Molecular dynamics studies of polyethylene oxide and polyethylene glycol: Hydrodynamic radius and shape anisotropy

A revision (C35r) to the CHARMM ether force field is shown to reproduce experimentally observed conformational populations of dimethoxyethane. Molecular dynamics simulations of 9, 18, 27, and 36-mers of polyethylene oxide (PEO) and 27-mers of polyethylene glycol (PEG) in water based on C35r yield a persistence length lambda = 3.7 angstrom, in quantitative agreement with experimentally obtained values of 3.7 angstrom for PEO and 3.8 angstrom for PEG; agreement with experimental values for hydrodynamic radii of comparably sized PEG is also excellent. The exponent nu relating the radius of gyration and molecular weight (R-g proportional to M-w(nu)) of PEO from the simulations equals 0.515 +/- 0.023, consistent with experimental observations that low molecular weight PEG behaves as an ideal chain. The shape anisotropy of hydrated PEO is 2.59:1.44:1.00. The dimension of the middle length for each of the polymers nearly equals the hydrodynamic radius R-h obtained from diffusion measurements in solution. This explains the correspondence of R-h and R-p, the pore radius of membrane channels: a polymer such as PEG diffuses with its long axis parallel to the membrane channel, and passes through the channel without substantial distortion.