Journal of Physical Chemistry B, Vol.116, No.24, 7102-7112, 2012
Impact of Ion Binding on Poly-L-Lysine (Un)folding Energy Landscape and Kinetics
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 pi-helix (bulge), pure alpha-helix, and turns) of PLL are slower than those of short alanine-based peptides. The PLL pure alpha-helix folding time is similar to that of short alanine-based peptides. We for the first time have directly observed that turn conformations are alpha-helix and pi-helix (bulge) unfolding intermediates. ClO4- binding to the Lys side chain -NH3+ groups and the peptide backbone slows the alpha-helix unfolding rate compared to that in pure water, but little impacts the folding rate, resulting in an increased alpha-helix stability. ClO4- 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-pi helix (bulge) unfolding and folding coordinates do not directly go through the alpha-helix energy well. Our results clearly demonstrate that PLL (un)folding is not a two-state process.