Selective polymerization of functional monomers with Novozym 435

Chiral stationary phases based on lactide derivatives for high-performance liquid chromatography

Lactide is a natural and renewable lactone cyclic ester-containing intrinsic chiral center, providing an affordable natural compound that is potential for the development of chiral polymers. In this work, we reported two novel chiral stationary phases (CSPs) based on lactide derivatives, methylene lactide (MLA), for high-performance liquid chromatography (HPLC). By using free radical polymerization, chemically bonded CSPs of poly(methylene lactide) (PMLA) and side-chain modified PMLA by aminolysis (N-PMLA) can be prepared. Also, poly(l-lactic acid) (PLLA) was prepared as a control. The chiral resolution performance of the chromatographic columns was examined in both reversed-phase and normal-phase modes. PMLA and N-PMLA CSPs exhibited fairly good chiral recognition ability, whereas the separation ability of PLLA is much weaker. This work provides a new platform for the development of high-performance CSPs from affordable natural products.

Lipase-Catalyzed Reactive Extrusion: Copolymerization of ε-Caprolactone and ω-Pentadecalactone

This study assesses the use of immobilized lipase catalyst N435 during reactive extrusion (REX) versus magnetically stirred bulk and solution reaction conditions for the copolymerization of ε-caprolactone with ω-pentadecalactone (CL/PDL 1:1 molar). N435-catalyzed REX for reaction times from 1 to 3 h results in total %-monomer conversion, M_n , and Đ values increase from 92.7% to 98.8%, 36.1 to 51.3 kDa, and 1.85 to 1.96, respectively. Diad fraction analysis by quantitative ¹³ C NMR reveals that, after just 1 h, rapid N435-catalyzed transesterification reactions occur that give random copolyesters. In contrast, for bulk polymerization with magnetic stirring in round bottom flasks, reaction times from 1 to 3 h result in the following: M_n  increases from 12.4 to 25.6 kDa, Đ decreases from 2.98 to 1.87, and the randomness index increases from 0.74 and 0.86 as PDL*-PDL diads are dominant. These results highlight that REX avoids problems associated with internal batch mixing that are encountered in bulk polymerizations. In sharp contrast to a previous study of 1:1 molar PDL/δ-valerolactone (VL) copolymerizations by N435-catalyzed REX, VL %-conversion increases to just 40.1% in 1 h whereas CL reaches 94.7%.

Main Structural Targets for Engineering Lipase Substrate Specificity

Microbial lipases represent one of the most important groups of biotechnological biocatalysts. However, the high-level production of lipases requires an understanding of the molecular mechanisms of gene expression, folding, and secretion processes. Stable, selective, and productive lipase is essential for modern chemical industries, as most lipases cannot work in different process conditions. However, the screening and isolation of a new lipase with desired and specific properties would be time consuming, and costly, so researchers typically modify an available lipase with a certain potential for minimizing cost. Improving enzyme properties is associated with altering the enzymatic structure by changing one or several amino acids in the protein sequence. This review detailed the main sources, classification, structural properties, and mutagenic approaches, such as rational design (site direct mutagenesis, iterative saturation mutagenesis) and direct evolution (error prone PCR, DNA shuffling), for achieving modification goals. Here, both techniques were reviewed, with different results for lipase engineering, with a particular focus on improving or changing lipase specificity. Changing the amino acid sequences of the binding pocket or lid region of the lipase led to remarkable enzyme substrate specificity and enantioselectivity improvement. Site-directed mutagenesis is one of the appropriate methods to alter the enzyme sequence, as compared to random mutagenesis, such as error-prone PCR. This contribution has summarized and evaluated several experimental studies on modifying the substrate specificity of lipases.

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.

Chemo-Enzymatic Synthesis of Poly(4-piperidine lactone- b-ω-pentadecalactone) Block Copolymers as Biomaterials with Antibacterial Properties

With increasing troubles in bacterial contamination and antibiotic-resistance, new materials possessing both biocompatibility and antimicrobial efficacy are supposed to be developed for future biomedical application. Herein, we demonstrated a chemo-enzymatic ring opening polymerization (ROP) approach for block copolyester, that is, poly(4-benzyl formate piperidine lactone- b-ω-pentadecalactone) (PNPIL- b-PPDL), in a one-pot two-step process. Afterward, cationic poly(4-piperidine lactone- b-ω-pentadecalactone) (PPIL- b-PPDL) with pendent secondary amino groups was obtained via acidic hydrolysis of PNPIL- b-PPDL. The resulting cationic block copolyester exhibited high antibacterial activity against Gram negative E. coli and Gram positive S. aureus, while showed low toxicity toward NIH-3T3 cells. Moreover, the antibacterial property, cytotoxicity and degradation behavior could be tuned simply by variation of PPIL content. Therefore, we anticipate that such cationic block copolymers could potentially be applied as biomaterials for medicine or implants.

Ring opening polymerization of macrolactones: high conversions and activities using an yttrium catalyst

The ring-opening polymerization of macrolactones (C₁₅–C₂₃) is reported using an yttrium catalyst which shows high rates and conversions in the production of long-chain aliphatic polyesters.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Lipases in polymer chemistry

Lipases are highly active in the polymerization of a range of monomers. Both ring-opening polymerization of cyclic monomers such as lactones and carbonates as well as polycondensation reactions have been investigated in great detail. Moreover, in combination with other (chemical) polymerization techniques, lipase-catalyzed polymerization has been employed to synthesize a variety of polymer materials. Major advantages of enzymatic catalysts are the often-observed excellent regio-, chemo- and enantioselectivity that allows for the direct preparation of functional materials. In particular, the application of techniques such as Dynamic Kinetic Resolution (DKR) in the lipase-catalyzed polymerization of racemic monomers is a new development in enzymatic polymerization. This paper reviews selected examples of the application of lipases in polymer chemistry covering the synthesis of linear polymers, chemoenzymatic polymerization and applications of enantioselective techniques for the synthesis and modification of polymers.

Synthesis of aliphatic suberin-like polyesters by ecofriendly catalytic systems

A rapid and ecofriendly microwave assisted p-dodecylbenzenesulfonic acid (DBSA) emulsion polycondensation of long-chain suberin model comonomers was successfully carried out for the first time. Microwave irradiation reduced drastically the reaction time to only 15 min, compared with the DBSA/water polycondensation under conventional heating. Bulk polycondensation using CALB lipase or Bi(OTf)3 were also carried out with isolation yields up to 93% and number-average molecular weights up to around 7300.

A smart micellar system with an amine-containing polycarbonate shell

The present paper reports the design and preparation of an amphiphilic triblock co-polymer poly(ε-caprolactone) (PCL)-poly(6,14-dimethyl-1,3,9,11-tetraoxa-6,14-diaza-cyclohexadecane-2,10-dione) (PADMC)-PCL and the use of micelles composed of them as carriers for pH-sensitive drug release. The triblock co-polymers were synthesized via two-step ring-opening polymerization with catalysis by Novozym-435 lipase. By adjusting the feed ratio, three co-polymers with different PCL lengths and the same PADMC length were produced. The block structure of the co-polymers obtained was confirmed by comparative studies on PCL-PADMC-PCLs and the corresponding random poly(ε-caprolactone-random-6,14-dimethyl-1,3,9,11-tetraoxa-6,14-diaza-cyclohexadecane-2,10-dione) (poly(CL-r-ADMC)) by means of nuclear magnetic resonance and differential scanning calorimetry. Cell cytotoxicity tests showed that the co-polymer displayed no apparent cytotoxicity to 293T and HeLa cells. Transmissions electron microscopy indicates that the self-assembled micelles exhibited a well-defined spherical shape with a diameter between ∼30 and 50 nm. The critical aggregation concentration was dependent on the block composition. Due to the presence of ionizable tertiary amine groups in the PADMC block, acid-induced variation in the micellar morphology was evident with respect to micelle size and size distribution. The size-pH curve was characterized by a smooth sigmoid form, and had a dramatic upward shift with decreasing pH from 6.5 to 4.5, which correlated well with the buffer range of hydrophilic PADMC. As a demonstration of the potential of PCL-PADMC-PCL micelles to control drug delivery, acid induced drug release for prednisone acetate-loaded micelles was explored. PCL-PADMC-PCL micelles show good promise as smart drug carriers, sensing the local specific pH decrease around lesion sites.

Functional aliphatic polyesters for biomedical and pharmaceutical applications

Functional aliphatic polyesters are biodegradable polymers with many possibilities to tune physico-chemical characteristics such as hydrophilicity and degradation rate as compared to traditional polyesters (e.g. PLLA, PLGA and PCL), making the materials suitable for drug delivery or as scaffolds for tissue engineering. Lately, a large number of polyesters have been synthesized by homopolymerization of functionalized monomers or co-polymerization with other monomers mainly via ring-opening polymerization (ROP) of cyclic esters. This review presents the recent trends in the synthesis of these materials and their application for protein delivery and tissue engineering.

Enzyme-catalysis breathes new life into polyester condensation polymerizations

Traditional chemical catalysts for polyester synthesis have enabled the generation of important commercial products. Undesirable characteristics of chemically catalyzed condensation polymerizations include the need to conduct reactions at high temperatures (150-280 degrees C) with metal catalysts that are toxic and lack selectivity. The latter is limiting when aspiring towards synthesis of increasingly complex and well-defined polyesters. This review describes an exciting technology that makes use of immobilized enzyme-catalysts for condensation polyester synthesis. Unlike chemical catalysts, enzymes function under mild conditions (< or =100 degrees C), which enables structure retention when polymerizing unstable monomers, circumvents the introduction of metals, and also provides selectivity that avoids protection-deprotection steps and presents unique options for structural control. Examples are provided that describe the progress made in enzyme-catalyzed polymerizations, as well as current limitations and future prospects for developing more efficient enzyme-catalysts for industrial processes.

Lipase-catalyzed polyester synthesis--a green polymer chemistry

This article is a short comprehensive review describing in vitro polyester synthesis catalyzed by a hydrolysis enzyme of lipase, most of which has been developed for these two decades. Polyesters are prepared by repeated ester bond-formation reactions; they include two major modes, ring-opening polymerization (ROP) of cyclic monomers such as cyclic esters (lactones) and condensation polymerization via the reaction between a carboxylic acid or its ester group and an alcohol group. Polyester synthesis is, therefore, a reaction in reverse way of in vivo lipase catalysis of ester bond-cleavage with hydrolysis. The lipase-catalyzed polymerizations show very high chemo-, regio-, and enantio-selectivities and involve various advantageous characteristics. Lipase is robust and compatible with other chemical catalysts, which allows novel chemoenzymatic processes. New syntheses of a variety of functional polyesters and a plausible reaction mechanism of lipase catalysis are mentioned. The polymerization characteristics are of green nature currently demanded for sustainable society, and hence, desirable for conducting 'green polymer chemistry'.