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that could be used in motion prediction work, and in part by a desire to deepen basic
understanding of protein motion, we asked ourselves the following question:
Would it be possible to approximately reproduce the observed motion by allowing
flexure at the hinge points but keeping the regions between hinges rigid?
In order for this question to be answered in the affirmative, the hinge selection should be
the one to best meet the following criteria:
The φ, ψ, and α (effective α-carbon to α-carbon) torsion angles of hinge residues may
often (but not always) be larger than those of their neighbors.
Amino acids on either side of the hinge residues must be co-moving with their respective
rigid regions.
Rotations of one of the rigid regions about the hinge region must not result in significant
and irreconcilable steric clashes.
In order to use (1) as a useful guide to selecting the hinge location, we made use of the
torsion angle charts and graphs in the structure analysis tools section on the morph page.
However often large rotations of the main chain are induced by multiple cooperative
torsions in the hinge, and these may be individually small, particularly in α-helices. The
usefulness of this flexibility measure is further limited by the frequent occurrence of large
torsion angles which do not coincide with hinges. Nonetheless, when the precise location
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