The enhanced stability of the cross-linked hylan structure to hydroxyl (OH) radicals compared with the uncross-linked hyaluronan

Abstract

A comparison has been made of the relative stabilities of hyaluronan and hylan to degradation by OH radicals produced by γ-irradiation of aqueous solutions in N 2O, when G (yield per 100 eV) for OH radicals is 5.6 and H atoms 0.6. Using low angle light scattering and viscometric methods, the change in molecular weight of the polysaccharides was measured with increasing dose. From the yield/dose curves (expressed as breaks per molecule), the initial G value for hyaluronan degradation is ∼ 4. A further slow post-irradiation decrease in molecular weight is observed, which can be brought to completion by incubating the solutions for 1 h at 60°C. Thereafter, the G value for degradation is ∼ 6. A similar post-irradiation degradation was found for hylan. A technique using tetranitromethane (TNM) has been used to distinguish between two types of radicals formed on the hyaluronan backbone. Radicals of the 1-hydroxy-2-alkoxy type (C-2, C-4, C-2 and C3 of the glucuronic acid) would induce strand breakage by alkoxy elimination. For the equivalent alkoxy radical at C6 of the acetamido monosaccharide, ring opening would occur with formation of a hemi-acetal, leading also to strand breakage. The C-2 and C-3 radicals would eliminate water rather than produce breaks by β-alkoxy elimination. Thus three out of the initially formed radicals would produce breaks by β-alkoxy formation. These can be stabilised with TNM and distinguished. It is concluded that these are the radicals involved in the post-irradiation thermal degradation process. Comparison of hylan and hyaluronan is, therefore, most valid when this post-irradiation process has been completed. Therefore, all G values for degradation were measured after incubation for 1 h at 60°. This investigation establishes the greater stability of hylan ( G degradation = 2) compared to hylan ( G degradation = 6). Therefore, in an environment such as supplementation of an inflammed joint where OH radicals are released, hylan is able to retain its integrity as a viscoelastic macromolecule three times better than hyaluronan. Its potential as a viscosupplementation material, or as an inflammatory drug release matrix inserted within the joint is, therefore, greater than non-cross-linked hyaluronan.