Hydration shell differentiates folded and disordered states of a Trp-cage miniprotein, allowing characterization of structural heterogeneity by wide-line NMR measurements

Hydration properties of folded and unfolded/disordered miniproteins were monitored in frozen solutions by wide-line (1)H-NMR. The amount of mobile water as function of T (−80 °C < T < 0 °C) was found characteristically different for folded (TC5b), semi-folded (pH < 3, TCb5(H+)) and disordered (TC5b_N1R) variants. Comparing results of wide-line (1)H-NMR and molecular dynamics simulations we found that both the amount of mobile water surrounding proteins in ice, as well as their thaw profiles differs significantly as function of the compactness and conformational heterogeneity of their structure. We found that (i) at around −50 °C ~50 H(2)Os/protein melt (ii) if the protein is well-folded then this amount of mobile water remains quasi-constant up to −20 °C, (iii) if disordered then the quantity of the lubricating mobile water increases with T in a constant manner up to ~200 H(2)Os/protein by reaching −20 °C. Especially in the −55 °C ↔ −15 °C temperature range, wide-line (1)H-NMR detects the heterogeneity of protein fold, providing the size of the hydration shell surrounding the accessible conformers at a given temperature. Results indicate that freezing of protein solutions proceeds by the gradual selection of the enthalpically most favored states that also minimize the number of bridging waters.