units either.

Answer 1

The unit is actually $\pu{nm^2}$. What is written there is not is not specific to GROMACS, but to the plotting program it is using Xmgrace. \S denotes the start of superscript font, which ends when it encounters a \N.

If you pass this file as an input to Xmgrace, it should produce a plot of surface area ($\pu{nm^2}$) vs time ($\pu{ps}$).