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 ( A 2 ), 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.