ESR SPIN PROBE STUDY OF WATER WITHIN COVALENTLY CROSS-LINKED, DEXTRAN-BASED MICROBEAD HYDROGELS (SEPHADEX(R)G)

Aqueous suspensions of dextran-based Sephadex(R) G microbeads of progressively increasing degrees of covalent crosslinking and concomitantly decreasing water content formed a two-phase system consisting of the water-swollen microbeads of diameter 100-300 mum, the hydrogel phase, and of the surrounding water, the aqueous phase. These suspensions were analyzed by ESR spectroscopy using a series of small amphipathic nitroxide spin probes which partitioned between the two phases. Probe partition coefficients increased with decreasing gel water content for the more hydrophobic probes and decreased for the more hydrophilic probes. The ESR spectra of the suspensions consisted of the superposition of the spectra for the probes in the two phases. From the partition coefficients and the spectral parameters of the probes in the suspension and in the aqueous phase, relative rotational correlation times, tau(C), and isotropic nitrogen nuclear hyperfine splitting constants, a(N), for the probes in the aqueous hydrogel interior were determined. For each probe, tau(C) increased and a(N) decreased with increasing crosslinking and decreasing gel water content, indicating increasing microviscosity and decreasing dielectric constant and hydrophilicity for the gel water. The alterations in the gel water properties resulted from increasing gel matrix-water interactions and were further enhanced for water within the restricted volumes (compartments) in the most highly crosslinked hydrogels (Sephadex G-15 and G-10). In addition, the a(N) values indicated that two hydrogen bonds were formed to the nitroxide group of the probes in water and that, in the hydrogels, the hydrogen bonding contribution to a(N) progressively increased as the water content decreased.