¹H‐¹⁹F REDOR‐filtered NMR spin diffusion measurements of domain size in heterogeneous polymers

Abstract

Solid state NMR spectroscopy is inherently sensitive to chemical structure and composition and thus makes an ideal method to probe the heterogeneity of multicomponent polymers. Specifically, NMR spin diffusion experiments can be used to extract reliable information about spatial domain sizes on multiple length scales, provided that magnetization selection of one domain can be achieved. In this paper, we demonstrate the preferential filtering of protons in fluorinated domains during NMR spin diffusion experiments using H- F heteronuclear dipolar dephasing based on rotational echo double resonance (REDOR) MAS NMR techniques. Three pulse sequence variations are demonstrated based on the different nuclei detected: direct H detection, plus both H➔ C cross polarization and H➔ F cross polarization detection schemes. This H- F REDOR-filtered spin diffusion method was used to measure fluorinated domain sizes for a complex polymer blend. The efficacy of the REDOR-based spin filter does not rely on spin relaxation behavior or chemical shift differences and thus is applicable for performing NMR spin diffusion experiments in samples where traditional magnetization filters may prove unsuccessful. This REDOR-filtered NMR spin diffusion method can also be extended to other samples where a heteronuclear spin pair exists that is unique to the domain of interest.

High-Sensitivity Detection of Nanometer 1H–19F Distances for Protein Structure Determination by 1H-Detected Fast MAS NMR

“…19−21 It also complements a 2D-resolved 13 REDOR technique, 22 which has a shorter distance upper limit of ∼1 nm. The combination of 1 H and 19 F for distance measurement was previously reported for the slow MAS condition, 23,24 which suffered from short 1 H T 2 relaxation times and the lack of possibility for high-sensitivity 1 H detection.…”

“…The choice of 1 H– 19 F heteronuclear distance measurement circumvents the difficulties in measuring homonuclear 1 H– 1 H distances, where the dense 1 H network in biomolecules causes dipolar truncation and relayed transfer. , Proton dilution by perdeuteration followed by back H/D exchange still leaves uncertainties in 1 H– 1 H distance measurements due to residual relayed polarization transfer, while an ultrafast MAS of ∼100 kHz for protonated samples − requires 3D and four-dimensional correlation experiments for 1 H chemical shift assignment. − The 1 H– 19 F distance approach demonstrated here circumvents these difficulties and complements the recently introduced homonuclear 19 F– 19 F distance measurements. − It also complements a 2D-resolved 13 C– 19 F REDOR technique, which has a shorter distance upper limit of ∼1 nm. The combination of 1 H and 19 F for distance measurement was previously reported for the slow MAS condition, , which suffered from short 1 H T 2 relaxation times and the lack of possibility for high-sensitivity 1 H detection.…”