alpha-Helix-double helix interaction shown in the structure of a protamine-transfer RNA complex and a nucleoprotamine model.

Single crystal X-ray diffraction and circular dichroism studies of protamine binding to a tRNA suggest that the protamine molecule changes its conformation from a random coil to a structure containing a helices on binding to tRNA, and that alpha-helical segment(s) of protamine bind approximately along a shallow groove of a double-helical portion of ttrna. based on these observations, a structural model for nucleoprotamine is proposed.