Microbial production of natural poly amino acid

Three kinds of poly amino acids, poly-γ-glutamic acid, poly(ε-L-lysine) and multi-L-arginyl-poly (L-aspartic acid) can be synthesized by enzymatic process independently from ribosomal protein biosynthesis pathways in microorganism. These biosynthesized polymers have attracted more and more attentions because of their unique properties and various applications. In this review, the current knowledge on the biosynthesis, biodegradations and applications of these three poly amino acids are summarized.     

Chemical–physical behavior of hydrogels of poly(vinyl alcohol) and poly(ethylene glycol)

New hydrogels based on polyethylene glycol (PEG) and poly(vinyl alcohol) (PVA) of different degrees of hydrolysis were synthesized. To form the network the PEG was modified at their ends with acyl chloride groups to be used as the crosslinking agent. The compositions of the hydrogels were between 50% and 90% by weight of PEG and PVA of various degrees of hydrolysis were used. It was found that the degree of hydrolysis of the PVA and the PEG content influence the equilibrium water content of the hydrogel. The process of swelling of all the hydrogels prepared followed a second-order kinetics.     

Thermal degradation of two classes of block copolymers based on poly(lactic-glycolic acid) and poly(ϵ-caprolactone) or poly(ethylene glycol)

Thermodegradative investigations of two classes of multi-block copolymers containing poly(D,L-lactic-glycolic acid) (PLGA) and either poly(ethylene glycol) (PEG) or poly(ϵ-caprolactone) diol-terminated (PCDT) segments were performed. In particular, the influence of the type and length of the segments as well as of the molar ratio between the D,L-lactic acid (LA) and glycolic acid (GA) residues was investigated at 180°C in air by viscometry, FT-IR analysis and isothermal thermogravimetry. The thermal oxidative degradation of these materials is largely affected by the LA/GA ratio, a higher LA content generally imparting higher stability. The FT-IR analysis suggests that, depending on the composition of the PLGA segments, degradative processes are triggered which can lead to a preferential degradation of the blocks.     

Thermal degradation of star-shaped poly(?-caprolactone)

Thermal degradation of a serials of star-shaped poly(?-caprolactone) (PCL) with well-defined arm numbers and arm length was investigated. The weight loss of star-shaped PCL during heating process showed a two-stage character, and its dependence on molecular weight and multi-armed structure was well discussed. It was found that the thermal stability could be improved not only by increasing molecular weight but also by increasing arm numbers when the molecular weight is in a certain range. Based on the analyses of pyrolytic products by ¹H NMR and TGA-FTIR, two mechanisms of thermal degradation for the random cleavage of ester bonds of PCL chains were proposed. Ester bonds were pyrolyzed into alkene and carboxyl functional groups in mechanism I while they were pyrolyzed into ketene and hydroxyl functional groups in mechanism II. The effects of multi-armed structure of star-shaped PCL on the cleavage of ester bonds of PCL chains were discussed in terms of the limitation of central “core” on mobility of each PCL arm. Combined the results of viscosity analysis with thermal analysis, it could be concluded that both thermal stability and processability of PCL materials can be improved by controlling the multi-armed structures.     

Development of orientation during electrospinning of fibres of poly(ε-caprolactone)

We explore the influence of a rotating collector on the internal structure of poly(ε-caprolactone) fibres electrospun from a solution in dichloroethane. We find that above a threshold collector speed, the mean fibre diameter reduces as the speed increases and the fibres are further extended. Small-angle and wide-angle X-ray scattering techniques show a preferred orientation of the lamellar crystals normal to the fibre axis which increases with collector speed to a maximum and then reduces. We have separated out the processes of fibre alignment on the collector and the orientation of crystals within the fibres. There are several stages to this behaviour which correspond to the situations (a) where the collector speed is slower than the fibre spinning rate, (b) the fibre is mechanically extended by the rotating collector and (c) where the deformation leads to fibre fracture. The mechanical deformation leads to a development of preferred orientation with extension which is similar to the prediction of the pseudo-affine deformation model and suggests that the deformation takes place during the spinning process after the crystals have formed.     

Preparation and characterization of microspheres based on blend of poly(lactic acid) and poly(ɛ-caprolactone) with poly(vinyl alcohol) as emulsifier

In this paper, microspheres were prepared by oil-in-water (o/w) emulsion solvent evaporation method. Biodegradable polymer such as blend of poly (lactic acid) (PLA) and poly(?-caprolactone) (PCL) with certain compositions and characteristics was used to prepare the microspheres with poly(vinyl alcohol) (PVA) as an emulsifier. This study observed the microspheres particle’s size distribution at various concentrations of PVA (1%, 1.5%, 2%, and 2.5% PVA). The PVA volume variations effects during the process (50, 100, 150, 200, and 250 mL) were also observed. The blend of PLA and PCL is formed only by physical interaction between them. This can be seen from the FTIR spectrum which shows both PLA and PCL component. The microspheres physical size and appearance were observed by optical microscope (MO). The overall results of this study showed that the formula which used 50–150 mL of 2.5% polyvinyl alcohol produced the microspheres with the most uniform size distribution.     

A facile synthesis of hydrophilic poly(ε-caprolactone)-based poly(ether-ester-amide)s

Segmented poly(ether-ester-amide)s, (PEEA)s, of controlled hydrophilicity degree, based on poly(ε-caprolactone) (PCL), were synthesized according to a facile two-step procedure using α,ω-dihydroxy oligomeric PCL, 4,7,10-trioxa-1,13-tridecanediamine and macromers prepared from poly(ethylene glycol)s and adipoyl chloride. The PEEAs showed M _n values in the range 5–11.5 kDa. A PCL-type crystallinity was found by WAXS. DSC indicated T_m values (49–51 °C) close to that of PCL macromer. Single glass transitions were observed both by DSC and DMTA techniques and the T_g values (−58–−50 °C by DSC) were slightly higher than that of PCL. The water uptake was in the range 4.8–26.0 wt.-% depending on the length of the poly(ethylene glycol) segment.

Monomers used to prepare the PEEAs.     

Molecular simulation of an amorphous poly(methyl methacrylate)–poly(tetrafluoroethylene) interface

Molecular dynamics calculations of an amorphous interfacial system of poly(methyl methacrylate) (PMMA) and poly(tetrafluoroethylene) (PTFE) containing about 10,000 interaction sites were performed for 15 ns under constant pressure and constant temperature conditions. The time evolutions of the thickness, density and number of atomic pairs in the interfaces suggested that the interfaces reached their equilibrium states with an interfacial thickness of about 2 nm at 500 K. The molecular motion in the interface and bulk was compared using mean square displacement and torsional autocorrelation function. The separation at a PMMA/PTFE interface was mimicked using non-equilibrium molecular dynamics calculations by applying the potential energy to the MD cell in a direction perpendicular to the interface. Initially, the PTFE layer close to the interface was deformed, and before complete separation, some segments of the PTFE molecules extended from the bulk to the surface of the PMMA layer, which were attached by the intermolecular interaction. The remaining PTFE molecules were entangled in the bulk, which probably prevented the transfer of the PTFE molecules to the surfaces of the PMMA layers. On the other hand, the PMMA layer was only slightly deformed. This separation behavior can be explained by taking into account the intermolecular interaction, the barrier to the conformational changes of the backbones and the entanglement of the PTFE molecules in the bulk.     

Enzymatic synthesis of poly(α-ethyl β-aspartate) by poly(ethylene glycol) modified poly(aspartate) hydrolase-1

We recently discovered that poly(aspartate) (PAA) hydrolase‐1 from Pedobacter sp. KP‐2 has a unique property of specifically cleaving the amide bond between β‐aspartate units in thermally synthesized PAA (tPAA). In the present study, the enzymatic synthesis of poly(α‐ethyl β‐aspartate) (β‐PAA) was performed by taking advantage of the substrate specificity of PAA hydrolase‐1. No polymerization of diethyl L ‐aspartate by native PAA hydrolase‐1 occurred because of the low dispersibility of the enzyme in organic solvent. Poly(ethylene glycol) (PEG) modification of the enzyme improved its dispersibility and enabled it to polymerize the monomer substrate. MALDI‐TOF MS analysis showed that the synthesized polymer was observed in the range of m/z = 750–2 500. This analysis also revealed that the polymer was composed of ethyl aspartate units, containing either an ethyl ester or a free carboxyl end group at its carboxyl terminus. ¹H NMR analysis demonstrated that the synthesized polymer consisted of only β‐amide linkages. Thus, the present results indicate that PAA hydrolase‐1 modified with PEG is useful for the synthesis of β‐PAA due to its unique substrate specificity and good dispersibility in organic solvent.

     

Effect of halloysite nanotubes on the thermal degradation behaviour of poly(ε-caprolactone)/poly(lactic acid) microfibrillar composites

This paper reports on the thermal degradation behaviour and kinetics of halloysite nanotubes containing microfibrillated poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blends. It was found that the nanotubes probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with halloysite nanotubes (HNT) catalysis. Drawing to form microfibrillated nanocomposites had little influence on the degradation behaviour of these materials, but pre-mixing of the HNT with PLA or PCL prior to melt-mixing and extrusion-drawing of the blends did influence the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had an influence on the degradation mechanism, as evidenced through a Fourier-transform infrared (FTIR) analysis of the degradation products.     

Preparation and polymerization mechanism of dihydroxy-capped PCL,poly(CL-b-PEG-b-CL) and poly(DTC-b-CL-b-DTC)

PCL possesses a wide range of medical applications, such as tissue engineering and controlled drug release, because of its good biodegradability and miscibility. In order to extend the use of PCL, researchers have been exploring its structural and chemica…     

Surface patterning and biological evaluation of semi-interpenetrated poly(HEMA)/poly(alkyl β-malolactonate)s

Semi-interpenetrating hydrogel networks were prepared by radical polimerization of monomer HEMA, co-monomer EGDMA as cross-linking agent and in the presence of 1 wt % of poly(alkyl β-malolactonate)s. Biological evaluation in terms of cell adhesion and proliferation of the prepared materials showed the ability of HEMA-based hydrogels to sustain a good cell adhesion and proliferation. Moreover, a method based on soft-lithography to obtained surface microstructured semi-interpenetrating hydrogel networks was developed, thus allowing for further investigation of the influence of surface topography on cell behavior.     

Studies on LCST of poly （N-isopropylacrylamide-co-acrylic acid-co-N-diacetone acrylamide）

Linear copolymers from N-isopropylacrylamide(NIPA),acrylic acid(AA)and diacetone acrylamide(DAA)have been prepared. The effect of composition,ionic strength and pH on their lower critical solution temperature(LCST)has been investigated.     

Morphological structures of poly(vinylidene fluoride)/montmorillonite nanocomposites

Poly(vinylidene fluoride) (PVDF)/montmorillonite (MMT) nanocomposites were prepared by melt blen- ding a kind of organically modified montmorillonite with PVDF. The morphological structures of the nanocomposites were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The re- sults indicate that organically modified montmorillonites are in the form of intercalation, exfoliation, and fragments in the PVDF matrix. For the composites, the (001) peak position of MMT was found to shift to a lower angle in XRD patterns, and some MMT fragments could be observed under TEM. MMT loading was favorable to producing the piezoelectric β phase in the PVDF matrix and caused internal stress in α crystals. At the same time, the crystallinity and spherulite size of PVDF decreased with the MMT content. MMT induced β phase is stable even at high temperatures (160℃). For these changes in morphological structures, some possible explanations were proposed based on the experimental re- sults.     

Selective assembly of cyclodextrins on poly(ethylene oxide)–poly(propylene oxide) block copolymers

This paper presents a computational study on the formation of a molecular necklace formed by specific threading of cyclodextrins (CDs) on block copolymers. Structural as well as energetic principles for the selective complexation of - and -cyclodextrin with poly(ethylene oxide)–poly(propylene oxide) block copolymers (PEO–PPO) are elucidated considering a diblock copolymer of equimolecular composition (PEO)4–(PPO)4 as guest. A non-statistical distribution of CDs, i.e. -CDs primarily located on the PEO chain and -CDs on PPO blocks of the polymer, is based on a variety of structural features and energetic preferences considering both potential as well as solvation energies. This selectivity becomes already obvious considering 1:1 complexes between PEO and PPO monomers and the two CDs, but is increasingly evident when calculating higher order ensembles. Besides the host–guest interaction, docking between CDs themselves is an important, also non-statistical, prerequisite for the self-assembly of highly ordered tubes. The formation of intermolecular hydrogen bonds between adjacent CDs in a tubular aggregate gives an important contribution to the overall stability of the molecular necklace. The net effect, based on the preferential interaction between host and guest as well as between the host molecules themselves, results in the formation of a stable, highly ordered macromolecular, multicomponent aggregate.     

Viscoelastic behaviors of poly(ε-caprolactone)/attapulgite nanocomposites

Nanocomposites based on biodegradable poly(ε-caprolactone) (PCL) and attapulgite (AT) were prepared by solution blend. The morphology and rheological behaviors of the blends were examined. SEM observation showed that AT nano-rods were well dispersed in the PCL matrix in a nanometer scale at low AT content and a severe aggregation would happen at high AT content. The rheological behaviors of the polymer/fibrous clay nanocomposite samples were investigated by an Advanced Rheometric Expansion System with parallel plate geometry. The effects of AT content on the viscoelastic behaviors of the PCL/AT nanocomposites were also confirmed by rheological analysis. The apparent activation energies of flow were calculated by the Arrhenius equation.     

Electrochemical preparation of composite of poly brilliant cresyl blue (PBCB)–poly 5-amino-2-napthalenesulfonic acid electrode and electrocatalytic application

Poly brilliant cresyl blue (PBCB) and poly 5-amino-2-napthalenesulfonic (PANS) polymer composite modified electrode was fabricated by the electrochemical polymerization of brilliant cresyl blue and 5-amino-2-napthalenesulfonic acid. When compared polymer composite electrodes with PBCB and PANS electrode, it showed enhanced electrochemical property. The morphology of the resulting composite electrode was characterized by AFM, and the electrochemical properties of the modified electrode were characterized by cyclic voltammetry and amperometry. The composite electrode showed surface-confined and pH-dependent electrochemical property. The composite electrode exhibited high catalytic behavior toward the reduction of hydrogen peroxide at low overpotential. The detection limit and sensitivity of the electrode toward H₂O₂ detection was 5 μM and 1 μA/mM, respectively, and response time was less than 10 s for hydrogen peroxide.     

Synthesis of poly(2,2′,3,3′-indole)

New approaches to the synthesis of poly(2,2′,3,3′-indole) were developed based on the photochemical dehydropolycondensation of indole in the presence of iodine and the photochemical polycondensation of 1-(1H-indol-3-yl)-2-iodo-1-ethanone in the absence of catalyst and solvent. A suggested mechanism for the formation of the oligomeric chain in these reactions includes the intermediate formation of 3,3′-diindole with subsequent polycondensation via elimination of hydrogen atoms at position 2 of the dimer pyrrole fragment.     

The endothermic “annealing peak” of poly(phenylene sulphide) and poly(ethylene terephthalate)

The appearance of an endothermic annealing peak in semicrystalline poly(phenylene sulphide) and semicrystalline poly(ethylene terephthalate) after annealing at or above the cold-crystallization temperature is investigated by temperature-modulated differential scanning calorimetry, thermo-mechanical analysis and dynamic-mechanical analysis. The results indicate relaxation processes in the interlamellar amorphous phase, which is in a strongly constrained state after cold crystallization. During the annealing treatments rearranging processes take place. These processes result in a separation of the amorphous phase into an interlamellar relaxed and a “pseudo-crystalline” phase. Received: 27 October 1998 Accepted in revised form: 19 January 1999