Persistence length of α-helical poly-L-lysine

The α-helix has a significant role in protein function and structure because of its rigidity. In this study, we investigate the persistence length, l_p, of α-helical poly-L-lysine, PLL, for two molecular weights. PLL experiences a random coil-helix transition as the pH is raised from 7 to 12. Using light scattering experiments to determine the radius of gyration (R_g), hydrodynamic radius, (R_h), the shape factor (R_g/R_h), and second virial coefficient (A₂), and circular dichroism to determine the helical content, we find the structure and l_p of PLL as a function of pH (7.4-11.4) and ionic strength (100-166 mM). With increasing pH, we find an increase in l_p from 2 nm to 15-21 nm because of α-helix formation. We performed dissipative particle dynamics (DPD) simulations and found a similar increase in l_p. While this l_p is less than that predicted by molecular dynamics simulations, it is consistent with other experimental results, which quantify the mechanics of α-helices. By determining the mechanics of helical polypeptides like PLL, we can further understand their implications to protein function.