Enzymatic syntheses of linear and hyperbranched poly-l-lactide using compressed R134a–ionic liquid media

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

The Effect of Solvent Hydrophilicity on the Enzymatic Ring-Opening Polymerization of L-Lactide by Candida rugosa Lipase

Contradictions have been reported on the effect of organic solvents, especially toluene, on enzymatic ring-opening polymerization (eROP) of L-lactide. Studies have shown that log P, a common measure of hydrophilicity, affects enzyme activity. This study examines the effect of solvents with various log P values on the eROP of L-lactide, performed using Candida rugosa lipase (CRL). N,N-dimethylacetamide (DMA), 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, diphenyl ether, and dodecane were used as the organic solvents. The eROP in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) was also conducted to compare its performance with the organic solvents. The results show that [BMIM][PF₆]-mediated eROP gave better conversion and molecular weight than the organic solvent-mediated eROP. In this study, the effects of solvents hydrophilicity are discussed, including the possibility of hexafluorophosphate ion ([PF₆]⁻) hydrolysis to occur.

Recent advances in the synthesis of biodegradable polyesters by sustainable polymerization: lipase-catalyzed polymerization

Over the past three decades, enzymatic polymerization has dramatically developed and gradually broadened as a creative methodology in the construction of polymeric materials with tailor-made structures and properties. Compared with transition metal catalyst polymerizations, enzymatic polymerization is more attractive in the biomedicine field due to the metal-free residue, good biocompatibility, and few by-products. Meanwhile, enzymatic polymerization has far more activity towards macrolides. In this review, the synthesis of lipase-catalyzed polymer materials is systematically summarized, focusing on the synthesis of the complex and well-defined polymers. The enzymatic polyester synthesis was then discussed concerning the different reaction types, including ring-opening polymerization, polycondensation, a combination of ring-opening polymerization with polycondensation, and chemoenzymatic polymerization. Besides, exploration of novel biocatalysts and reaction media was also described, with particular emphasis on the enzymes obtained via immobilization or protein engineering strategies, green solvents, and reactors. Finally, recent developments in catalytic kinetics and mechanistic studies through the use of spectroscopy, mathematics, and computer techniques have been introduced. Besides, we addressed the remaining central issues in enzymatic polymerization and discussed current studies aimed at providing answers.

Spectroscopic Studies of a Phosphonium Ionic Liquid in Supercritical CO2

Fluorescence spectroscopy was used to study a solution comprised of coumarin 153 (C153)+ trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P6,6,6,14]+ [Tf2N]−)+ supercritical CO2 (scCO2). We compare the spectroscopy of C153 in neat scCO2 to that of C153/scCO2 with the addition of ionic liquid (IL). Excitation and emission peak frequencies of C153 in scCO2 and in IL/scCO2 diverged at reduced densities (ρr = ρ/ρc) below the CO2 critical density. At low fluid density, spectral changes in the IL/scCO2 solutions showed evidence that C153 experiences a very different microenvironment—one that is unlike neat scCO2. The data show that the presence of IL clearly influences the C153 excitation and emission profiles. Excitation was broadened and red shifted by >2000 cm−1 and the presence of an additional low-energy emission component that was red shifted by ~3000 cm−1 was clearly visible and not observed in neat scCO2. The solution heterogeneity was controlled by changing the scCO2 density and at high fluid density, both the excitation and emission spectra were more similar to those in neat scCO2. Steady-state anisotropy also showed that at low fluid density, the C153 emission was significantly polarized. Aggregation of C153 has been reported in the literature and this led us to hypothesize the possibility that C153 dimer (aggregation) formation may be occurring in scCO2. Another possible explanation is that dye–IL aggregates may dissolve into the scCO2 phase due to C153 acting as a “co-solvent” for the IL. Time-resolved intensity decay measurements yielded only slightly non-exponential decays with accompanying time constants of ~3–4 ns that were significantly shorter than the 5–6 ns time constants in neat scCO2, which are suggestive of C153–IL interactions. However, these data did not conclusively support dimer formation. Pre-exponential factors of the time constants showed that almost all of the emission was due to monomeric C153.

Chemoenzymatic synthesis of polypeptides in neat 1,1,1,2-tetrafluoroethane solvent

Chemoenzymatic polypeptide synthesis offers several advantages over chemical or other biological routes, however, the use of aqueous-based media suffers from reverse hydrolysis reactions that challenge peptide chain propagation. Herein, the protease from subtilisin Carlsberg biocatalyzed the synthesis of poly-l-PheOEt, poly-l-LeuOEt, and the copolymers poly-l-PheOEt-co-l-LeuOEt from their amino acid ethyl ester substrates in a neat liquid 1,1,1,2-tetrafluoroethane solvent. The products, achieved in acceptable yields (ca. 50%), were fully characterized showing relatively high molar mass (ca. 20 000 Da for poly-l-PheOEt). This non-toxic low-boiling hydrofluorocarbon enhances enzymatic peptide propagation by limiting hydrolysis owing to its hydrophobic and relatively polar characteristics that sustain the protease activity and solubilize substrates and products. Computational molecular dynamic calculations were used to assess the l-PheOEt/l-LeuOEt-solvent and polypeptide-solvent interactions in this system. Additionally, the homopolypeptides displayed higher crystallinity than the copolypeptides with random incorporation of amino acid ethyl esters, notwithstanding the significantly highest specificity for Phe in this system. Interestingly, secondary structure characterization of the products by FTIR and circular dichroism suggests a non-common peptide folding.

Enantio-, Regio-, and Chemoselective Lipase-Catalyzed Polymer Synthesis

In contrast to chemical routes, enzymatic polymerization possesses favorable characteristics of mild reaction conditions, few by-products, and high activity toward cyclic lactones which make it a promising technique for constructing polymeric materials. Meanwhile, it can avoid the trace residue of metallic catalysts and potential toxicity, and thus exhibits great potential in the biomedical fields. More importantly, lipase-catalyzed polymer synthesis usually shows favorable enantio-, regio-, and chemoselectivity. Here, the history and recent developments in lipase-catalyzed selective polymerization for constructing polymers with unique structures and properties are highlighted. In particular, the synthesis of polymeric materials which are difficult to prepare in a chemical route and the construction of polymers through the combination of selective enzymatic and chemical methods are focused. In addition, the future direction is proposed especially based on the rapid developments in computational chemistry and protein engineering techniques.

Enzymatic hydrolysis of chitin pretreated by rapid depressurization from supercritical 1,1,1,2-tetrafluoroethane toward highly acetylated oligosaccharides

The hydrolysis of chitin treated under supercritical conditions was successfully carried out using chitinases obtained by an optimized fermentation of the fungus Lecanicillium lecanii. The biopolymer was subjected to a pretreatment based on suspension in supercritical 1,1,1,2-tetrafluoroethane (scR134a), which possesses a critical temperature and pressure of 101°C and 40bar, respectively, followed by rapid depressurization to atmospheric pressure and further fibrillation. This methodology was compared to control untreated chitins and chitin subjected to steam explosion showing improved production of reducing sugars (0.18mg/mL), enzymatic hydrolysis and high acetylation (FA of 0.45) in products with degrees of polymerization between 2 and 5.

Radical-Mediated Enzymatic Polymerizations

Polymerization reactions are commonly effected by exposing monomer formulations to some initiation stimulus such as elevated temperature, light, or a chemical reactant. Increasingly, these polymerization reactions are mediated by enzymes--catalytic proteins--owing to their reaction efficiency under mild conditions as well as their environmental friendliness. The utilization of enzymes, particularly oxidases and peroxidases, for generating radicals via reduction-oxidation mechanisms is especially common for initiating radical-mediated polymerization reactions, including vinyl chain-growth polymerization, atom transfer radical polymerization, thiol-ene step-growth polymerization, and polymerization via oxidative coupling. While enzyme-mediated polymerization is useful for the production of materials intended for subsequent use, it is especially well-suited for in situ polymerizations, where the polymer is formed in the place where it will be utilized. Such polymerizations are especially useful for biomedical adhesives and for sensing applications.