"Clicking" trimeric peptides onto hybrid T8POSS nanocages and identifying synthesis limitations

Macromolecule branching upon polyhedral oligomeric silsesquioxanes (POSS) via "click" chemistry has previously been reported for promoting natural biological responses in vitro, particularly when regarding their demonstrated biocompatibility and structural robustness as potential macromolecule anchoring points. However, "clicking" of large molecules around POSS structures uncovers two main challenges: (1) a synthetic challenge encompassing multi-covalent attachment of macromolecules to a single nanoscale-central position, and (2) purification and separation of fully adorned nanocages from those that are incomplete due to their similar physical characteristics. Here we present peptide decoration to a T8POSS nanocage through the attachment of azido-modified trimers. Triglycine- and trialanine-methyl esters "clicked" to 97% and 92% completion, respectively, resulting in 84% and 68% yields of the fully-adorned octamers. The "clicks" halt within 27-h of the reaction time, and efforts to further increase the octamer yield were of negligible benefit. Exploration of reaction conditions reveals multiple factors preventing full octa-arm modification to all available POSS nanocages, and offers insights into macromolecule attachment between both peptides and small inorganic-organic structures, all of which require consideration for future work of this nature.

Catalyzing Benzoxazine Polymerization with Titanium-Containing POSS to Reduce the Curing Temperature and Improve Thermal Stability

Trisilanolphenyl-polyhedral oligomeric silsesquioxane titanium (Ti-Ph-POSS) was synthesized through the corner-capping reaction, and Ti-Ph-POSS was dispersed in benzoxazine (BZ) to prepare Ti-Ph-POSS/PBZ composite materials. Ti-Ph-POSS could catalyze the ring-opening polymerization (ROP) of BZ and reduce the curing temperature of benzoxazine. In addition, Ti immobilized on the Ti-Ph-POSS cage could form covalent bonds with the N or O atoms on polybenzoxazine, improving the thermal stability of PBZ. The catalytic activity of the Ti-Ph-POSS/BZ mixtures was assessed and identified through ¹H nuclear magnetic resonance (¹H-NMR) and Fourier-transform infrared (FTIR) analyses, while thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) were used to determine the thermal properties of the composite. It was found that PBZ exhibited a higher glass transition temperature (T_g) and better thermal stability when Ti-Ph-POSS was added. The curing behavior of the Ti-Ph-POSS/BZ mixtures showed that the initial (T_i) and peak (T_p) curing temperatures sharply decreased as the content of Ti-Ph-POSS and the heating rate increased. The curing kinetics of these Ti-Ph-POSS/BZ systems were analyzed using the Kissinger method, and the morphology of Ti-Ph-POSS/PBZ was determined via scanning electron microscopy (SEM). It was found that the Ti-Ph-POSS particles were well distributed in the composites. When the content exceeded 2 wt%, several Ti-Ph-POSS particles could not react with benzoxazine and were only dispersed within the PBZ matrix, resulting in aggregation of the Ti-Ph-POSS molecules.

A comparison of the chemical bonding and reactivity of Si8H8O12 and Ge8H8O12: A theoretical study

We have analyzed the chemical bonding and reactivity in the cubic molecule octahydridosilsesquioxane, Si₈H₈O₁₂, and its counterpart Ge₈H₈O₁₂ by means of ab initio quantum chemical methods and group theory. Density functional theory and MP2 methods combined with the basis sets 6-311+G(d) and 6-311++G(2d,p) were used for geometry optimization and vibrational frequency analysis. The geometries of Si₈H₈O₁₂ and Ge₈H₈O₁₂ are unstable under O_h symmetry and distort to the rare T_h molecular symmetry. The energy gained from this pseudo-Jahn-Teller distortion ranges from 0.78 to 6.14 kcal mol^-1 depending on methodological treatment. The Fukui functions and the molecular electrostatic potential were both used as DFT-based reactivity descriptors. Our study shows that Si₈H₈O₁₂ and Ge₈H₈O₁₂ are both hard amphoteric molecules. The cavity within each cage is acidic and able to encapsulate hard small bases such as F^-. The exterior of the cages is basic and can form stable exohedral complexes with hard acids, as in the case of H⁺. The insertion of F^- in Si₈H₈O₁₂ and Ge₈H₈O₁₂ cages gives the most stable endohedral complexes of the series studied, characterized by formation energies of -3.50 and -3.45 eV at CAM-B3LYP/6-311+G(d) and -3.61 and -3.68 eV at the MP2/6-311++G(d,p) level, respectively. The calculated formation energies of the exohedral and endohedral complexes align with the DFT reactivity descriptor analysis.