Riboflavin-Photosensitized Changes in Aqueous Solutions of Alginate. Rheological Studies

Abstract

Interactions between photoexcited riboflavin (RF), promoted by irradiation in the range of 310-800 nm, and alginate have been studied in air equilibrated aqueous solutions with the aid of rheological methods. Light irradiation of RF causes under aerobic conditions fragmentation of alginate and a decrease in the shear viscosity and other rheological parameters of its solutions. The decrease is most pronounced in concentrated polymer solutions. The photochemical degradation of alginate is inhibited in the presence of the quenchers/scavengers d-mannitol, glutathione, potassium iodide, and sodium azide and in excess oxygen. The addition of thiourea to alginate-RF solutions leads to enhanced degradation of the polymer. Significant shear-thinning effects and deviations from the Cox-Merz rule are observed at higher polymer concentrations.

Characterization of Riboflavin‐Photosensitized Changes in Aqueous Solutions of Alginate by Dynamic Light Scattering

“…This diffusion process reflects a concerted motion of polymer chains relative to the solvent. In a previous study [5] the overlap concentration c*, where a transition to the semidilute regime occurs and a transient network is formed, was estimated to be 0.16 wt.-%. The results in Figure 4a for the fast relaxation time at low polymer concentrations indicate that the average size of the clusters for the alginate/RF system is smaller.…”

“…In all experiments, the time of irradiation was 15 min, which is sufficient for a substantial photochemical degradation of the polymer. [5] The alginate-RF samples were kept in the dark after irradiation, and this measure prevented further photo-induced degradation of the polymer.…”

“…investigated [5,6] with the aid of rheology. The results revealed photochemical degradation of the polymer with significant impact on the rheological properties.…”

“…[2,3,5,6] In photochemical degradation of RF under neutral aqueous conditions, lumichrome is probably a dominant product. [4,8] It is known [2,3] that RF in combination with light can cause scission of polysaccharide molecules, such as hyaluronic acid chains, but the mechanism seems to be complex.…”

“…Both types of reaction can take place simultaneously. In a recent rheological investigation [5] of RF photosensitizer-induced alginate degradation, a simplified schematic representation of two possible reaction pathways was given and effects of quenchers and scavengers on alginate degradation were analyzed. Although the detailed mechanism of the RF-sensitized photochemical degradation of alginate has not yet been settled, there is a large body of evidence that the final step is a free radical-induced cleavage of the glycosidic linkage.…”