Plasticization of poly(hydroxybutyrate) in vivo

Novel unexpected functions of PHA granules

Polyhydroxyalkanoates (PHA), polyesters accumulated by numerous prokaryotes in the form of intracellular granules, have been for decades considered being predominantly storage molecules. However, numerous recent discoveries revealed and emphasized their complex biological role for microbial cells. Most of all, it was repeatedly reported and confirmed that the presence of PHA granules in prokaryotic cells enhances stress resistance and robustness of microbes against various environmental stress factors such as high or low temperature, freezing, oxidative, and osmotic pressure. It seems that protective mechanisms of PHA granules are associated with their extraordinary architecture and biophysical properties as well as with the complex and deeply interconnected nature of PHA metabolism. Therefore, this review aims at describing novel and unexpected properties of PHA granules with respect to their contribution to stress tolerance of various prokaryotes including common mesophilic heterotrophic bacteria, but also extremophiles or photo-autotrophic cyanobacteria. KEY POINTS: • PHA granules present in bacterial cells reveal unique properties and functions. • PHA enhances stress robustness of bacterial cells.

Poly(3-hydroxyalkanoate)-based drug formulations: the micro- and nanostructure

Biodegradable and biocompatible polymers referred to as polyhydroxyalkanoates (PHAs) are extensively used in the production of pharmaceutical drugs to ensure sustained release, targeted delivery, reduced toxicity, and increased stability of the drug substance. Although the pharmaceutical industry ordinarily exploits chemically synthesized PHAs, bioengineered polymers are also starting to enjoy growing interest. This article focuses on the research and development of drug formulations based on natural PHAs that act as auxiliary substances for antibacterial, anti-inflammatory, anticancer, and hormonal medications, as well as pain killers, and discusses the association between their properties and the micro/nano structure of the synthetic drug. The problems associated with the poor performance of active components in traditional dosage forms can be overcome in PHAs-based formulations.

Reduction of enthalpy relaxation in gelatine films by addition of polyols

The aim of this study was to evaluate the effect of plasticisers with different molecular weights (glycerol and sorbitol) on the structural relaxation kinetics of bovine gelatine films stored under the glass transition temperature (Tg). Plasticisers were tested at weight fractions of 0.0, 0.06 and 0.10. Films conditioned in environments under ∼44% relative humidity gave moisture contents (w/w) in the range 0.14-0.18. The enthalpy relaxation (ΔH) was determined using differential scanning calorimetry (DSC). Samples used had Tg values in the range 24-49 °C. After removing the thermal history (30 °C above Tg, 15 min), samples were isothermally stored at 10 °C below Tg for between 2 and 80 h. The addition of plasticisers induced a significant reduction in the rate of structural relaxation. The linearisation of ΔH by plotting against the logarithm of ageing time showed a reduction in the slope of samples plasticised with both polyols. The reduction in relaxation kinetics may be related to the ability of polyols to act as enhancers of molecular packing, as recently reported using positron spectroscopy (PALS). However, a direct correlation between the relaxation kinetics and the plasticiser's molecular weight could not be established, suggesting that this phenomenon may be governed by complex molecular gelatin-plasticiser-water interactions.

Feasibility study of an alkaline-based chemical treatment for the purification of polyhydroxybutyrate produced by a mixed enriched culture

This study focused on investigating the feasibility of purifying polyhydroxybutyrate (PHB) from mixed culture biomass by alkaline-based chemical treatment. The PHB-containing biomass was enriched on acetate under non-sterile conditions. Alkaline treatment (0.2 M NaOH) together with surfactant SDS (0.2 w/v% SDS) could reach 99% purity, with more than 90% recovery. The lost PHB could be mostly attributed to PHB hydrolysis during the alkaline treatment. PHB hydrolysis could be moderated by increasing the crystallinity of the PHB granules, for example, by biomass pretreatment (e.g. freezing or lyophilization) or by effective cell lysis (e.g. adjusting alkali concentration). The suitability of the purified PHB by alkaline treatment for polymer applications was evaluated by molecular weight and thermal stability. A solvent based purification method was also performed for comparison purposes. As result, PHB produced by mixed enriched cultures was found suitable for thermoplastic applications when purified by the solvent method. While the alkaline method resulted in purity, recovery yield and molecular weight comparable to values reported in literature for PHB produced by pure cultures, it was found unsuitable for thermoplastic applications. Given the potential low cost and favorable environmental impact of this method, it is expected that PHB purified by alkaline method may be suitable for other non-thermal polymer applications, and as a platform chemical.

Biocompatibility of polyhydroxybutyrate microspheres: in vitro and in vivo evaluation

Microspheres have been prepared from the resorbable linear polyester of beta-hydroxybutyric acid (polyhydroxybutyrate, PHB) by the solvent evaporation technique and investigated in vitro and in vivo. Biocompatibility of the microspheres has been proved in tests in the culture of mouse fibroblast cell line NIH 3T3 and in experiments on intramuscular implantation of the microspheres to Wistar rats for 3 months. Tissue response to the implantation of polymeric microspheres has been found to consist in a mild inflammatory reaction, pronounced macrophage infiltration that increases over time, involving mono- and poly-nuclear foreign body giant cells that resorb the polymeric matrix. No fibrous capsules were formed around polymeric microparticles; neither necrosis nor any other adverse morphological changes and tissue transformation in response to the implantation of the PHB microparticles were recorded. The results of the study suggest that polyhydroxybutyrate is a good candidate for fabricating prolonged-action drugs in the form of microparticles intended for intramuscular injection.

Fourier-transform infrared (FTIR) spectroscopy for monitoring and determining the degree of crystallisation of polyhydroxyalkanoates (PHAs)

FTIR spectroscopy has been used to monitor and determine the degree of crystallisation in a sample of polyhydroxybutyrate-co-14%valerate (PHB-co-14%HV). Time series spectra of solution-cast films of the polymer revealed spectral changes attributed to the onset of crystallisation. Curve fitting was used to obtain an absolute measure of crystallinity. Mean centred principal-component analysis (PCA) revealed that 99.9% of the spectral variance could be attributed to factor 1. The loadings plot for factor 1 contained features attributable to crystalline and amorphous phases. These features were opposite in sign, indicating that changes in the spectra with the onset of crystallisation are simultaneous and opposite in direction, i.e. as the crystalline band increases the amorphous band decreases. Cross-peaks in asynchronous 2D correlation maps indicate there are likely to be very minor components that are changing out of phase. The presence of these minor components is supported by examination of the loadings of higher factors in the PCA model. PCA has been shown to be suitable for determining the number of dynamic spectral features and has enabled relative and objective monitoring of crystallisation kinetics.

Long-acting delivery microspheres of levo-norgestrol-poly(3-hydroxybutyrate): their preparation, characterization and contraceptive tests on mice

The preparative technology for sustained release drug delivery microspheres of levo-norgestrol-poly(3-hydroxybutyrate) was optimized based on the in-liquid-drying method. The formation of the drug microspheres was confirmed with differential thermal analysis. The appearance, particle size and distribution, residual CHCl3, drug content, drug release characteristics in vitro, stability and anticonceptive effect on mice of the microspheres were all examined. The average particle size was 64 microm with over 90% of the microspheres being in the range of 28.7-85.8 microm. The residual CHCl3 was lower than 0.001%. The drug release behaviour in vitro could be described by the Higuchi equation and the drug release t1/2 was prolonged by 1.8 times, compared with the original drug LNG. The microspheres were stable for 3 months and showed significant sustained release and anticonceptive effect in mice, and lower toxicity compared with the original drug.

Artificial granule suspensions of long side chain poly(3-hydroxyalkanoate)

In vitro preparation of long side chain poly(3-hydroxyalkanoate) artificial granule suspensions that mimic the "as biosynthesized" inclusions in vivo is reported. Elastomeric poly(3-hydroxyalkanoate) can be made into an aqueous suspension of noncrystalline, submicron-sized particles. Light-scattering measurements on these suspensions showed polymer particles in the range of 90–200 nm in diameter. High-resolution 13C-NMR studies demonstrated the noncrystalline character of the in vitro artificial granules. Upon drying at room temperature, the suspensions yielded a continuous film resulting from the coalescence of these polymers with a low glass transition temperature and low Young's modulus. Aqueous suspensions of poly(3-hydroxyoctanoate) are ideal substrates for enzymatic degradation studies because of their stability and purity, since they are self-stabilized. The range of poly(3-hydroxyalkanoates) that can be converted to artificial granules and the methods of preparation are described.Key words: poly(3-hydroxyalkanoate), artificial granules, in vitro bacterial inclusions, enzyme model substrates, poly(3-hydroxyoctanoate).

Physical properties of bacterial poly((R)-3-hydroxyalkanoates)

Poly((R)-3-hydroxyalkanoates) are bacterial storage polyesters, currently receiving much attention because of their potential application as biodegradable and biocompatible plastics. Methods and skills from both microbiology and polymer science can be used to manipulate these materials to make their physical properties meet the requirements for specific applications. The present paper reviews the physical properties of the most promising poly((R)-3-hydroxyalkanoates) as well as the opportunities offered by polymer technology to improve them, emphasizing the mechanical properties in relation to structure and processing.Key words: biopolymers, poly(hydroxyalkanoates), PHB, PHA.

Considerations on the structure and biochemistry of bacterial polyhydroxyalkanoic acid inclusions

Some mathematical calculations were done that provided information about the structure and biochemistry of polyhydroxyalkanoic acid (PHA) granules and about the amounts of the different constituents that contribute to the PHA granules. The data obtained from these calculations are compared with data from the literature, which show that PHA granules consist not only of the polyester but also of phospholipids and proteins. The latter are referred to as granule-associated proteins, and they are always located at the surface of the PHA granules. A concept is proposed that distinguishes four classes of structurally and functionally different granule-associated proteins: (i) class I comprises the PHA synthases, which catalyze the formation of ester linkages between the constituents; (ii) class II comprises the PHA depolymerases, which are responsible for the intracellular degradation of PHA, (iii) class III comprises a new type of protein, which is referred to as phasins and which has most probably a function analogous to that of oleosins in oilseed plants, and (iv) class IV comprises all other proteins, which have been found to be associated with the granules but do not belong to classes I-III. Particular emphasis is placed on the phasins, which constitute a significant fraction of the total cellular protein. Phasins are assumed to form a close protein layer at the surface of the granules, providing the interface between the hydrophilic cytoplasm and the much more hydrophobic core of the PHA inclusion.

Poly(β-hydroxybutyrate) extrusion from pleomorphic cells ofAzotobacter vinelandiiUWD

Azotobacter vinelandii UWD cells fill with up to 80% (per dry mass) poly(β-hydroxybutyrate) (PHB) after 24 h growth in medium containing sugars and fish peptone. However, peptones were not usually added to Azotobacter culture as they induced pleomorphism and compromised cell wall strength. This study examines the morphology of these PHB-producing pleomorphic cells in the transmission electron microscope. PHB-producing cells incubated for 18–24 h were most frequently 2–3 μm diameter spheres containing up to 20 PHB inclusions/cross section, or a calculated ≈ 100 inclusions/cell volume. These inclusions tended to be of small size (≈ 0.5 μm diameter) and became fewer and larger in older cells. The most striking feature of these pleomorphic cells was the apparent extrusion of polymer from the cells. It is unlikely that PHB extrusion is an active process from a viable cell as there was considerable cell wall damage at the point of polymer extrusion. The results suggest that the extrusion of PHB may be the result of polymer expansion, caused by the dehydration of the specimen for transmission electron microscopy, coupled with the inability of the pleomorphic cell wall to retain the expanding polymer. Thus, freeze-substituted sections of similar cells that were prepared without chemical dehydration did not extrude PHB. However, lysed cells prepared for transmission electron microscopy by chemical dehydration also did not extrude PHB, which suggests differences in the fluidity of the PHB in intact cell inclusions and lysed cell granules.Key words: poly(β-hydroxybutyrate), inclusions, polymer expansion, dehydration artifact.

Biomimetic, amorphous granules of polyhydroxyalkanoates: composition, mobility, and stabilization in vitro by proteins

It is a remarkable feature of poly(3-hydroxybutyrate) (PHB) that although the isolated polymer is highly crystalline, native PHB storage granules in cells are only found in an amorphous, mobile state. It has recently been proposed that the failure of PHB granules to crystallize is simply the result of slow nucleation kinetics that are operative for small, isolated particles. In support of this new model, we present here a straightforward procedure by which pure crystalline PHB can be reconstituted into submicron-size, detergent-coated artificial granules. The artificial granules are amorphous and stable in suspension, and they are essentially indistinguishable from their native counterparts in terms of size, morphology, molecular mobility, and density. Furthermore, when the surfactant coating is removed from the artificial granules by dialysis, the granules crystallize, verifying the nucleation hypothesis. In vivo, the PHB granule surface is likely to consist of both polypeptide and lipid; in vitro it is possible to prepare amorphous PHB granules that are stabilized solely by phospholipids or by any of several common proteins (serum albumin, casein, or ovalbumin). Artificial amorphous granules may be prepared from a variety of different bacterial PHAs and from blends of incompatible polyesters.Key words: polyhydroxyalkanoate, poly(3-hydroxybutyrate), granules, nucleation, latex.

Nuclear magnetic resonance relaxation studies of poly(hydroxybutyrate) in whole cells and in artificial granules

The physical state of poly(hydroxybutyrate) (PHB) in whole cells and in the form of artificial biomimetic granules has been probed using 13C nuclear magnetic resonance (NMR) spectroscopy. Studies on varying concentrations of whole cells of Alcaligenes eutrophus show that changes in the line widths of PHB in whole cells do not correlate with changes in transverse relaxation times. Solid-state magic-angle spinning NMR studies demonstrate that the line broadening results from a reduction in the static field homogeneity rather than from intrinsic properties of the PHB within the cells. Transverse and longitudinal relaxation times of PHB in whole cells and in artificial granules are similar, indicating similarities in structure and mobility.

Immunocytochemical analysis of poly-beta-hydroxybutyrate (PHB) synthase in Alcaligenes eutrophus H16: localization of the synthase enzyme at the surface of PHB granules

Antibodies raised against the Alcaligenes eutrophus poly-beta-hydroxybutyrate (PHB) synthase polypeptide were used for immunocytochemical localization of the synthase enzyme in whole cells and purified PHB granules. The data presented demonstrate for the first time that the synthase enzyme is located on the surface of the PHB granule rather than being incorporated inside the granule during its formation. From these basic observations and data from the recent literature, a model of granule assembly is proposed.