The default ribbon path is a smooth bspline (semitransparent tan in the figure), which can diverge from the true positions of the backbone atoms (α-carbons shown as gray balls). A cardinal spline allows tracking the backbone more closely. Without smoothing (light blue), it follows the α-carbons exactly, or it can be combined with some “compromise” smoothing of strand and/or coil. Ribbon spline options can be set with the ribspline command or in the molecule model attributes.

The image shows the structure of the human OX2 orexin receptor bound to the insomnia drug suvorexant, Protein Data Bank entry 4s0v. The drug is shown as spheres colored by element, and the receptor as ribbons with secondary structure elements rainbow-colored from blue at the N-terminus to red at the C-terminus. (More samples...)