The debate over whether to include torsion angles in determining secondary structure is an old one (DSSP vs. STRIDE). It turns out that on high-quality x-ray structures it doesn't make much difference what algorithm you use---the disagreements come down to matters of opinion about where the edges of secondary structures are. The biggest differences are on NMR structures, where insufficient data or insufficient refinement often results in failure to get Hbonds within the recognition range of algorithms like DSSP. Then using the phi and psi angles can help compensate for the noisy positions of the donor and acceptor atoms. Currently, most crystallographers seem to use the DSSP definitions of secondary structure---not because they are "right" but because they are acceptable and do not need to be defended. I doubt that anyone would quibble if the STRIDE definitions were used instead---the main effect would be changes of +-1 residue on the ends of helices. ------------------------------------------------------------ Kevin Karplus karplus at soe.ucsc.edu http://www.soe.ucsc.edu/~karplus Professor of Biomolecular Engineering, University of California, Santa Cruz Undergraduate and Graduate Director, Bioinformatics (Senior member, IEEE) (Board of Directors, ISCB) life member (LAB, Adventure Cycling, American Youth Hostels) Effective Cycling Instructor #218-ck (lapsed) Affiliations for identification only.