Synthesis of Novel Polymers with Biodegradability by Main-Chain Editing of Chiral Polyketones.

Although the use of biodegradable plastics is suitable for unrecoverable, single-use plastic, their high production cost and much lower variety compared to commodity plastics limit their application. In this study, we developed a new polymer with potential biodegradability, poly(ketone/ester), synthesized from propylene and carbon monoxide. Propylene and carbon monoxide are easily available at low costs from fossil resources, and they can also be derived from biomass. Using an atom insertion reaction to the main chain of the polymer, the main-chain editing of the polymer molecule proceeded with up to 89% selectivity for atom insertion over main-chain cleavage.

Coastal and deep-sea biodegradation of polyhydroxyalkanoate microbeads.

Microbeads find widespread usage in personal care items and cosmetics, serving as exfoliants or scrubbing agents. Their micro-scale size poses challenges in effective drainage capture and given their origin from non-biodegradable oil-based plastics, this contributes substantially to marine pollution. In this study, microbeads were prepared by a simple yet scalable melt homogenization method using four types of polyhydroxyalkanoates (PHA), namely poly[(R)-3-hydroxybutyrate] (P(3HB)), poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (P(3HB-co-3HV)), poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (P(3HB-co-3HHx)) and poly[(R)-3-hydroxybutyrate-co-(R)-4-hydroxyvalerate] (P(3HB-co-4HB)). Microbeads with different surface smoothness, compressive strength (6.2-13.3 MPa) and diameter (from 1 ~ 150 µm) could be produced. The microbeads were subjected to a comprehensive degradation analysis using three techniques: enzymatic, Biochemical Oxygen Demand (BOD) evaluations, and in situ degradation tests in the deep-sea off Misaki Port in the northern Pacific Ocean (depth of 757 m). Qualitatively, results from enzymatic and in situ degradation demonstrated significant degradation within one week and five months, respectively. Quantitatively, BOD findings indicated that all PHA microbeads degraded similarly to cellulose (~ 85% biodegradability in 25 days). In conclusion, PHA microbeads from this study exhibit promising potential as alternatives to conventional non-biodegradable microbeads.

Microbial decomposition of biodegradable plastics on the deep-sea floor.

Microbes can decompose biodegradable plastics on land, rivers and seashore. However, it is unclear whether deep-sea microbes can degrade biodegradable plastics in the extreme environmental conditions of the seafloor. Here, we report microbial decomposition of representative biodegradable plastics (polyhydroxyalkanoates, biodegradable polyesters, and polysaccharide esters) at diverse deep-sea floor locations ranging in depth from 757 to 5552 m. The degradation of samples was evaluated in terms of weight loss, reduction in material thickness, and surface morphological changes. Poly(L-lactic acid) did not degrade at either shore or deep-sea sites, while other biodegradable polyesters, polyhydroxyalkanoates, and polysaccharide esters were degraded. The rate of degradation slowed with water depth. We analysed the plastic-associated microbial communities by 16S rRNA gene amplicon sequencing and metagenomics. Several dominant microorganisms carried genes potentially encoding plastic-degrading enzymes such as polyhydroxyalkanoate depolymerases and cutinases/polyesterases. Analysis of available metagenomic datasets indicated that these microorganisms are present in other deep-sea locations. Our results confirm that biodegradable plastics can be degraded by the action of microorganisms on the deep-sea floor, although with much less efficiency than in coastal settings.

Recyclable Polycarbosilane from a Biomass-Derived Bifuran-Based Monomer.

Two of the most fundamental principles for the development of next-generation polymers are production from renewable biomass and well-designed recyclability. Bifuran derivatives represent promising building blocks for functional polymers on account of their high rigidity, strong interchain interactions, and extended π-conjugation. In this study, a polycarbosilane containing a bifuran-based repeat unit was prepared via the hydrosilylation of dihydrosilylbifuran and 1,5-hexadiene. The crystallinity and thermal properties of the bifuran-containing polycarbosilane were superior to those of a corresponding polycarbosilane containing a single-furan-based repeat unit and comparable to those of the benzene-based analogue due to the rigidity and interchain interactions of the poly(bifurancarbosilane) unit. The bifuran moiety in the repeat unit causes a red-shift and strong UV absorption of the polycarbosilane compared to that containing the single-furan-based and benzene-based repeat units. The bifuran moiety also renders the resulting polycarbosilane strongly fluorescent, while the polycarbosilanes containing the benzene-based and single-furan-ring-based repeat units did not emit fluorescence. These desirable photoproperties result from the extension of the σ-π conjugation in the repeat unit. Furthermore, the chemical recyclability is a unique and attractive property of the bifuran-based polycarbosilane; upon treatment with trifluoroacetic acid, bifuran can be regenerated as the monomer, while trifluoroacetate silane can be up-cycled to the corresponding polysiloxane. Thus, the bifuran motif endows polycarbosilane with improved thermal, optical, and recycling properties.

Degradation of ester linkages in rice straw components by Sphingobium species recovered from the sea bottom using a non-secretory tannase-family α/ß hydrolase.

Microbial decomposition of allochthonous plant components imported into the aquatic environment is one of the vital steps of the carbon cycle on earth. To expand the knowledge of the biodegradation of complex plant materials in aquatic environments, we recovered a sunken wood from the bottom of Otsuchi Bay, situated in northeastern Japan in 2012. We isolated Sphingobium with high ferulic acid esterase activity. The strain, designated as OW59, grew on various aromatic compounds and sugars, occurring naturally in terrestrial plants. A genomic study of the strain suggested its role in degrading hemicelluloses. We identified a gene encoding a non-secretory tannase-family α/ß hydrolase, which exhibited ferulic acid esterase activity. This enzyme shares the consensus catalytic triad (Ser-His-Asp) within the tannase family block X in the ESTHER database. The molecules, which had the same calculated elemental compositions, were produced consistently in both the enzymatic and microbial degradation of rice straw crude extracts. The non-secretory tannase-family α/ß hydrolase activity may confer an important phenotypic feature on the strain to accelerate plant biomass degradation. Our study provides insights into the underlying biodegradation process of terrestrial plant polymers in aquatic environments.

Environmental biodegradability of recombinant structural protein.

Next generation polymers needs to be produced from renewable sources and to be converted into inorganic compounds in the natural environment at the end of life. Recombinant structural protein is a promising alternative to conventional engineering plastics due to its good thermal and mechanical properties, its production from biomass, and its potential for biodegradability. Herein, we measured the thermal and mechanical properties of the recombinant structural protein BP1 and evaluated its biodegradability. Because the thermal degradation occurs above 250 °C and the glass transition temperature is 185 °C, BP1 can be molded into sheets by a manual hot press at 150 °C and 83 MPa. The flexural strength and modulus of BP1 were 115 ± 6 MPa and 7.38 ± 0.03 GPa. These properties are superior to those of commercially available biodegradable polymers. The biodegradability of BP1 was carefully evaluated. BP1 was shown to be efficiently hydrolyzed by some isolated bacterial strains in a dispersed state. Furthermore, it was readily hydrolyzed from the solid state by three isolated proteases. The mineralization was evaluated by the biochemical oxygen demand (BOD)-biodegradation testing with soil inocula. The BOD biodegradability of BP1 was 70.2 ± 6.0 after 33 days.

Biobased Poly(Schiff-Base) Composed of Bifurfural.

In this study, bifurfural, an inedible biobased chemical and a second-generation biomass, was polymerized with several diamines using an environmentally benign process, and the chemical structures of the resulting poly(Schiff base)s were analyzed. Because furan rings, which are only produced from biomass and not from fossil resources, endow polymers with unique properties that include high rigidity and expanded π-conjugation, bifurfural, which contains two furan rings, is of significant interest as a biobased building block. 1H NMR, IR, and matrix assisted laser desorption ionization-time of flight mass spectra of the poly(Schiff base)s reveal that they are composed of mixtures of linear and cyclic structures. The UV-vis spectroscopy and molecular orbital theory confirm the extended π-conjugation in the bifurfural/p-phenylenediamine poly(Schiff base) system. Poly(Schiff base)s composed of bifurfural and 1,3-propanediamine, 1,4-butandiamine, 1,5-pentanediamine, and 1,6-hexanediamine were molded at 120 °C into films that exhibited good strengths and were tough to bend. These results indicate that bifurfural-based poly(Schiff base)s are promising biobased materials.

Synthesis and verification of biobased terephthalic acid from furfural.

Exploiting biomass as an alternative to petrochemicals for the production of commodity plastics is vitally important if we are to become a more sustainable society. Here, we report a synthetic route for the production of terephthalic acid (TPA), the monomer of the widely used thermoplastic polymer poly(ethylene terephthalate) (PET), from the biomass-derived starting material furfural. Biobased furfural was oxidised and dehydrated to give maleic anhydride, which was further reacted with biobased furan to give its Diels-Alder (DA) adduct. The dehydration of the DA adduct gave phthalic anhydride, which was converted via phthalic acid and dipotassium phthalate to TPA. The biobased carbon content of the TPA was measured by accelerator mass spectroscopy and the TPA was found to be made of 100% biobased carbon.

Crystalline-structure-dependent enzymatic degradation of polymorphic poly(3-hydroxypropionate).

The crystalline structure dependence of enzymatic degradation behavior was investigated for the polymorphic poly(3-hydroxypropionate) (P3HP), which has a basic backbone chemical structure of bacterial poly(3-hydroxyalkanoate)s (P3HAs). The P3HP films consisting of the beta-, gamma-, and/or delta-form crystal were cast or melt-crystallized as reported previously (Macromolecules 2005, 38, 6455; Macromolecules 2006, 39, 194-203) by controlling the molecular weight, crystallization temperature, and/or temperature of the melt. Their thermal properties, crystalline structures, morphologies, and (13)C solid spin-lattice relaxation dynamics were characterized by the differential scanning calorimetry, the wide-angle X-ray diffraction, the small-angle X-ray scattering (SAXS), and the (13)C solid-state NMR spectra (SNMR), respectively. Both the crystallinities and the lamellar thicknesses of P3HP films were found to decrease roughly in the order of beta-form > (or approximately) gamma-form > delta-form. From previous work, which indicates that the P3HA enzymatic degradation depends only on the degree of crystallinity and the lamellar thickness, their enzymatic degradation rates are then expected to increase in the order of beta-form < (or approximately) gamma-form < delta-form. Unexpectedly, their experimental P3HP enzymatic degradation rates in the presence of P3HA depolymerase isolated from Ralstonia pickettii T1 increase in the reverse order, i.e., delta-form < gamma-form < beta-form. The weight loss rate of the delta-form film is almost 1 order of magnitude smaller than that of the fastest degraded beta-form film. It is then strongly indicated that the crystalline structure plays a strikingly decisive role in the enzymatic degradation of P3HP. In particular, only when the conformation of crystalline chain accords with that of the bacterial poly(3-hydroxybutyrate) (P3HB) sample, i.e., the 2 1 helix conformation, is the P3HP sample degraded as slow as the P3HB sample. The inherent reason responsible for the unique P3HP enzymatic degradation behavior has been further clarified by comparing the molecular interaction and dynamics of polymorphic P3HP crystals.

Fine structure and enzymatic degradation of poly[(R)-3-hydroxybutyrate] and stereocomplexed poly(lactide) nanofibers.

Fiber morphology and crystalline structure of poly[(R)-3-hydroxybutyrate] (P(3HB)) and stereocomplexed poly(lactide) (PLA) nanofibers were investigated by using scanning and transmission electron microscopies and X-ray and electron diffractions. In the P(3HB) nanofibers spun from less than 1 wt% 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solution, planar zigzag conformation (beta-form) as well as 2(1) helix conformation (alpha-form) structure was formed. Based on the electron diffraction measurement of single P(3HB) nanofiber, it was revealed that the molecular chains of P(3HB) align parallel to the fiber direction. From the enzymatic degradation test of P(3HB) nanofiber, it was shown that beta-form molecular chains are degraded more preferentially than alpha-form chains. Stereocomplexed PLA nanofibers were electrospun from 1 wt% poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) solution in HFIP, which contains equal amounts of PLLA and PDLA. While as-spun stereocomplexed PLA nanofiber was amorphous, PLA nanofiber annealed at 100 degrees C contained only racemic crystal. It was supposed that the crystallization behavior of stereocomplexed PLA in the nanofiber is affected by the electrospinning process, which forcibly exerts the strain onto the polymer chains.

Adsorption and hydrolysis reactions of poly(hydroxybutyric acid) depolymerases secreted from Ralstonia pickettii T1 and Penicillium funiculosum onto poly[(R)-3-hydroxybutyric acid].

Reaction processes of poly[(R)-3-hydroxybutyric acid] (P(3HB)) with two types of poly(hydroxybutyric acid) (PHB) depolymerases secreted from Ralstonia pickettii T1 and Penicillium funiculosum were characterized by means of atomic force microscopy (AFM) and quartz crystal microbalance (QCM). The PHB depolymerase from R. pickettii T1 consists of catalytic, linker, and substrate-binding domains, whereas the one from P. funiculosum lacks a substrate-binding domain. We succeeded in observing the adsorption of single molecules of the PHB depolymerase from R. pickettii T1 onto P(3HB) single crystals and the degradation of the single crystals in a phosphate buffer solution at 37 degrees C by real-time AFM. On the contrary, the enzyme molecule from P. funiculosum was hardly observed at the surface of P(3HB) single crystals by real-time AFM, even though the enzymatic degradation of the single crystals was surely progressed. On the basis of the AFM observations in air of the P(3HB) single crystals after the enzymatic treatments, however, not only the PHB depolymerase from R. pickettii T1 but also that from P. funiculosum adsorbed onto the surface of P(3HB) crystals, and both concentrations of the enzymes on the surface were nearly identical. This means both enzymes were adsorbed onto the surface of P(3HB) single crystals. Moreover, QCM measurements clarified quantitatively the differences in detachment behavior between two types of PHB depolymerases, namely the enzyme from R. pickettii T1 was hardly detached but the enzyme from P. funiculosum was released easily from the surface of P(3HB) crystals under an aqueous condition.

The crystal structure of polyhydroxybutyrate depolymerase from Penicillium funiculosum provides insights into the recognition and degradation of biopolyesters.

Polyhydroxybutyrate is a microbial polyester that can be produced from renewable resources, and is degraded by the enzyme polyhydroxybutyrate depolymerase. The crystal structures of polyhydroxybutyrate depolymerase from Penicillium funiculosum and its S39 A mutant complexed with the methyl ester of a trimer substrate of (R)-3-hydroxybutyrate have been determined at resolutions of 1.71 A and 1.66 A, respectively. The enzyme is comprised of a single domain, which represents a circularly permuted variant of the alpha/beta hydrolase fold. The catalytic residues Ser39, Asp121, and His155 are located at topologically conserved positions. The main chain amide groups of Ser40 and Cys250 form an oxyanion hole. A crevice is formed on the surface of the enzyme, to which a single polymer chain can be bound by predominantly hydrophobic interactions with several hydrophobic residues. The structure of the S39A mutant-trimeric substrate complex reveals that Trp307 is responsible for the recognition of the ester group adjacent to the scissile group. It is also revealed that the substrate-binding site includes at least three, and possibly four, subsites for binding monomer units of polyester substrates. Thirteen hydrophobic residues, which are exposed to solvent, are aligned around the mouth of the crevice, forming a putative adsorption site for the polymer surface. These residues may contribute to the sufficient binding affinity of the enzyme for PHB granules without a distinct substrate-binding domain.

Correlation between solid-state structures and enzymatic degradability of cocrystallized blends.

Solid-state structures and enzymatic degradability have been investigated for cocrystallized blends between poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHBV] and poly(3-hydroxybutyrate-co-3-hydroxypropionate) [PHBP]. From wide-angle X-ray diffraction patterns, small-angle X-ray scattering data, and the comparison of the enzymatic degradability of these blends, the solid-state structures of PHBV/PHBP blend samples, in which the PHBV component has higher isothermal crystal growth rate (G) value than the PHBP one, might be similar to those of the component PHBVs; while those of the PHBP/PHBV blend samples, in which PHBP component has higher G value, were similar to the component PHBPs. Normalized one-dimensional correlation functions gamma(x) of PHBV/PHBP binary blends crystallized at 90 degrees C.

Cloning, expression and characterization of a poly(3-hydroxybutyrate) depolymerase from Marinobacter sp. NK-1.

A DNA fragment carrying the gene encoding poly(3-hydroxybutyrate) (P(3HB)) depolymerase was cloned from the genomic DNA of Marinobacter sp. DNA sequencing analysis revealed that the Marinobacter sp. P(3HB) depolymerase gene is composed of 1734bp and encodes 578 amino acids with a molecular mass of 61,757Da. A sequence homology search showed that the deduced protein contains the signal peptide, catalytic domain (CD), cadherin-type linker domain (LD), and two substrate-binding domain (SBD). The fusion proteins of glutathione S-transferase (GST) with the CD showed the hydrolytic activity for denatured P(3HB) (dP(3HB)), P(3HB) emulsion (eP(3HB)) and p-nitrophenylbutyrate. On the other hand, the fusion proteins lacking the SBD showed much lower hydrolytic activity for dP(3HB) compared to the proteins containing both CD and SBD. In addition, binding tests revealed that the SBDs are specifically bound not to eP(3HB) but dP(3HB). These suggest that the SBDs play a crucial role in the enzymatic hydrolysis of dP(3HB) that is a solid substrate.

Change of surface structure of poly(3-hydroxybutyrate) film upon enzymatic hydrolysis by PHB depolymerase.

The change in the surface structure of poly[(R)-3-hydroxybutyrate] [PHB] films upon the enzymatic hydrolysis was analyzed by attenuated total reflection infrared [ATR/IR] spectrometry. As enzymes, PHB depolymerases isolated from Ralstonia pickettii T1 and Pseudomonas stutzeri were used. By curve decomposition of the carbonyl stretching band of ATR/IR spectra, the change in the surface crystallinity of PHB films by exposure to buffer containing 0, 1, and 4 microg of PHB depolymerases was estimated. It has been widely believed that the enzymatic hydrolysis first occurs in the amorphous phase, followed by the degradation in the crystalline phase, and extracellular PHB depolymerase can degrade only polymer chains in the surface layer of the film. Therefore, the surface crystallinity had been expected to increase upon the enzymatic degradation. However, the results were contrary to this expectation. The surface crystallinity was decreased by the enzymatic attack. Because ATR/IR spectrometry is sensitive to a small change in molecular structure of the sample surface, the decrease in the crystallinity shown by ATR/IR experiments probably does not indicate the complete loss of regularity of the crystalline phase. Because the chains at crystalline surface are more mobile than those inside the crystals, the C=O band for crystalline surface may appear at a position similar to those of the amorphous or interfacial phase in ATR/IR spectra of PHB. Only the chains inside the crystals may contribute to the C=O band of the crystalline phase. Thus, we rather suppose that the decrease in the crystalline peak of the ATR/IR spectra reflects the change in chain mobility or the increase of crystalline surface area by cracking of lamellas at the surface layers of PHB films or both.

Enzymatic hydrolysis of chemosynthesized atactic poly(3-hydroxybutyrate) by poly(3-hydroxyalkanoate) depolymerase from Acidovorax Sp. TP4 and Ralstonia pickettii T1.

The enzymatic degradability of chemosynthesized atactic poly([R,S]-3-hydroxybutyrate) [a-P(3HB)] by two types of extracellular poly(3-hydroxyalkanoate) (PHA) depolymerases purified from Ralstonia pickettii T1 (PhaZ(ral)) and Acidovorax Sp. TP4 (PhaZ(aci)), defined respectively as PHA depolymerase types I and II according to the position of the lipase box in the catalytic domain, were studied. The enzymatic degradation of a-P(3HB) by PhaZ(aci) depolymerase was confirmed from the results of weight loss and the scanning electron micrographs. The degradation products were characterized by one- and two-dimension (1)H NMR spectroscopy. It was found that a-P(3HB) could be degraded into monomer, dimer, and trimer by PhaZ(aci) depolymerase at temperatures ranging from 4 to 20 degrees C, while a-P(3HB) could hardly be hydrolyzed by PhaZ(ral) depolymerase in the same temperature range. These results suggested that the chemosynthesized a-P(3HB) could be degraded in the pure state by natural PHA depolymerase.

Enzymatic hydrolysis of bacterial poly(3-hydroxybutyrate-co-3-hydroxypropionate)s by poly(3-hydroxyalkanoate) depolymerase from Acidovorax Sp. TP4.

Enzymatic degradability has been investigated for a series of bacterial poly(3-hydroxybutyrate-co-3-hydroxypropionate)s (P(3HB-co-3HP)s) with 3-hydroxypropionate (3HP) unit contents from 11 to 86 mol % as well as poly(3-hydroxybutyrate) (P(3HB)) and chemosynthesized poly(3-hydroxypropionate) (P(3HP)). The behavior of degradation by two types of extracellular poly(3-hydroxyalkanoate) (PHA) depolymerases purified from Ralstonia pikettii T1 and Acidovorax Sp. TP4, defined respectively as PHA depolymerase types I and II according to the position of the lipase box in the catalytic domain, were compared in relation to the thermal properties and crystalline structures of the PHA samples elucidated by differential scanning calorimetry and wide-angle X-ray diffraction. The degradation products were characterized by high-performance liquid chromatography and one- (1D) and two-dimension (2D) (1)H NMR spectroscopy. It was found that the PHA depolymerase of Acidovorax Sp. TP4 showed degradation behavior different from that shown by depolymerase of R. pikettii T1. PHA depolymerase from Acidovorax Sp. TP4 degraded the P(3HB-co-3HP) films with lower crystallinity in higher rates than those with higher crystallinity, no matter what kinds of crystalline structures they formed. In contrast, PHA depolymerase from R. pikettii T1 degraded P(3HB-co-3HP) films forming P(3HB) crystalline structure in higher rates than those forming P(3HP)s. The increase in amorphous nature of the P(3HB-co-3HP) films with P(3HB)-homopolymer-like crystalline structure increases and then decreases the rate of degradation by depolymerase from R. pikettii T1. The 3-hydroxybutyrate (3HB) monomer was produced as a major product by the hydrolysis of P(3HB) film by PHA depolymerase from Acidovorax Sp. TP4. The P(3HB-co-3HP) films could be degraded into 3HB and 3-hydroxypropionate (3HP) monomer at last, indicating that the catalytic domain of the enzyme recognized at least two monomeric units as substrates. While the PHA depolymerase from R. pikettii T1 hydrolyzed P(3HB) film into 3HB dimer as a major product, and the catalytic domain recognized at least three monomeric units. The degradation behavior of P(3HB-co-3HP) films by the PHA depolymerase of Acidovorax Sp. TP4 could be distinguished from that by the depolymerase of R. pikettii T1.