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Hi Brad,
when you say 'volume of a ligand', then you think of the volume of its
electron density cluster depending on data resolution and contour level,
right? Since most ligands (in a difference electron density map) appear
nicest between contour levels 1.4 and 3 sigma (to our experience of
ligand building) while still depending on the resolution, the volume is
expected to be between 2.5 and 5 times the number of non-hydrogen atoms
in cubic Ångstroms. If you want to spend a little more time on this, you
can simulate electron densities for these ligands (by e.g. using sfall),
then tune the contour level and count the map grid points inside,
finally multiply this number with the volume of a grid cell and you have
an estimation.
Cheers,
Gerrit.
------------------------
Gerrit G. Langer
Postdoctoral Researcher
Lamzin-Group
EMBL-Hamburg
