Fig. 1. : Default DSSP assignment for 1c3y fragment. The variations between the secondary structure assignments for different NMR models of the same protein illustrate the impact of fluctuations on structure and highlight the difficulty of predicting protein structure. (a) The default DSSP [58] assignments for all 23 models of the THP12-carrier protein (PDB:1c3y [59] , residues ). The structure models were calculated using 13C/15N labelled protein and 3D/4D NMR spectroscopy with 13 NOE's per residue. (b) DSSPcont assignments for the first NMR model alone; the core of the helix (residues 24-28) are assigned as H by default DSSP although the entire a-helix switched to a 310-helix when applying a hydrogen bond threshold of -1 kcal/mol. A 'fuzzy' helix capping, as seen here, is common and was observed for approximately one in four N-caps and half the C-caps in our data sets. Dissecting the continuous assignment shows that a 0.1 kcal/mol looser hydrogen bond threshold in the default DSSP would extend the helix by one residue (residue 29). If the default threshold instead had been tightened by 0.2 kcal/mol, the helix would lose one residue (residue 28).