End-to-End Distance Probability Distributions of Dilute Poly(ethylene oxide) in Aqueous Solution

We introduce a powerful, widely applicable approach to characterizing polymer conformational distributions, specifically the end-to-end distance distributions, P(R_ee), accessed through double electron-electron resonance (DEER) spectroscopy in conjunction with molecular dynamics (MD) simulations. The technique is demonstrated on one of the most widely used synthetic, disordered, water-soluble polymers: poly(ethylene oxide) (PEO). Despite its widespread importance, no systematic experimental characterization of PEO's R_ee conformational landscape exists. The evaluation of P(R_ee) is particularly important for short polymers or (bio)polymers with sequence complexities that deviate from simple polymer physics scaling laws valid for long chains. In this study, we characterize the R_ee landscape by measuring P(R_ee) for low molecular weight (MW: 0.22-2.6 kDa) dilute PEO chains. We use DEER with end-conjugated spin probes to resolve R_ee populations from ∼2-9 nm and compare them with full distributions from MD. The P( R_ee)'s from DEER and MD show remarkably good agreement, particularly at longer chain lengths where populations in the DEER-unresolvable range (<1.5 nm) are low. Both the P(R_ee) and the root-mean-square R̃_ee indicate that aqueous PEO is a semiflexible polymer in a good solvent, with the latter scaling linearly with molecular weight up to its persistence length (l_p ∼ 0.48 nm), and rapidly transitioning to excluded volume scaling above l_p. The R̃_ee scaling is quantitatively consistent with that from experimental scattering data on high MW (>10 kDa) PEO and the P(R_ee)'s crossover to the theoretical distribution for an excluded volume chain.