The low-pH unfolded state of the C-terminal domain of the ribosomal protein L9 contains significant secondary structure in the absence of denaturant but is no more compact than the low-pH urea unfolded state

There is considerable interest in the properties of the unfolded states of proteins, particularly unfolded states which can be populated in the absence of high concentrations of denaturants. Interest in the unfolded state ensemble reflects the fact that it is the starting point for protein folding as well as the reference state for protein stability studies and can be the starting state for pathological aggregation. The unfolded state of the C-terminal domain (residues 58-149) of the ribosomal protein L9 (CTL9) can be populated in the absence of denaturant at low pH. CTL9 is a 92-residue globular α, β protein. The low-pH unfolded state contains more secondary structure than the low-pH urea unfolded state, but it is not a molten globule. Backbone ( ¹H, ¹³C, and ¹⁵N) NMR assignments as well as side chain ¹³C_β and ¹H_β assignments and ¹⁵N R₂ values were obtained for the pH 2.0 unfolded form of CTL9 and for the urea unfolded state at pH 2.5. Analysis of the deviations of the chemical shifts from random coil values indicates that residues that comprise the two helices in the native state show a clear preference for adopting helical φ and ψ angles in the pH 2.0 unfolded state. There is a less pronounced but nevertheless clear tendency for residues 107-124 to preferentially populate helical φ and ψ values in the unfolded state. The urea unfolded state has no detectable tendency to populate any type of secondary structure even though it is as compact as the pH 2.0 unfolded state. Comparison of the two unfolded forms of CTL9 provides direct experimental evidence that states which differ significantly in their secondary structure can have identical hydrodynamic properties. This in turn demonstrates that global parameters such as R_h or R_g are very poor indicators of "random coil" behavior.