Conformational changes of α-helical peptides with different hydrophobic residues induced by metal-ion binding

We designed peptides that formed helix bundle structures upon binding of the metal-ions to His residues to form a stable hydrophobic core, in order to analyze the effects of Ala, Val, Ile, and Leu residues, located in the hydrophobic core, together with His, on the conformational changes in respective peptides designated as HA, HV, HI, and HL, respectively. Circular dichroism measurements showed that HV and HI changed from random coil to helix bundle structures upon Zn²⁺ binding, similar to that observed for HA, while HL existed in the helix bundle structure even in the absence of Zn²⁺. Electron spin resonance measurements showed that Cu²⁺ coordination of HI and HL was quite different from that of HA and HV, indicating that HA and HV fluctuated to a greater extent in the solution, despite that their apparent α-helical contents being similar to those of HI and HL. This was also supported by the results obtained from the analyses of thermal stabilities. The change in the structural fluctuation for each peptide upon Zn²⁺ binding was evaluated based on binding thermodynamics using isothermal titration calorimetry. The structural flexibility in the metal-ion-bound state was found to be in the order HA > HV > HI, and that in the metal-ion-unbound state was found to be greater for HI than that for HL.