Analysis of ß-strand Twist from the 3-dimensional Image of a Protein

Electron cryo-microscopy (Cryo-EM) technique produces density maps that are 3-dimensional (3D) images of molecules. It is challenging to derive atomic structures of proteins from 3D images of medium resolutions. Twist of a α-strand has been studied extensively while little of the known information has been directly obtained from the 3D image of a α-sheet. We describe a method to characterize the twist of α-strands from the 3D image of a protein. An analysis of 11 α-sheet images shows that the Averaged Minimum Twist (AMT) angle is larger for a close set than for a far set of α-traces.