First observation of metal ion-induced structural fluctuations of α-helical peptides by using diffracted X-ray...

In order to analyze protein structural dynamics, we designed simple model peptides whose structures changed from random-coil to helix-bundle structures by forming stable hydrophobic core in the presence of metal ions. The strategy involved destabilizing a de novo designed three helix-bundle protein by substituting the residues present in its hydrophobic core with histidine and small amino acids. The conformational changes of peptides induced upon binding of Zn²⁺ to histidine were analyzed using circular dichroism spectroscopy, which revealed peptides, HA and HG, to be good candidates for further analyses. The diffracted X-ray tracking experiments showed that the structural fluctuations of both HA and HG were suppressed upon binding of Zn²⁺. We succeeded in observing the differences in fluctuations of HA and HG in solution between random-coil like and helix-bundle structures. The metal-binding energies determined using the angular diffusion coefficients were in good agreement with those determined using isothermal titration calorimetry.