Impact of Ion Binding on Poly-l-Lysine (Un)folding Energy Landscape and Kinetics

2012-06-21T00:00:00Z (GMT) by Kan Xiong Sanford A. Asher
We utilize T-jump UV resonance Raman spectroscopy (UVRR) to study the impact of ion binding on the equilibrium energy landscape and on (un)­folding kinetics of poly-l-lysine (PLL). We observe that the relaxation rates of the folded conformations (including π-helix (bulge), pure α-helix, and turns) of PLL are slower than those of short alanine-based peptides. The PLL pure α-helix folding time is similar to that of short alanine-based peptides. We for the first time have directly observed that turn conformations are α-helix and π-helix (bulge) unfolding intermediates. ClO<sub>4</sub><sup>–</sup> binding to the Lys side chain −NH<sub>3</sub><sup>+</sup> groups and the peptide backbone slows the α-helix unfolding rate compared to that in pure water, but little impacts the folding rate, resulting in an increased α-helix stability. ClO<sub>4</sub><sup>–</sup> binding significantly increases the PLL unfolding activation barrier but little impacts the folding barrier. Thus, the PLL folding coordinate(s) differs from the unfolding coordinate(s). The-π helix (bulge) unfolding and folding coordinates do not directly go through the α-helix energy well. Our results clearly demonstrate that PLL (un)­folding is not a two-state process.