Conformations of Prolyl-Peptide Bonds in the Bradykinin 1-5 Fragment in Solution and in the Gas Phase.

The dynamic nature of intrinsically disordered peptides makes them a challenge to characterize by solution-phase techniques. In order to gain insight into the relation between the disordered state and the environment, we explore the conformational space of the N-terminal 1-5 fragment of bradykinin (BK[1-5](2+)) in the gas phase by combining drift tube ion mobility, cold-ion spectroscopy, and first-principles simulations. The ion-mobility distribution of BK[1-5](2+) consists of two well-separated peaks. We demonstrate that the conformations within the peak with larger cross-section are kinetically trapped, while the more compact peak contains low-energy structures. This is a result of cis-trans isomerization of the two prolyl-peptide bonds in BK[1-5](2+). Density-functional theory calculations reveal that the compact structures have two very different geometries with cis-trans and trans-cis backbone conformations. Using the experimental CCSs to guide the conformational search, we find that the kinetically trapped species have a trans-trans configuration. This is consistent with NMR measurements performed in a solution, which show that 82% of the molecules adopt a trans-trans configuration and behave as a random coil.