NMR chemical shifts in proteins depend strongly on local structure. The program TALOS establishes an empirical relation between
13C,
15N and
1H chemical shifts and backbone torsion angles
ϕ and
ψ (Cornilescu et al. J Biomol NMR 13 289–302,
1999). Extension of the original 20-protein database to 200 proteins increased the fraction of residues for which backbone angles
could be predicted from 65 to 74%, while reducing the error rate from 3 to 2.5%. Addition of a two-layer neural network filter
to the database fragment selection process forms the basis for a new program, TALOS+, which further enhances the prediction
rate to 88.5%, without increasing the error rate. Excluding the 2.5% of residues for which TALOS+ makes predictions that strongly
differ from those observed in the crystalline state, the accuracy of predicted
ϕ and
ψ angles, equals ±13°. Large discrepancies between predictions and crystal structures are primarily limited to loop regions,
and for the few cases where multiple X-ray structures are available such residues are often found in different states in the
different structures. The TALOS+ output includes predictions for individual residues with missing chemical shifts, and the
neural network component of the program also predicts secondary structure with good accuracy.
Keywords Heteronuclear chemical shift - Secondary structure - Order parameter - Dynamics - TALOS