  Soss proved that it is NP-hard to find the maximum 2D span of a fixed-angle
polygonal chain: the largest distance achievable between the endpoints in a
planar embedding. These fixed-angle chains can serve as models of protein
backbones. The corresponding problem in 3D is open. We show that three special
cases of particular relevance to the protein model are solvable in polynomial
time. When all link lengths and all angles are equal, the maximum 3D span is
achieved in a flat configuration and can be computed in constant time. When all
angles are equal and the chain is simple (non-self-crossing), the maximum flat
span can be found in linear time. In 3D, when all angles are equal to 90 deg
(but the link lengths arbitrary), the maximum 3D span is in general nonplanar
but can be found in quadratic time.
