This shows the backbone of a protein macromolecule by
creating artificial 'backbone' bonds between alpha carbons.
Backbone radius can be specified in angstroms.
By default, the backbone bond color is inherited from the
atoms (as with other bonds). You can explicitly change
the backbone color.
This shows a spline trace of a protein backbone.
The hermite spline curve passes through the mid-points
between alpha carbon atoms
With color by 'structure' you get different colors for
HELIX, TURN, and SHEET. Color by 'amino' and color by
'shapely' assign different colors
to different amino acids.