Characterization of an (R)-specific enoyl-CoA hydratase from Streptomyces sp. strain CFMR 7: A metabolic tool for enhancing the production of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

Poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] [P(3HB-co-3HHx)] has a high potential to serve as a commercial bioplastic due to its biodegradability, thermoplastic and mechanical properties. The properties of this copolymer are greatly affected by the composition of 3HHx monomer. One of the most efficient ways to modulate the composition of 3HHx monomer in P(3HB-co-3HHx) is by manipulating the (R)-3HHx-CoA monomer supply. In this study, a new (R)-specific enoyl-CoA hydratase originating from a non-PHA producer, Streptomyces sp. strain CFMR 7 (PhaJSs), was characterized and found to be effective in supplying 3HHx monomer during in vivo production of P(3HB-co-3HHx) copolymer. The P(3HB-co-3HHx) copolymer produced from the Cupriavidus necator transformant that harbors phaJSs, PHB-4/pBBR1-CBP-M-CPF4JSs, showed enhanced 3HHx incorporation of up to 11 mol% without affecting the P(3HB-co-3HHx) production when palm oil was used as the carbon source. In addition, both kcat and kcat/Km of PhaJSs were higher toward the C6 than the shorter C4 substrates, underscoring the preference for 3-hydroxyhexanoyl-CoA. These results suggest that PhaJSs has a significant ability to supply 3HHx monomers for PHA biosynthesis via ß-oxidation and can be applied for metabolic engineering of robust PHA-producing strains.

Two-Trace Two-Dimensional Correlation Spectroscopy Study of the Crystallization Behavior of Bioplastics.

A pair of attenuated total reflection infrared (ATR IR) spectra obtained during the crystallization of bioplastic copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] or PHBHx from the melt by spontaneous cooling were examined using the two-trace two-dimensional correlation spectroscopy (2T2D-COS) analysis. Unlike conventional difference spectra, 2T2D spectra showed unexpected details for the patterns of spectral intensity changes, clearly revealing the existence of two distinct populations of crystalline contributions, attributed to the well-ordered primary crystals and the less ordered secondary crystals, in addition to the amorphous component. The 2T2D spectrum sorts out highly overlapped bands associated with different constituents of the system, based on the fundamental properties and constraint imposed on a pair of spectra. Hetero-mode asynchronous 2T2D correlation analysis between the congested CH-stretching region and better resolved carbonyl-stretching region further indicates that the increase in the intensities of certain methyl and methylene bands during the crystallization process is mostly associated with the growth of the well-ordered primary crystals of PHBHx instead of the formation of the secondary crystals in the confined inter-lamellar space.

Safety assessment of the substance poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) for use in food contact materials.

The EFSA Panel on Food Contact Materials, Enzymes and Processing Aids (CEP Panel) assessed the safety of poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) (PHBH), CAS No 147398-31-0 and food contact material (FCM) substance No 1059. This biodegradable copolymer is produced by fermentation of palm oil using a genetically modified microorganism (Cupriavidus necator). Overall migration was up to 5.4 mg/kg. Oligomers are hydroxyl-terminated or with crotyl- and hexenyl end-groups from dehydration of hydroxyl end-groups. In the absence of calibration standards, the total oligomer migration was set at the overall migration values. Other degradation products are crotonic acid and (E)-2-hexenoic acid. Crotonic acid is authorised for use in FCMs with a specific migration limit (SML) of 0.05 mg/kg food. For (E)-2-hexenoic acid, no indication for genotoxicity was identified by the EFSA CEF Panel in its group evaluation of flavouring substances in FGE.05Rev2 (EFSA CEF Panel, 2010b). The other migrating substances detected, ■■■■■, are from the authorised substance 'palm oil and/or palm fatty acid distillate' (FCM substance No 9) used as a carbon source for the fermentation and do not give rise to safety concern. A PHBH oligomer mixture was synthesized to simulate that migrating. It did not give rise to concern for genotoxicity. From the repeated dose 90-day oral toxicity study in rats, the Panel identified the no-observed-adverse-effect level (NOAEL) at the highest dose tested in males, 1,364 mg/kg body weight (bw) per day. The Panel concluded that the potential for bioaccumulation of oligomers is low. Overall, the CEP Panel concluded that the substance PHBH is not of safety concern for the consumer if it is used alone or blended with other polymers in contact with all kinds of food during more than 6 months at room temperature or below, including hot-fill or a short heating up phase. The specific migration of all oligomers < 1,000 Da should not exceed 5 mg/kg food. The migration of crotonic acid should not exceed the SML of 0.05 mg/kg food. As the migration of (E)-2-hexenoic acid can be expected to be always lower than that of crotonic acid, no individual restriction is necessary.

SUPERSEDED: Safety assessment of the substance poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) for use in food contact materials.

This opinion of the EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids (CEF Panel) deals with the safety assessment of poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate), Chemical Abstracts Service (CAS) No 147398-31-0 and food contact material (FCM) substance No 1059, for contact with dry/solid food. This biodegradable (co)polymer is produced by fermentation of palm oil using a genetically modified microorganism (Cupriavidus necator). No migration of oligomers into food simulant E (10 days at 40 and 60°C) was found at a detection limit per single oligomer of 5 µg/kg food. Migration of the degradation product crotonic acid was 8 and 25 µg/kg at the two test temperatures, respectively. The other migrating substances detected, ■■■■■, likely originated from or are related to the authorised substance (FCM No. 9) 'palm oil and/or palm fatty acid distillate' used as carbon source for the fermentation. At the migration levels reported, these migrants do not give rise to safety concern. No genotoxicity data are required for poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) because of its high molecular weight. The fraction below 1,000 Da is 0.5%. The major monomeric unit in the copolymer, 3-hydroxybutyric acid, is an intermediate in fatty acid metabolism. The minor monomeric unit, 3-hydroxyhexanoic acid, tested negative for bacterial gene mutations. Degradation products, which may be present in the (co)polymer, are crotonic acid and (E)-2-hexenoic acid. Crotonic acid is authorised for use in FCM with a specific migration limit (SML) of 0.05 mg/kg food; for (E)-2-hexenoic acid, no indication for genotoxicity was identified by the EFSA CEF Panel in its 2010 group evaluation of flavouring substances in FGE.05Rev2. The CEF Panel concluded that the substance poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) is not of safety concern if used alone or in blends with other polymers for contact with dry/solid food. If the SML of crotonic acid is met, migration of (E)-2-hexenoic acid will also not exceed 0.05 mg/kg food.

Two-Dimensional Raman Correlation Spectroscopy Study of Poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] Copolymers.

Two-dimensional correlation analysis was applied to the time-dependent evolution of Raman spectra during the isothermal crystallization of bioplastic, poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] or PHBHx copolymer. Simultaneous Raman measurement of both carbonyl stretching and low-frequency crystalline lattice mode regions made it possible to carry out the highly informative hetero-mode correlation analysis. The crystallization process of PHBHx involves: (1) the early nucleation stage; (2) the primary growth of well-ordered crystals of PHBHx; and (3) the secondary crystal growth phase. The latter stage probably occurs in the inter-lamellar region, with an accompanying reduction of the amorphous component, which occurs most dominantly during the primary crystal growth. The development of a fully formed lamellar structure comprising the 21 helices occurs after the primary growth of crystals. In the later stage, secondary inter lamellar space crystallization occurs after the full formation of packed helices comprising the lamellae.

Additive manufacturing of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] scaffolds for engineered bone development.

A wide range of poly(hydroxyalkanoate)s (PHAs), a class of biodegradable polyesters produced by various bacteria grown under unbalanced conditions, have been proposed for the fabrication of tissue-engineering scaffolds. In this study, the manufacture of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (or PHBHHx) scaffolds, by means of an additive manufacturing technique based on a computer-controlled wet-spinning system, was investigated. By optimizing the processing parameters, three-dimensional scaffolds with different internal architectures were fabricated, based on a layer-by-layer approach. The resulting scaffolds were characterized by scanning electron microscopy, which showed good control over the fibre alignment and a fully interconnected porous network, with porosity in the range 79-88%, fibre diameter 47-76 µm and pore size 123-789 µm. Moreover, the resulting fibres presented an internal porosity connected to the external fibre surface as a consequence of the phase-inversion process governing the solidification of the polymer solution. Scaffold compressive modulus and yield stress and strain could be varied in a certain range by changing the architectural parameters. Cell-culture experiments employing the MC3T3-E1 murine pre-osteoblast cell line showed good cell proliferation after 21 days of culture. The PHBHHx scaffolds demonstrated promising results in terms of cell differentiation towards an osteoblast phenotype. Copyright © 2014 John Wiley & Sons, Ltd.