Stretch-Induced Ordering of Prochiral Dimethyl Sulfoxide in Anisotropic Hydrogels Analysed by 1H and 2H Nuclear Magnetic Resonance.

Abstract

Nuclear spins in small molecules dissolved in stretched hydrogels typically have population-averaged residual interactions. The nuclear magnetic resonance (NMR) spectra of these systems often show additional peaks and splittings compared with free solutions. Residual dipolar couplings (RDCs) and quadrupolar couplings (RQCs) are observed for guest 1H or 2H nuclear spins, respectively. Dimethyl sulfoxide (DMSO) is an exquisitely sensitive probe of such biologically relevant environments since it is prochiral and becomes effectively chiral when embedded in anisotropic gelatin-based hydrogels. Measured 1H RDCs and 2H RQCs were used to estimate bond order parameters over a wide range of stretching extents. At the largest extent of stretching, the 2H splittings were -73.0 and -9.4 Hz, similar to those found for guest molecules in liquid crystals. Inhomogeneous line broadening of the 2H resonances was related to the size of the RQC due to a spatial distribution of RQCs, which was revealed using a one-dimensional slice selective imaging experiment along the stretching direction. 1H NMR spectra exhibited homogeneous line broadening, with resonance integrals that indicated concealed multiplet structure. Understanding molecular bond ordering in mechanically oriented environments provides a conceptual framework for investigating more complex systems including zeolites and those found in vivo.