Assignment of secondary structure from Cα coordinates

A multiple regression analysis has established a nonlinear relationship between the backbone dihedral angles and the Cα coordinates obtained from the x-ray crystal structures of 14 proteins. The regression equations have been applied to predict specific dihedral angles of each residue in the backbone of 24 proteins. Overall this method (Nonlinear Regression Distance Torsion) predicts values of ϕ and ψ within a ±45° window of those found in the x-ray structure with an accuracy of 94 and 91% and within a ±30° window of 88 and 81%. Two methods for the assignment of motif from Cα coordinates are reported. For the first method, motif is assigned from the dihedral angles predicted using the regression equations. By the second method, motif of the ith residue is assigned from the distance Cαi-1 to Cαi+2 (v6) and torsional angle Cαi-1, Cαi, Cαi+1, Cαi+2 (v13). For the 24 proteins, 23.7% of the residues by the former method and 19.6% by the latter method are assigned differently than in the Protein Data Bank. © 1997 John Wiley & Sons, Inc.