Non-covalent interactions between unsolvated peptides: helical complexes based on acid-base interactions

We have used ion-mobility mass spectrometry to examine the conformations of the protonated complex formed between AcA(7)KA(6)KK and AcEA(7)EA(7), helical alanine-based peptides that incorporate glutamic acid (E) and lysine (K). Designed interactions between the acidic E and basic K residues help to stabilize the complex, which is generated by electrospray and studied in the gas phase. There are two main conformations: (1) a coaxial linear arrangement where the helices are tethered together by an EKK interaction between the pair of lysines at the C-terminus of the AcA(7)KA(6)KK peptide and a glutamic acid at the N-terminus of the AcEA(7)EA(7) peptide and (2) a coiled-coil arrangement with side-by-side antiparallel helices where there is an additional EK interaction between the E and K residues in the middle of the helices. The coiled-coil opens up to the coaxial linear structure as the temperature is raised. Entropy and enthalpy changes for the opening of the coiled-coil were derived from the measurements. The enthalpy change indicates that the interaction between the E and K residues in the middle of the helices is a weak neutral hydrogen bond. The EKK interaction is significantly stronger.