Cis–Trans Isomerizations of Proline Residues Are Key to Bradykinin Conformations

A recent ion mobility-mass spectrometry (IM-MS) study of the nonapeptide bradykinin (BK, amino acid sequence Arg¹-Pro²-Pro³-Gly⁴-Phe⁵-Ser⁶-Pro⁷-Phe⁸-Arg⁹) found evidence for 10 populations of conformations that depend upon the solution composition [J. Am. Chem. Soc. 2011, 133, 13810]. Here, the role of the three proline residues (Pro², Pro³, and Pro⁷) in establishing these conformations is investigated using a series of seven analogue peptides in which combinations of alanine residues are substituted for prolines. IM-MS distributions of the analogue peptides, when compared to the distribution for BK, indicate the multiple structures are associated with different combinations of cis and trans forms of the three proline residues. These data are used to assign the structures to different peptide populations that are observed under various solution conditions. The assignments also show the connectivity between structures when collisional activation is used to convert one state into another.