Metal Ion Interactions with Polyalanine Peptides

Electrospray mass spectrometry and ion mobility measurements have been used to perform a systematic study of complex formation between metal ions and polyalanine peptides. Monovalent metal ions (Li+, Na+, K+, Cs+, and Rb+) are known to form complexes with polyalanine peptides. In the unsolvated complex, the polyalanine peptide adopts a helical conformation that is stabilized by coordination of the metal ion to the C-terminus. Complexes are also observed between polyalanine peptides and the dications of alkali earth metals (Mg2+, Ca2+, Sr2+, and Ba2+), though they are substantially less abundant than with the monovalent ions. Ion mobility measurements for the unsolvated Ala n +M2+ complexes are consistent with an α-helical conformation, but with a substantial disruption of the helix at the C-terminus due to much stronger coordination to the dication. Attempts to observe complex formation with trivalent metal ions (In3+, Sc3+, and Y3+) were not successful.