Singularities of Hinge Structures
  Motivated by the hinge structure present in protein chains and other molecular conformations, we study the singularities of certain maps associated to body-and-hinge and panel-and-hinge chains. These are sequentially articulated systems where two consecutive rigid pieces are connected by a hinge, that is, a codimension two axis.   The singularities, or critical points, correspond to a dimensional drop in the linear span of the axes, regarded as points on a Grassmann variety in its Pl\"{u}cker embedding. These results are valid in arbitrary dimension. The three dimensional case is also relevant in robotics. 
Ciprian S. Borcea and Ileana Streinu