Redox-Responsive Disulfide Cross-Linked PLA–PEG Nanoparticles

We have developed a strategy for the preparation of redox-responsive PEG–PLA-based nanoparticles containing disulfide bonds that can be disassembled in the presence of cellular concentrations of glutathione. Functionalized poly­(lactide)­s were prepared by ring-opening copolymerization of l-lactide and 3-methyl-6-(tritylthio­methyl)-1,4-dioxane-2,5-dione, a monomer bearing a pendant trityl-thiol group, followed by the postpolymerization modification of trityl-thiol into pyridyl disulfide groups. Polymeric networks composed of PLA and PEG blocks linked by disulfide bonds were prepared by a disulfide exchange reaction between the functionalized PLAs and telechelic PEG having thiol groups at both ends, HS-PEG-SH, in DMF. When dialyzed against water, they assembled into dispersible nanoparticles, with a flowerlike structure having a hydrophobic core and a hydrophilic shell, with sizes in the range 167–300 nm that are suitable for drug delivery. The effects of the number of functional groups, molecular weight, and concentration on the nanoparticle size were evaluated. The stability of the nanoparticles after dilution and the redox-responsive behavior in the presence of different concentrations of glutathione were assessed. The hydrophobic molecule Nile red could be encapsulated in the nanoparticles and then released in the presence of glutathione at cellular concentration

A Route to Aliphatic Poly(ester)s with Thiol Pendant Groups: From Monomer Design to Editable Porous Scaffolds

Biodegradable aliphatic polyesters such as poly­(lactide) and poly­(ε-caprolactone), largely used in tissue engineering applications, lack suitable functional groups and biological cues to enable interactions with cells. Because of the ubiquity of thiol groups in the biological environment and the pliability of thiol chemistry, we aimed to design and synthesize poly­(ester) chains bearing pendant thiol-protected groups. To achieve this, 3-methyl-6-(tritylthiomethyl)-1,4-dioxane-2,5-dione, a lactide-type monomer possessing a pendant thiol-protected group, was synthesized. This molecule, when used as a monomer in controlled ring-opening polymerization in combination with lactide and ε-caprolactone, appeared to be a convenient “building block” for the preparation of functionalized aliphatic copolyesters, which were easily modified further. A polymeric sample bearing pyridyl disulfide groups, able to bind a cysteine-containing peptide, was efficiently obtained from a two-step modification reaction. Porous scaffolds were then prepared by blending this latter copolymer sample with poly­(l-lactide<i>-<i>co</i>-</i>ε-caprolactone) followed by salt leaching. A further disulfide exchange reaction performed in aqueous medium formed porous scaffolds with covalently linked arginine-glycine-aspartic acid sequences. The scaffolds were characterized by thermal and mechanical tests, and scanning electron microscopy surface images revealed a highly porous morphology. Moreover, a cytotoxicity test indicated good cell viability

Random Copolymerization of ε‑Caprolactone and Lactides Promoted by Pyrrolylpyridylamido Aluminum Complexes

The monomethylaluminum complexes <b>1</b> and <b>2</b>, bearing pyrrolylpyridylamido as dianionic [⁻NNN^–] tridentate ligands with general formula [NNN]­AlMe, were synthesized and tested as initiators in the ring-opening polymerization (ROP) of ε-caprolactone, l-lactide, and d,l-lactide. In the presence of 1 equiv of alcohol, compounds <b>1</b> and <b>2</b> were highly active initiators in the ROP of ε-CL (TOF up to 4000 mol_CL mol_Al^–1 h^–1), and they showed moderate activity in the ROP of lactides (TOF up to 1.7 mol_LA mol_Al^–1 h^–1). The polymerization processes proceeded with a living mechanism; moreover, the obtained PLAs resulted isotactic-enriched with <i>P</i>_m values up to 76%. More interestingly, this class of catalysts promoted the random copolymerization of ε-caprolactone and lactides. In particular, compound <b>1</b> allowed excellently controlled random copolymerization of ε-caprolactone and d,l-lactide as indicated by both the values of the reactivity ratios of the two monomers (<i>r</i>_LA = 1.17; <i>r</i>_CL = 1.36) and the average lengths of the caproyl and lactidyl sequences (<i>L</i>_CL = 2.0; <i>L</i>_LA = 2.5)

Template-Assisted Enzymatic Synthesis of Oligopeptides from a Polylactide Chain

Peptides are often attached to polymer materials, as bioactive components, for the control of interactions between the material and its surrounding proteins and cells. However, synthesizing peptides and attaching them to polymers can be challenging and laborious. Herein, we describe the grafting of oligopeptides to an aliphatic polyester, using a one-step chemo-enzymatic synthesis with papain as the biocatalyst. To enable enzyme-mediated functionalization of the polyester, ethyl hept-6-enoylalaninate (grafter) was synthesized and attached to polylactide chains using thiol–ene click reactions. The oligopeptides were grafted onto the polylactide chains using two different synthetic routes: the grafting from strategy, in which the grafter was attached to the polyester prior to oligopeptide synthesis, or the grafting to strategy, in which oligopeptides were synthesized on the grafter first, then attached to the polymer chain. The final products were analyzed and their structures were confirmed using nuclear magnetic resonance (NMR). The peptide attachment was evaluated using size exclusion chromatography (SEC), contact angle measurement and energy-dispersive X-ray spectroscopy-scanning electron microscopy (EDS-SEM). Furthermore, the mechanistic aspects of the synthesis of the oligopeptides on the grafter were studied using molecular dynamics (MD) simulations. The simulation revealed that hydrogen bonding (between the P1 amide nitrogen of the grafter backbone and the carbonyl oxygen of D158 in the papain) maintain the grafter in a productive conformation to stabilize the transition state of nitrogen inversion, a key step of the biocatalytic mechanism. Apart from being biologically relevant, both experimental and computational results suggest that the designed grafter is a good template for initiating chemo-enzymatic synthesis. The results also showed that the grafting to strategy was more successful compared to the grafting from strategy. Overall, a successful synthesis of predefined peptide functionalized polylactide was prepared, where the oligopeptides were grafted in an easy, time efficient, and environmentally friendly way

Anilidopyridyl-Pyrrolide and Anilidopyridyl-Indolide Group 3 Metal Complexes: Highly Active Initiators for the Ring-Opening Polymerization of <i>rac</i>-Lactide

Three new group 3 metal complexes, bearing anilidopyridyl-pyrrolide (<b>L</b>^<b>1</b>) and anilidopyridyl-indolide (<b>L</b>^<b>2</b>) as dianionic tridentate ligands, with the general formula LMN­(SiHMe₂)₂ were synthesized (complex <b>1</b>, M = Y, L = <b>L</b>^<b>1</b>; complex <b>2</b>, M = Sc, L = <b>L</b>^<b>1</b>; complex <b>3</b>, M = Y, L = <b>L</b>^<b>2</b>). All complexes were fully characterized and tested as initiators for the ROPs of <i>rac</i>-lactide. The yttrium complexes <b>1</b> and <b>3</b> resulted in highly active catalysts (TOF up to 10⁴ mol_lactide mol_Y^–1 h^–1), whereas the scandium complex showed moderate activities. This class of catalysts allowed a good control of the macromolecular architecture of the polymer, namely, the nature of end groups, the molecular weights, and their distribution. Moreover, the obtained PLAs showed <i>P</i>_r values in the range of 0.57–0.84, depending on the nature of the initiator and solvent. Well-controlled and rapid ROPs of <i>rac</i>-lactide were obtained in the solvent-free polymerizations at 130 °C as well, suggesting that complexes <b>1</b>–<b>3</b> are stable at high temperature. Finally, in the presence of 2-propanol, complex <b>1</b> promoted the <i>immortal</i> ROP of <i>rac</i>-lactide, showing a remarkable TOF of 3.5 × 10⁴ mol_lactide mol_Y^–1h^–1

Phenoxy-Thioether Aluminum Complexes as ε‑Caprolactone and Lactide Polymerization Catalysts

A series of new phenoxy-thioether (OS) proligands have been synthesized. They were found to readily react with 1 equiv of AlMe₃ to afford the corresponding Al chelate complexes {4,6-tBu₂-OC₆H₂-2-CH₂S­(2-R-C₆H₄)}­AlMe₂ (R = H (<b>1</b>), Br (<b>2</b>), CH₃ (<b>3</b>), CF₃ (<b>4</b>)) in quantitative yields. All the aluminum methyl complexes are stable monomeric species. In the solid state, as determined from X-ray crystallographic studies, complex <b>2</b> consists of a four-coordinate aluminum species in which the metal center is chelated by the sulfur and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and l- and <i>rac</i>-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of <i>rac</i>-LA were converted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by <b>1</b>–<b>4</b> are first order with respect to monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters, increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide

Phenoxy-Thioether Aluminum Complexes as ε‑Caprolactone and Lactide Polymerization Catalysts

A series of new phenoxy-thioether (OS) proligands have been synthesized. They were found to readily react with 1 equiv of AlMe₃ to afford the corresponding Al chelate complexes {4,6-tBu₂-OC₆H₂-2-CH₂S­(2-R-C₆H₄)}­AlMe₂ (R = H (<b>1</b>), Br (<b>2</b>), CH₃ (<b>3</b>), CF₃ (<b>4</b>)) in quantitative yields. All the aluminum methyl complexes are stable monomeric species. In the solid state, as determined from X-ray crystallographic studies, complex <b>2</b> consists of a four-coordinate aluminum species in which the metal center is chelated by the sulfur and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and l- and <i>rac</i>-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of <i>rac</i>-LA were converted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by <b>1</b>–<b>4</b> are first order with respect to monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters, increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide

Anilidopyridyl-Pyrrolide and Anilidopyridyl-Indolide Group 3 Metal Complexes: Highly Active Initiators for the Ring-Opening Polymerization of <i>rac</i>-Lactide

Three new group 3 metal complexes, bearing anilidopyridyl-pyrrolide (<b>L</b>^<b>1</b>) and anilidopyridyl-indolide (<b>L</b>^<b>2</b>) as dianionic tridentate ligands, with the general formula LMN­(SiHMe₂)₂ were synthesized (complex <b>1</b>, M = Y, L = <b>L</b>^<b>1</b>; complex <b>2</b>, M = Sc, L = <b>L</b>^<b>1</b>; complex <b>3</b>, M = Y, L = <b>L</b>^<b>2</b>). All complexes were fully characterized and tested as initiators for the ROPs of <i>rac</i>-lactide. The yttrium complexes <b>1</b> and <b>3</b> resulted in highly active catalysts (TOF up to 10⁴ mol_lactide mol_Y^–1 h^–1), whereas the scandium complex showed moderate activities. This class of catalysts allowed a good control of the macromolecular architecture of the polymer, namely, the nature of end groups, the molecular weights, and their distribution. Moreover, the obtained PLAs showed <i>P</i>_r values in the range of 0.57–0.84, depending on the nature of the initiator and solvent. Well-controlled and rapid ROPs of <i>rac</i>-lactide were obtained in the solvent-free polymerizations at 130 °C as well, suggesting that complexes <b>1</b>–<b>3</b> are stable at high temperature. Finally, in the presence of 2-propanol, complex <b>1</b> promoted the <i>immortal</i> ROP of <i>rac</i>-lactide, showing a remarkable TOF of 3.5 × 10⁴ mol_lactide mol_Y^–1h^–1

Different Insight into Amphiphilic PEG-PLA Copolymers: Influence of Macromolecular Architecture on the Micelle Formation and Cellular Uptake

One constrain in the use of micellar carriers as drug delivery systems (DDSs) is their low stability in aqueous solution. In this study “tree-shaped” copolymers of general formula mPEG-(PLA)_n (<i>n</i> = 1, 2 or 4; mPEG = poly­(ethylene glycol) monomethylether 2K or 5K Da; PLA = atactic or isotactic poly­(lactide)) were synthesized to evaluate the architecture and chemical composition effect on the micelles formation and stability. Copolymers with mPEG/PLA ratio of about 1:1 wt/wt were obtained using a “core-first” synthetic route. Dynamic Light Scattering (DLS), Field Emission Scanning Electron Microscopy (FESEM), and Zeta Potential measurements showed that mPEG_2K-(PD,LLA)₂ copolymer, characterized by mPEG chain of 2000 Da and two blocks of atactic PLA, was able to form monodisperse and stable micelles. To analyze the interaction among micelles and tumor cells, FITC conjugated mPEG-(PLA)_<i>n</i> were synthesized. The derived micelles were tested on two, histological different, tumor cell lines: HEK293t and HeLa cells. Fluorescence Activated Cells Sorter (FACS) analysis showed that the FITC conjugated mPEG_2K-(PD,LLA)₂ copolymer stain tumor cells with high efficiency. Our data demonstrate that both PEG size and PLA structure control the biological interaction between the micelles and biological systems. Moreover, using confocal microscopy analysis, the staining of tumor cells obtained after incubation with mPEG_2K-(PD,LLA)₂ was shown to be localized inside the tumor cells. Indeed, the mPEG_2K-(PD,LLA)₂ paclitaxel-loaded micelles mediate a potent antitumor cytotoxicity effect