[gmx-users] Ellipticity 222 - negative values?

[Anh Mai]

Hi all,
I have a question about the *positive value* of the output of the *ellipticity
222* Θ_222 , as an output from the *gmx helix* package.
Specifically, I run

"gmx helix -s top.tpr -n index.ndx -f traj.xtc -b 400000 -cz conf.gro"

and got the "cd222.xvg" file like this:
"
# gmx helix is part of G R O M A C S:
#
# Great Red Oystrich Makes All Chemists Sane
#
@    title "Ellipticity at 222 nm"
@    xaxis  label "Time (ps)"
@    yaxis  label "nm"
@TYPE xy
    400000       59.57
    400050       60.32
    400100       57.64
    400150       59.94
    400200       60.63
    400250       56.91
    400300       56.67
...
".
Why is the output value of the ellipticity is positive, instead of being
negative?
According to Hirst and Brooks (1994) (DOI: 10.1006/jmbi.1994.1644
<https://doi.org/10.1006/jmbi.1994.1644>), the ellipticity is negative
(Table 2/ page 176).

Any suggestions are greatly appreciated.
Thanks,
Anh Mai