Persistence length of α-helical poly-L-lysine.

Abstract

The α-helix has a significant role in protein function and structure because of its rigidity. In this study, we investigate the persistence length, <i>l</i>_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 (<i>R</i>_g), hydrodynamic radius, (<i>R</i>_h), the shape factor (<i>R</i>_g/<i>R</i>_h), and second virial coefficient (<i>A</i>₂), and circular dichroism to determine the helical content, we find the structure and <i>l</i>_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 <i>l</i>_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 <i>l</i>_p. While this <i>l</i>_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.