Polyproline-13 (Pro13) is a peptide with two distinct conformations depending on solvent conditions. It adopts a condensed helical structure known as polyproline I (PPI) consisting of entirely <i>cis</i> peptide bonds in less polar solvents. In more polar solvents, all peptide bonds adopt a <i>trans</i> configuration forming an elongated polyproline II (PPII) helix. Due to this solvent dependance, changing the polarity of the solvent will induce a conformational change in the peptide. Recent work has shown that this transition can be affected by the presence of divalent metal ions in solution which bind exclusively to the PPII form of Pro13, resulting in additional PPII-like conformations. The current study uses ion mobility mass spectrometry (IMS-MS) to examine monovalent metals binding to polyproline. Unlike multivalent metals, these ions coordinate with both the PPI and PPII conformations, indicating that binding occurs via a different mechanism. Bound ions also do not inhibit solvent-driven conformational changes. In addition, multiple monovalent ions are observed to simultaneously bind to Pro13 with smaller ions such as lithium and sodium binding upwards of eight individual metal ions while heavier metals like rubidium and cesium only bind one. Upon the introduction of multiple species of metal ions, Pro13 exhibits preferential binding of some cations over others. Specifically, potassium outcompetes other metals such as sodium and rubidium, suggesting that a balance of size and charge density determines binding affinity. Characterization of these proline-metal complexes provides novel insight into the coordination of metal ions with peptide backbones and their resulting effect on overall protein structure.

Monovalent metal binding to polyproline 13 by ion mobility mass spectrometry

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