Effect of pH and Molecular Length on the Structure and Dynamics of Linear and Short-Chain Branched Poly(ethylene imine) in Dilute Solution: Scaling Laws from Detailed Molecular Dynamics Simulations

Abstract

Atomistic molecular dynamics (MD) simulations are carried out to examine the effect of molecular weight M w (= 0.6, 0.86, 1.12, and 2.15 kDa) and pH (or equivalently, degree of ionization, α + = 0, 50, and 100%) on the structure, state of hydration, and dynamics of linear and branched poly(ethylene imine) (PEI) chains in infinitely dilute salt-free aqueous solutions. It is found that the degree of ionization is the key factor determining the type of molecular conformation adopted by PEI, regardless of molecular architecture and chain length, resulting in a stable trans conformation for fully ionized solutions and in a stable gauche + /gauche − state for neutral or alternate ionized ones; in the latter case, a strong electrolyte behavior is verified for both linear and branched PEI. Linear PEI is observed to be significantly stiffer than branched PEI of the same molecular weight at 100% degree of ionization, but the effect subsides as the degree of ionization decreases. Also, linear PEI diffuses markedly slower than branched PEI of the same M w . From the MD results, scaling exponents are deduced and reported for the conformation, solvent-accessible surface area, and dynamics of the two different PEI structures with M w .

Branched polyethyleneimine: CHARMM force field and molecular dynamics simulations

“…As expected, we find the branched PEIs to be more compact than the linear ones, as also indicated by the time evolutions in Figure 7. This is in accordance with the findings of Sun et al 15 and the study of Mintis et al 12 (regarding the effect of pH and molecular length), who equally obtained lower R g values for branched polymers, irrespective of protonation. However, we note that the FFs employed in these studies were only partially specific for PEI.…”

“…The optimized force constant for the NC3CH2 bond is lower (by ~2.4%) than the one extracted by Mintis et al 12 from the general GAFF force field. By contrast, our k θ values for the backbone angles involving NC3 are systematically higher (with ~2.8% for the CCN angle and ~24% for the CNC angle), rendering our B PEI model significantly stiffer.…”

“…By contrast, using atomic charges resulted from ab initio calculations on a larger set of four trimers with different protonation fractions, Kondinskaia et al 11 investigated the DNA‐polycation binding behavior. Notably, Mintis et al 12 recently reported scaling laws for the gyration radius and diffusion coefficient as functions of the PEI mass and conformation (linear/branched), derived from molecular dynamics (MD) simulations using the Amber FF with modified charges for B PEI, and the FF of Basu et al 13 for L PEI.…”

Molecular Dynamics Study of the Conformation, Ion Adsorption, Diffusion, and Water Structure of Soluble Polymers in Saline Solutions

“…In this work, we chose to use the Generalized Amber Force Field (GAFF) because it successfully simulates polymers in solution [ 28 , 34 , 35 ]. To do this, the Antechamber program [ 36 , 37 ] was used first to generate the topology of the polymers to be studied; this includes the van der Waals potentials, bonding, angles, and dihedrals.…”

Complexation of Alkali and Alkaline-Earth Metal Cations at Spodumene-Saltwater Interfaces by Molecular Simulation: Impact on Oleate Adsorption

“…This force field has been parameterized and tested on various minerals. For the oleate molecule, we used the AMBER 99sb force field [27] for the bonded and nonbonded contributions in combination with the restrained electrostatic potential (RESP) charge fitting method [28,29]. The SPC/E water model [30] constrained with SETTLE was used to describe water [31].…”