Helical Persistence Length of Poly-L-Lysine

In order to understand the role of α-helical flexibility in protein function, we investigate the helical and electrostatic contributions to the persistence length (l_p) of poly-L-lysine (PLL) independently by inducing a coil-helix transition by changing the pH, and probing the electrostatic effects by changing the solution ionic strength. Using static and dynamic light scattering, l_p is determined from the salt- and pH-dependent R_g. In addition to R_g, hydrodynamic radius, (R_h), the shape factor (R_g/R_h), and second virial coefficient (A₂) of the PLL for a range of salt and pH conditions are reported. In neutral pH, the polymer is a random coil, swollen by charge interactions. Increasing the ionic strength reduces the polyelectrolyte l_p and increases chain flexibility due to greater salt screening. Increasing the pH leads to the partial and complete formation of helices and dominance of the helical contribution to l_p, which leads to an increase in rigidity, but not to the extent predicted by molecular dynamics simulations. By understanding the contributions to helical l_p, we can further understand their implications to protein flexibility and function.