Rapid protein fold determination using secondary chemical shifts and cross-hydrogen bond N-15-C-13 ' scalar couplings ((3hb)J(NC '))

The possibility of generating protein folds at the stage of backbone assign ment using structural restraints derived from experimentally measured cross -hydrogen bond scalar couplings and secondary chemical shift information is investigated using as a test case the small alpha/beta protein chymotrypsi n inhibitor 2. Dihedral angle restraints for the phi and psi angles of 32 o ut of 64 residues could be obtained from secondary chemical shift analysis with the TALOS program (Corneliscu et al., 1999a). This information was sup plemented by 18 hydrogen-bond restraints derived from experimentally measur ed cross-hydrogen bond (3hb)J(NC') coupling constants. These experimental d ata were sufficient to generate structures that are as close as 1.0 Angstro m backbone rmsd from the crystal structure. The fold is, however, not uniqu ely defined and several solutions are generated that cannot be distinguishe d on the basis of violations or energetic considerations. Correct folds cou ld be identified by combining clustering methods with knowledge-based poten tials derived from structural databases.