Electrospray ionization (ESI) is the softest ionization method that is currently available and it is widely accepted, that ESI generated ions of proteins and protein assemblies at certain conditions retain characteristic aspects of their solution-state conformation. ESI mass spectrometry (MS) therefore evolved as a useful tool to obtain information on composition, stoichiometry, and dynamics of non-covalently associated protein complexes. While tertiary structure information of proteins can be obtained from ion mobility spectrometry (IMS), only a few techniques yield direct information on the secondary structure of gas-phase peptides and proteins. We present here the mid-IR spectroscopic secondary structural analysis of three de novo designed α-helical coiled coil model peptides and their non-covalently associated complexes in the gas-phase. The conformational stability of such coiled coil peptides in solution is primarily driven by aggregation. Isolated monomers usually remain unfolded. Two of the investigated peptides were designed to assemble into stable α-helical complexes in acidic solution, while the third one remains monomeric and unfolded at these conditions. Monomer ions of all three peptides show comparable photodissociation IR spectra and therefore suggest an unfolded conformation in the gas phase. In contrast, considerable C=O stretch (amide-I) and N–H bend (amide-II) band shifts have been observed for the dimers which is consistent with an elevated H-bond content. These findings provide evidence that at least a fraction of the condensed phase α-helical structure is retained in the gas-phase coiled coil complexes.
Gas-phase IR spectra of intact α-helical coiled coil protein complexes
Pagel, K.*; Kupser, P.; Bierau, F.; Polfer, N.C.; Steill, J.D.; Oomens, J.; Meijer, G.; Koksch, B.; von Helden, G.