Lipase-catalyzed polyester synthesis--a green polymer chemistry

Biocatalyst immobilization on magnetic nano-architectures for potential applications in condensation reactions

In this review article, a perspective on the immobilization of various hydrolytic enzymes onto magnetic nanoparticles for synthetic organic chemistry applications is presented. After a first part giving short overview on nanomagnetism and highlighting advantages and disadvantages of immobilizing enzymes on magnetic nanoparticles (MNPs), the most important hydrolytic enzymes and their applications were summarized. A section reviewing the immobilization techniques with a particular focus on supporting enzymes on MNPs introduces the reader to the final chapter describing synthetic organic chemistry applications of small molecules (flavour esters) and polymers (polyesters and polyamides). Finally, the conclusion and perspective section gives the author's personal view on further research discussing the new idea of a synergistic rational design of the magnetic and biocatalytic component to produce novel magnetic nano-architectures.

Enzymes in "Green" Synthetic Chemistry: Laccase and Lipase

Enzymes play an important role in numerous natural processes and are increasingly being utilized as environmentally friendly substitutes and alternatives to many common catalysts. Their essential advantages are high catalytic efficiency, substrate specificity, minimal formation of byproducts, and low energy demand. All of these benefits make enzymes highly desirable targets of academic research and industrial development. This review has the modest aim of briefly overviewing the classification, mechanism of action, basic kinetics and reaction condition effects that are common across all six enzyme classes. Special attention is devoted to immobilization strategies as the main tools to improve the resistance to environmental stress factors (temperature, pH and solvents) and prolong the catalytic lifecycle of these biocatalysts. The advantages and drawbacks of methods such as macromolecular crosslinking, solid scaffold carriers, entrapment, and surface modification (covalent and physical) are discussed and illustrated using numerous examples. Among the hundreds and possibly thousands of known and recently discovered enzymes, hydrolases and oxidoreductases are distinguished by their relative availability, stability, and wide use in synthetic applications, which include pharmaceutics, food and beverage treatments, environmental clean-up, and polymerizations. Two representatives of those groups-laccase (an oxidoreductase) and lipase (a hydrolase)-are discussed at length, including their structure, catalytic mechanism, and diverse usage. Objective representation of the current status and emerging trends are provided in the main conclusions.

Development of Poly(sorbitol adipate)-g-poly(ethylene glycol) Mono Methyl Ether-Based Hydrogel Matrices for Model Drug Release

Hydrogels were prepared by Steglich esterification and by crosslinking pre-synthesized poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG) using different-chain-length-based disuccinyl PEG. PSA and PSA-g-mPEG were investigated for polymer degradation as a function of time at different temperatures. PSA-g-mPEG hydrogels were then evaluated for their most crucial properties of swelling that rendered them suitable for many pharmaceutical and biomedical applications. Hydrogels were also examined for their Sol-Gel content in order to investigate the degree of cross-linking. Physical structural parameters of the hydrogels were theoretically estimated using the modified Flory-Rehner theory to obtain approximate values of polymer volume fraction, the molecular weight between two crosslinks, and the mesh size of the hydrogels. X-ray diffraction was conducted to detect the presence or absence of crystalline regions in the hydrogels. PSA-g-mPEG hydrogels were then extensively examined for higher and lower molecular weight solute release through analysis by fluorescence spectroscopy. Finally, the cytotoxicity of the hydrogels was also investigated using a resazurin reduction assay. Experimental results show that PSA-g-mPEG provides an option as a biocompatible polymer to be used for pharmaceutical applications.

Exploiting Enzyme in the Polymer Synthesis for a Remarkable Increase in Thermal Conductivity

The interest in polymers with high thermal conductivity increased much because of their inherent properties such as low density, low cost, flexibility, and good chemical resistance. However, it is challenging to engineer plastics with good heat transfer characteristics, processability, and required strength. Improving the degree of the chain alignment and forming a continuous thermal conduction network is expected to enhance thermal conductivity. This research aimed to develop polymers with a high thermal conductivity that can be interesting for several applications. Two polymers, namely poly(benzofuran-co-arylacetic acid) and poly(tartronic-co-glycolic acid), with high thermal conductivity containing microscopically ordered structures were prepared by performing enzyme-catalyzed (Novozyme-435) polymerization of the corresponding α-hydroxy acids 4-hydroxymandelic acid and tartronic acid, respectively. A comparison between the polymer's structure and heat transfer obtained by mere thermal polymerization before and enzyme-catalyzed polymerization will now be discussed, revealing a dramatic increase in thermal conductivity in the latter case. The polymer structures were investigated by FTIR spectroscopy, nuclear magnetic resonance (NMR) spectroscopy in liquid- and solid-state (ss-NMR), and powder X-ray diffraction. The thermal conductivity and diffusivity were measured using the transient plane source technique.

Different methods of synthesizing poly(glycerol sebacate) (PGS): A review

Poly(glycerol sebacate) (PGS) is a biodegradable elastomer that has attracted increasing attention as a potential material for applications in biological tissue engineering. The conventional method of synthesis, first described in 2002, is based on the polycondensation of glycerol and sebacic acid, but it is a time-consuming and energy-intensive process. In recent years, new approaches for producing PGS, PGS blends, and PGS copolymers have been reported to not only reduce the time and energy required to obtain the final material but also to adjust the properties and processability of the PGS-based materials based on the desired applications. This review compiles more than 20 years of PGS synthesis reports, reported inconsistencies, and proposed alternatives to more rapidly produce PGS polymer structures or PGS derivatives with tailor-made properties. Synthesis conditions such as temperature, reaction time, reagent ratio, atmosphere, catalysts, microwave-assisted synthesis, and PGS modifications (urethane and acrylate groups, blends, and copolymers) were revisited to present and discuss the diverse alternatives to produce and adapt PGS.

Recent Advances in the Enzymatic Synthesis of Polyester

Polyester is a kind of polymer composed of ester bond-linked polybasic acids and polyol. This type of polymer has a wide range of applications in various industries, such as automotive, furniture, coatings, packaging, and biomedical. The traditional process of synthesizing polyester mainly uses metal catalyst polymerization under high-temperature. This condition may have problems with metal residue and undesired side reactions. As an alternative, enzyme-catalyzed polymerization is evolving rapidly due to the metal-free residue, satisfactory biocompatibility, and mild reaction conditions. This article presented the reaction modes of enzyme-catalyzed ring-opening polymerization and enzyme-catalyzed polycondensation and their combinations, respectively. In addition, the article also summarized how lipase-catalyzed the polymerization of polyester, which includes (i) the distinctive features of lipase, (ii) the lipase-catalyzed polymerization and its mechanism, and (iii) the lipase stability under organic solvent and high-temperature conditions. In addition, this article also focused on the advantages and disadvantages of enzyme-catalyzed polyester synthesis under different solvent systems, including organic solvent systems, solvent-free systems, and green solvent systems. The challenges of enzyme optimization and process equipment innovation for further industrialization of enzyme-catalyzed polyester synthesis were also discussed in this article.

N-Hydroxyethyl acrylamide as a functional eROP initiator for the preparation of nanoparticles under "greener" reaction conditions

N-Hydroxyethyl acrylamide was used as a functional initiator for the enzymatic ring-opening polymerisation of ε-caprolactone and δ-valerolactone. N-Hydroxyethyl acrylamide was found not to undergo self-reaction in the presence of Lipase B from Candida antarctica under the reaction conditions employed. By contrast, this is a major problem for 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate which both show significant transesterification issues leading to unwanted branching and cross-linking. Surprisingly, N-hydroxyethyl acrylamide did not react fully during enzymatic ring-opening polymerisation. Computational docking studies helped us understand that the initiated polymer chains have a higher affinity for the enzyme active site than the initiator alone, leading to polymer propagation proceeding at a faster rate than polymer initiation leading to incomplete initiator consumption. Hydroxyl end group fidelity was confirmed by organocatalytic chain extension with lactide. N-Hydroxyethyl acrylamide initiated polycaprolactones were free-radical copolymerised with PEGMA to produce a small set of amphiphilic copolymers. The amphiphilic polymers were shown to self-assemble into nanoparticles, and to display low cytotoxicity in 2D in vitro experiments. To increase the green credentials of the synthetic strategies, all reactions were carried out in 2-methyl tetrahydrofuran, a solvent derived from renewable resources and an alternative for the more traditionally used fossil-based solvents tetrahydrofuran, dichloromethane, and toluene.

Advancements in Biodegradable Active Films for Food Packaging: Effects of Nano/Microcapsule Incorporation

Food packaging plays a fundamental role in the modern food industry as a main process to preserve the quality of food products from manufacture to consumption. New food packaging technologies are being developed that are formulated with natural compounds by substituting synthetic/chemical antimicrobial and antioxidant agents to fulfill consumers' expectations for healthy food. The strategy of incorporating natural antimicrobial compounds into food packaging structures is a recent and promising technology to reach this goal. Concepts such as "biodegradable packaging", "active packaging", and "bioactive packaging" currently guide the research and development of food packaging. However, the use of natural compounds faces some challenges, including weak stability and sensitivity to processing and storage conditions. The nano/microencapsulation of these bioactive compounds enhances their stability and controls their release. In addition, biodegradable packaging materials are gaining great attention in the face of ever-growing environmental concerns about plastic pollution. They are a sustainable, environmentally friendly, and cost-effective alternative to conventional plastic packaging materials. Ultimately, a combined formulation of nano/microencapsulated antimicrobial and antioxidant natural molecules, incorporated into a biodegradable food packaging system, offers many benefits by preventing food spoilage, extending the shelf life of food, reducing plastic and food waste, and preserving the freshness and quality of food. The main objective of this review is to illustrate the latest advances in the principal biodegradable materials used in the development of active antimicrobial and antioxidant packaging systems, as well as the most common nano/microencapsulated active natural agents incorporated into these food-packaging materials.

Effect of ultrasound on protein functionality

The review focus on the effect of ultrasound on protein functionality. The presence of transient ultrasonic mechanical waves induce various sonochemical and sonomechanical effects on a protein. Sonochemical effects include the breakage of chains and/or the modification of side groups of aminoacids. Sonomechanical modifications by enhanced molecular agitation, might lead to the transient or permanent modification of the 3D structure of the folded protein. Since the biological function of proteins depends on the maintenance of its 3D folded structure, both sonochemical and sonomechanical effects might affect its properties. A protein might maintain its 3D structure and functionality after minor sonochemical effects, however, the enhanced mass transfer by sonomechanical effects might expose internal hydrophobic residues of the protein, making protein unfolding to an irreversible denatured state. Ultrasound enhanced mass transport effects are unique pathways to change the 3D folded structure of proteins which lead to a new functionality of proteins as support shield materials during the formation microspheres. Enzymes are proteins and their reactions should be conducted in a reactor set-up where enzymes are protected from sonic waves to maximize their catalytic efficiency. In this review, focused examples on protein dispersions/emulsions and enzyme catalysis are given.

Enzyme-catalyzed synthesis of malonate polyesters and their use as metal chelating materials

Following the environmental problems caused by non-degradable plastics there is a need to synthesise greener and more sustainable polymers. In this work we describe, for the first time, the facile enzyme-catalysed synthesis of linear polyesters using dimethyl malonate as the diester. These polymers, containing a different aliphatic diol component (C4, C6 or C8), were synthesised in solventless conditions using immobilized Candida antarctica lipase B as the biocatalyst. The potential of enzymes for catalysing this reaction is compared with the unsuccessful antimony- and titanium-catalysed synthesis (T > 150 °C). The application of the synthesized polymers as effective metal chelators in biphasic, green solvent systems was also described, together with the characterisation of the synthesised materials.

Sophorolipid-Based Oligomers as Polyol Components for Polyurethane Systems

Due to reasons of sustainability and conservation of resources, polyurethane (PU)-based systems with preferably neutral carbon footprints are in increased focus of research and development. The proper design and development of bio-based polyols are of particular interest since such polyols may have special property profiles that allow the novel products to enter new applications. Sophorolipids (SL) represent a bio-based toolbox for polyol building blocks to yield diverse chemical products. For a reasonable evaluation of the potential for PU chemistry, however, further investigations in terms of synthesis, derivatization, reproducibility, and reactivity towards isocyanates are required. It was demonstrated that SL can act as crosslinker or as plasticizer in PU systems depending on employed stoichiometry. (ω-1)-hydroxyl fatty acids can be derived from SL and converted successively to polyester polyols and PU. Additionally, (ω-1)-hydroxyl fatty acid azides can be prepared indirectly from SL and converted to A/B type PU by Curtius rearrangement.

Prospects of Using Biocatalysis for the Synthesis and Modification of Polymers

Trends in the dynamically developing application of biocatalysis for the synthesis and modification of polymers over the past 5 years are considered, with an emphasis on the production of biodegradable, biocompatible and functional polymeric materials oriented to medical applications. The possibilities of using enzymes not only as catalysts for polymerization but also for the preparation of monomers for polymerization or oligomers for block copolymerization are considered. Special attention is paid to the prospects and existing limitations of biocatalytic production of new synthetic biopolymers based on natural compounds and monomers from biomass, which can lead to a huge variety of functional biomaterials. The existing experience and perspectives for the integration of bio- and chemocatalysis in this area are discussed.

Achievements and Trends in Biocatalytic Synthesis of Specialty Polymers from Biomass-Derived Monomers Using Lipases

New technologies for the conversion of biomass into high-value chemicals, including polymers and plastics, is a must and a challenge. The development of green processes in the last decade involved a continuous increase of the interest towards the synthesis of polymers using in vitro biocatalysis. Among the remarkable diversity of new bio-based polymeric products meeting the criteria of sustainability, biocompatibility, and eco-friendliness, a wide range of polyesters with shorter chain length were obtained and characterized, targeting biomedical and cosmetic applications. In this review, selected examples of such specialty polymers are presented, highlighting the recent developments concerning the use of lipases, mostly in immobilized form, for the green synthesis of ε-caprolactone co-polymers, polyesters with itaconate or furan units, estolides, and polyesteramides. The significant process parameters influencing the average molecular weights and other characteristics are discussed, revealing the advantages and limitations of biocatalytic processes for the synthesis of these bio-based polymers.

Enzymatic Synthesis of Poly(alkylene succinate)s: Influence of Reaction Conditions

Application of lipases (preferentially Candida antarctica Lipase B, CALB) for melt polycondensation of aliphatic polyesters by transesterification of activated dicarboxylic acids with diols allows to displace toxic metal and metal oxide catalysts. Immobilization of the enzyme enhances the activity and the temperature range of use. The possibility to use enzyme-catalyzed polycondensation in melt is studied and compared to results of polycondensations in solution. The experiments show that CALB successfully catalyzes polycondensation of both, divinyladipate and dimethylsuccinate, respectively, with 1,4-butanediol. NMR spectroscopy, relative molar masses obtained by size exclusion chromatography, MALDI-TOF MS and wide-angle X-ray scattering are employed to compare the influence of synthesis conditions for poly(butylene adipate) (PBA) and poly(butylene succinate) (PBS). It is shown that the enzymatic activity of immobilized CALB deviates and influences the molar mass. CALB-catalyzed polycondensation of PBA in solution for 24 h at 70 °C achieves molar masses of up to Mw~60,000 g/mol, higher than reported previously and comparable to conventional PBA, while melt polycondensation resulted in a moderate decrease of molar mass to Mw~31,000. Enzymatically catalyzed melt polycondensation of PBS yields Mw~23,400 g/mol vs. Mw~40,000 g/mol with titanium(IV)n-butoxide. Melt polycondensation with enzyme catalysis allows to reduce the reaction time from days to 3–4 h.

Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol)

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.

Biocatalytic Approach for Novel Functional Oligoesters of ε-Caprolactone and Malic Acid

Biocatalysis has developed in the last decades as a major tool for green polymer synthesis. The particular ability of lipases to catalyze the synthesis of novel polymeric materials has been demonstrated for a large range of substrates. In this work, novel functional oligoesters were synthesized from ε-caprolactone and D,L/L-malic acid by a green and sustainable route, using two commercially available immobilized lipases as catalysts. The reactions were carried out at different molar ratios of the comonomers in organic solvents, but the best results were obtained in solvent-free systems. Linear and cyclic oligomeric products with average molecular weights of about 1500 Da were synthesized, and the formed oligoesters were identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis. The oligoester synthesis was not enantioselective in the studied reaction conditions. The operational stability of both biocatalysts (Novozyme 435 and GF-CalB-IM) was excellent after reutilization in 13 batch reaction cycles. The thermal properties of the reaction products were investigated by thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis. The presence of polar pendant groups in the structure of these oligomers could widen the possible applications compared to the oligomers of ε-caprolactone or allow the conversion to other functional materials.

Microwave-assisted synthesis of the lipase-catalyzed ring-opening copolymerization of ε-caprolactone and ω-pentadecanolactone: Thermal and FTIR characterization

Microwave-assisted synthesis of the lipase-catalyzed ring opening polymerization of ε-caprolactone (ε-CL) and ω-pentadecanolactone (ω-PDL) monomers was studied. A series of P(CL-co-PDL), with different molar feed ratios, including (ε-CL/ω-PDL) 100/0, 75/25, 50/50, 25/75, and 0/100, were synthesized. The resulting polyesters were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The microwave-assisted polymerization of the monomers reached high conversions (91–95%) within 60 min. FTIR spectra showed the typical absorption bands of these polyesters. A very intense band in the carbonyl region, which was shifted from 1,720 cm−1 for PCL to 1,732 cm−1 for PPDL homopolymer, as well as peaks owing to methylene groups in the 2,990–2,850 cm−1 range. DSC results revealed that all polyester samples were semi-crystalline. Interestingly, the copolymers exhibited only one melting peak (Tm), and their Tm values linearly increased from 57°C to 95°C as PPDL concentration was increased. Thermal stability of polyesters also depended on PDL content; an increase in PDL concentration increases polymer degradation temperature (Td).

Lipase-Catalyzed Transamidation of Urethane-Bond-Containing Ester

Significant improvement in mechanical properties and shape recovery in polyurethanes can be obtained by cross-linking, usually performed in a traditional chemical fashion. Here, we report model studies of enzymatic transamidations of urethane-bond-containing esters to study the principles of an enzymatic build-up of covalent cross-linked polyurethane networks via amide bond formation. The Lipase-catalyzed transamidation reaction of a urethane-bond-containing model ester ethyl 2-(hexylcarbamoyloxy)propanoate with various amines is discussed. A side product was formed, that could be successfully identified, and its synthesis reduced to a minimum (<1%). Furthermore, a noncatalyzed transamidation that is performed without CalB as the catalyst could be observed. Both observations are due to the known high reactivity of amines with urethane bonds.

(Co)polymerization of (−)-menthide and β-butyrolactone with yttrium-bis(phenolates): tuning material properties of sustainable polyesters

Sustainable thermoplastic elastomers derived from block copolymers of syndiotactic poly(3-hydroxybutyrate) and poly((−)-menthide) were synthesized via yttrium-mediated ring-opening polymerization.

Enzymatic Polymerization of PCL-PEG Co-polymers for Biomedical Applications

Biodegradable polymers, obtained via chemical synthesis, are currently employed in a wide range of biomedical applications. However, enzymatic polymerization is an attractive alternative because it is more sustainable and safer. Many lipases can be employed in ring-opening polymerization (ROP) of biodegradable polymers. Nevertheless, the harsh conditions required in industrial context are not always compatible with their enzymatic activity. In this work, we have studied a thermophilic carboxylesterase and the commonly used Lipase B from Candida antarctica (CaLB) for tailored synthesis of amphiphilic polyesters for biomedical applications. We have conducted Molecular Dynamics (MD) and Quantum Mechanics/Molecular Mechanics (QM/MM) MD simulations of the synthesis of Polycaprolactone-Polyethylene Glycol (PCL-PEG) model co-polymers. Our insights about the reaction mechanisms are important for the design of customized enzymes capable to synthesize different polyesters for biomedical applications.

Preparation and Characterization of Poly(δ-Valerolactone)/TiO2 Nanohybrid Material with Pores Interconnected for Potential Use in Tissue Engineering

Titanium dioxide/poly(δ-valerolactone) (TiO₂/Pδ-VL) nanohybrid material containing interconnected pores with sizes in the range 80–150 μm were prepared by the solvent casting and polymer melting routes, and the dispersion of the TiO₂ nanofiller in the Pδ-VL matrix and its adhesion were characterized by X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. A significant depression in the glass transition temperature (T_g) and melting temperature (T_m) values were revealed for the polymer nanocomposites prepared by the solvent casting technique. For the potential application of the prepared materials in the biomedical domain, complementary analyses were performed to examine the dynamic mechanical properties, and cell adhesion (using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay), and the results obtained for the samples prepared by the two methods were compared. Interconnected pores were successively produced in the new material by employing naphthalene microparticles as a porogen for the first time, and the results obtained were very promising.

Polymer conjugation of proteins as a synthetic post-translational modification to impact their stability and activity

For more than 40 years, protein-polymer conjugates have been widely used for many applications, industrially and biomedically. These bioconjugates have been shown to modulate the activity and stability of various proteins while introducing reusability and new activities that can be used for drug delivery, improve pharmacokinetic ability, and stimuli-responsiveness. Techniques such as RDRP, ROMP and "click" have routinely been utilized for development of well-defined bioconjugate and polymeric materials. Synthesis of bioconjugate materials often take advantage of natural amino acids present within protein and peptide structures for a host of coupling chemistries. Polymer modification may elicit increased or decreased activity, activity retention under harsh conditions, prolonged activity in vivo and in vitro, and introduce stimuli responsiveness. Bioconjugation has resulted to modulated thermal stability, chemical stability, storage stability, half-life and reusability. In this review we aim to provide a brief state of the field, highlight a wide range of behaviors caused by polymer conjugation, and provide areas of future work.

Immobilized Candida antarctica lipase B catalyzed synthesis of biodegradable polymers for biomedical applications

Biomedical applications of biodegradable polymers synthesized via the catalysis of immobilized Candida antarctica lipase B (CALB).

Enzymatic synthesis and characterization of novel terpolymers from renewable sources

2,5-Furandicarboxylic acid and itaconic acid are both important biobased platform chemicals and their terpolymer with 1,6-hexanediol (HDO) can be the starting point for a new class of reactive polyesters, with important applications. The green synthetic route developed in this study involves a biocatalytic condensation polymerization reaction of dimethyl furan-2,5-dicarboxylate (DMFDC) and dimethyl itaconate (DMI) with HDO in toluene at 80°C, using commercial immobilized lipases from Candida antarctica B. In the best conditions, the formed polymer product was isolated with more than 80% yield, containing about 85% terpolymer with average molecular mass of about 1200 (Mn, calculated from MALDI-TOF MS data) and 15% DMFDC_HDO copolymer. Considering the higher reactivity of DMFDC, the composition of the synthesized polymer can be directed by adjusting the molar ratio of DMFDC and DMI, as well as by extending the reaction time. Structural analysis by NMR demonstrated the regioselective preference for the carbonyl group from DMI adjacent to the methylene group. The biocatalyst was successfully reused in multiple reaction cycles.

Molecular bionics - engineering biomaterials at the molecular level using biological principles

Life and biological units are the result of the supramolecular arrangement of many different types of molecules, all of them combined with exquisite precision to achieve specific functions. Taking inspiration from the design principles of nature allows engineering more efficient and compatible biomaterials. Indeed, bionic (from bion-, unit of life and -ic, like) materials have gained increasing attention in the last decades due to their ability to mimic some of the characteristics of nature systems, such as dynamism, selectivity, or signalling. However, there are still many challenges when it comes to their interaction with the human body, which hinder their further clinical development. Here we review some of the recent progress in the field of molecular bionics with the final aim of providing with design rules to ensure their stability in biological media as well as to engineer novel functionalities which enable navigating the human body.

Functional biodegradable polymers via ring-opening polymerization of monomers without protective groups

Biodegradable polymers are of current interest and chemical functionality in such materials is often demanded in advanced biomedical applications. Functional groups often are not tolerated in the polymerization process of ring-opening polymerization (ROP) and therefore protective groups need to be applied. Advantageously, several orthogonally reactive functions are available, which do not demand protection during ROP. We give an insight into available, orthogonally reactive cyclic monomers and the corresponding functional synthetic and biodegradable polymers, obtained from ROP. Functionalities in the monomer are reviewed, which are tolerated by ROP without further protection and allow further post-modification of the corresponding chemically functional polymers after polymerization. Synthetic concepts to these monomers are summarized in detail, preferably using precursor molecules. Post-modification strategies for the reported functionalities are presented and selected applications highlighted.

Effect of Operating Variables and Kinetics of the Lipase Catalyzed Transesterification of Ethylene Carbonate and Glycerol

Glycerol carbonate (GC) is a value-added product originating from the valorization of widely available glycerol (Gly), a side stream from the production of biodiesel. Here we approach the production of this chemical comparing two reactions based on the transesterification of Gly with dimethyl carbonate (DMC) and ethylene carbonate (EC). When using DMC, it was observed that the free enzyme CALB (lipase B from Candida antarctica) gave the best results, whereas Eversa Transform (a genetic modification of Thermomyces lanuginosus lipase) performed better than the rest if EC was the reagent. With the selected catalysts, their immobilized analogous enzymes Novozym 435 and Lypozyme TL IM, respectively, were also tested. Observing that the yields for the reaction with EC were significantly faster, other operating variables were evaluated, resulting the best performance using a closed system, tert-butanol as solvent, a concentration of enzyme Eversa Transform of 3% w/w, a molar excess of EC:Gly of 9:1 and a temperature of 60 °C. Finally, several runs were conducted at different temperatures and molar ratios of EC:Gly, fitting a kinetic model to all experimental data for the reaction catalyzed with Eversa Transform. This model included the bimolecular transesterification reaction of Gly and EC catalyzed by the lipase and a reversible ring-opening polymerization of EC.

Efficient Physisorption of Candida Antarctica Lipase B on Polypropylene Beads and Application for Polyester Synthesis

In the present work, Candida antarctica lipase B (CaLB) was adsorbed onto polypropylene beads using different reaction conditions, in order to investigate their influence on the immobilization process and the enzyme activity of the preparations in polymerization reactions. In general, lower salt concentrations were more favorable for the binding of enzyme to the carrier. Polymerisation of dimethyl adipate (DMA) and 1,4-butanediol (BDO) was investigated in thin-film systems at 70 °C and at both atmosphere pressure (1000 mbar) and 70 mbar. Conversion rates and molecular masses of the reaction products were compared with reactions catalyzed by CaLB in its commercially available form, known as Novozym 435 (CaLB immobilized on macroporous acrylic resin). The best results according to molecular weight and monomer conversion after 24 h reaction time were obtained with CaLB immobilized in 0.1 M Na2HPO4\NaH2PO4 buffer at pH 8, producing polyesters with 4 kDa at conversion rates of 96% under low pressure conditions. The stability of this preparation was studied in a simulated continuous polymerization process at 70 °C, 70 mbar for 4 h reaction time. The data of this continuous polymerizations show that the preparation produces lower molecular weights at lower conversion rates, but is comparable to the commercial enzyme concerning stability for 10 cycles. However, after 24 h reaction time, using our optimum preparation, higher molecular weight polyesters (4 kDa versus 3.1 kDa) were obtained when compared to Novozym 435.

Lipase-Catalyzed Synthesis of Furan-Based Oligoesters and their Self-Assembly-Assisted Polymerization

We investigate the synthesis of bio-based hydrophilic and hydrophobic oligoesters, which in turn are derived from easily accessible monomers from natural resources. In addition to the selection of renewable monomers, Novozyme 435, an immobilized lipase B from Candida antarctica was used for the oligomerization of monomers. The reaction conditions for oligomerization using Novozyme 435 were established to obtain a moderate-to-good yield. The average number of repeating units and the molecular weight distribution of hydrophilic and hydrophobic oligoester were identified by using NMR spectroscopy, gel-permeation chromatography, and MS. The oligoester derived from a hydrophilic monomer self-assembled to form a viscous solution, which upon further heating resulted in the formation of a polymer by the intermolecular Diels-Alder reaction. The viscosity of the solution and the assembly of oligoester to form a fibrous structure were investigated by using rheological studies, XRD, and SEM. The molecular weight of the cross-linked polymer was identified by using matrix-assisted laser desorption/ionization-MS. The thermal properties of the bio-based polymers were investigated by using thermogravimetric analysis and differential scanning calorimetry. For the first time, the self-assembly-assisted polymerization of an oligoester is reported using the intermolecular Diels-Alder reaction, which opens a new avenue in the field of polymer science.

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.

A Novel Delivery System for the Controlled Release~of Antimicrobial Peptides: Citropin 1.1 and Temporin A

Antimicrobial peptides (AMPs) are prospective therapeutic options for treating multiple-strain infections. However, clinical and commercial development of AMPs has some limitations due to their limited stability, low bioavailability, and potential hemotoxicity. The purpose of this study was to develop new polymeric carriers as highly controlled release devices for amphibian peptides citropin 1.1 (CIT) and temporin A (TEMP). The release rate of the active pharmaceutical ingredients (APIs) was strongly dependent on the API characteristics and the matrix microstructure. In the current work, we investigated the effect of the polymer microstructure on in vitro release kinetics of AMPs. Non-contact laser profilometry, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) were used to determine the structural changes during matrix degradation. Moreover, geno- and cytotoxicity of the synthesized new carriers were evaluated. The in vitro release study of AMPs from the obtained non-toxic matrices shows that peptides were released with near-zero-order kinetics. The peptide “burst release” effect was not observed. New devices have reached the therapeutic concentration of AMPs within 24 h and maintained it for 28 days. Hence, our results suggest that these polymeric devices could be potentially used as therapeutic options for the treatment of local infections.