α-helical conformations are an important part of protein function and contribute to assembly and folding. In order to understand the role of flexibility of these regions, 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 of the solvent reduces the polyelectrolyte l_pand increases chain flexibility due to greater salt screening. Increasing the pH of the solvent leads to the partial and complete formation of helices and the 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_pand their implications to polymer or protein flexibility, we can further understand the function of biomacromolecules.

Persistence length, <i>l</i><sub>p</sub>, changes with introducing helical structure.

Persistence length, l_p, changes with introducing helical structure.

Kathryn G Wilcox

NSF Graduate Research Fellow, Case Western Reserve University

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