Gelation Behaviour of Pluronic F127/Polysaccharide Systems Revealed via Thioflavin T Fluorescence

Fast, reliable methods for characterizing the micelle-to-gel transition in emerging Pluronic F127/polysaccharide materials are essential for tailoring their applications as in situ gelling delivery systems. This study describes a simple fluorimetric method based on the response to gelation of the molecular probe thioflavin T (ThT). The techniques employed are (second derivative) steady-state and synchronous fluorescence. The capabilities of ThT as gelation reporter are tested for three model systems: Pluronic F127 (P16.6%), Pluronic F127/alginate (P16.6%ALG2%) and Pluronic F127/hyaluronic acid (P16.6%HA0.5%). We demonstrate that the changes in the short and long wavelength emissions of ThT allow accurate determination of the critical gelation temperatures in the investigated systems. The spectroscopic data providing information at molecular level are complemented with differential scanning microcalorimetric results revealing additional macroscopic insight into the micellization process. The gelation study is preceded by a solvatochromic analysis of ThT.

Model Systems for Evidencing the Mediator Role of Riboflavin in the UVA Cross-Linking Treatment of Keratoconus

Riboflavin under UVA radiation generates reactive oxygen species (ROS) that can induce various changes in biological systems. Under controlled conditions, these processes can be used in some treatments for ocular or dermal diseases. For instance, corneal cross-linking (CXL) treatment of keratoconus involves UVA irradiation combined with riboflavin aiming to induce the formation of new collagen fibrils in cornea. To reduce the damaging effect of ROS formed in the presence of riboflavin and UVA, the CXL treatment is performed with the addition of polysaccharides (dextran). Hyaluronic acid is a polysaccharide that can be found in the aqueous layer of the tear film. In many cases, keratoconus patients also present dry eye syndrome that can be reduced by the application of topical solutions containing hyaluronic acid. This study presents physico-chemical evidence on the effect of riboflavin on collagen fibril formation revealed by the following methods: differential scanning microcalorimetry, rheology, and STEM images. The collagen used was extracted from calf skin that contains type I collagen similar to that found in the eye. Spin trapping experiments on collagen/hyaluronic acid/riboflavin solutions evidenced the formation of ROS species by electron paramagnetic resonance measurements.

Correlation of the hierarchical structure with rheological behavior of polypseudorotaxane gel composed of pluronic and β-cyclodextrin

We have identified the hierarchical (primary, secondary, tertiary and quaternary) structures of a polypseudorotaxane (PPR) gel composed of the Pluronic F108 and β-cyclodextrin system to be β-cyclodextrin crystalline, lamellar sheets, lamellar stacks and "grains", respectively. The correlation between the rheological properties and the proposed structures under shear flows was rationalized. Alignment of lamellar stacks and reorganization of grain boundaries under shear flows were investigated by rheo-SANS, small angle X-ray scattering and small-angle light scattering. The relaxation of highly aligned lamellar stacks is slow (>2 h) after flow cessation compared to that of the regrouped grains (a few minutes). The main contribution to thixotropic behavior is likely from the faster relaxation of the reorganized grains containing highly oriented lamellar stacks. The comprehensive understanding of structure-function relationship of the PPR gel will facilitate the rational design for its applications.

Conformational preferences of TEMPO type radicals in complexes with cyclodextrins revealed by a combination of EPR spectroscopy, induced circular dichroism and molecular modeling

Electron paramagnetic resonance (EPR) spectroscopy is the main tool for evidencing the formation of inclusion complexes of cyclodextrins with paramagnetic guests, based on changes in the EPR parameters. In-depth information on complexation can only be obtained by a combination of physico-chemical methods. Herein we report on the interaction of three TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) type radicals with cyclodextrins by collecting and analysing data provided experimentally by EPR and circular dichroism spectroscopies and theoretically by density functional theory and molecular docking. The study focused on the pH influence on the complexation of three paramagnetic probes with cyclodextrins. The EPR spectra revealed that the type and protonation state of the substituent linked to the TEMPO structure influences the affinity of the paramagnetic group for the cyclodextrin cavity. Neutral radical species favour stronger association with cyclodextrins and inclusion of the nitroxide group into the cavity, especially in the case of 4-carboxy-TEMPO. Induced circular dichroism signals of neutral species varied in sign and intensity as a function of substituent and cyclodextrin type. Density functional theory and molecular docking results supported the experimental data regarding the conformational preferences of TEMPO radicals in complexes with cyclodextrins.

1-Naphthol as an ESPT fluorescent molecular probe for sensing thermotropic microenvironmental changes of pluronic F127 in aqueous media

Thermotropic microenvironmental changes and the level of hydration in different microenvironments of pluronic F127 (PF127), (PEO₁₀₆ PPO₇₀ PEO₁₀₆, average molar mass 13 000) in aqueous media have been studied using 1-naphthol, which is an ESPT fluorescent molecular probe.