Polyethylenimine: Experimental and Simulation Study of Polymer Biophysics During Ph Buffering

Abstract

Dynamic Light Scattering (DLS) and all atom molecular dynamics (MD) simulation were used in complementary ways to study the effect of protonation (simulation) and pH changes (experiment) on the polymer chain dynamics of a 2.4kDa linear PEI (∼50mer). Simulations were performed with a modified CHARMM potential for a variety of charge states and ionic strengths to generate conformational statistics. DLS yielded a phase map of PEI aggregation to select for conditions in which PEI was in the dilute and free polymer state. Hydrodynamic radii from the experiments and simulations were within 3A agreement. The results indicate that PEI absorbs protons with minimal change in its elongation length, except at high protonation where the chain elongates and deviates from worm chain distributions. Salt screening introduced changes in the protonation/pH response. Our observations are consistent with PEI studies showing that intra-chain charge repulsion raised the free energy of subsequent protonation.