Surface nanostructure of Hevea brasiliensis natural rubber latex particles

Metabolic engineering for enhanced oil in biomass

The world is hungry for energy. Plant oils in the form of triacylglycerol (TAG) are one of the most reduced storage forms of carbon found in nature and hence represent an excellent source of energy. The myriad of applications for plant oils range across foods, feeds, biofuels, and chemical feedstocks as a unique substitute for petroleum derivatives. Traditionally, plant oils are sourced either from oilseeds or tissues surrounding the seed (mesocarp). Most vegetative tissues, such as leaves and stems, however, accumulate relatively low levels of TAG. Since non-seed tissues constitute the majority of the plant biomass, metabolic engineering to improve their low-intrinsic TAG-biosynthetic capacity has recently attracted significant attention as a novel, sustainable and potentially high-yielding oil production platform. While initial attempts predominantly targeted single genes, recent combinatorial metabolic engineering strategies have focused on the simultaneous optimization of oil synthesis, packaging and degradation pathways (i.e., 'push, pull, package and protect'). This holistic approach has resulted in dramatic, seed-like TAG levels in vegetative tissues. With the first proof of concept hurdle addressed, new challenges and opportunities emerge, including engineering fatty acid profile, translation into agronomic crops, extraction, and downstream processing to deliver accessible and sustainable bioenergy.

A Green Approach for Preparing High-Loaded Sepiolite/Polymer Biocomposites

Global industry is showing a great interest in the field of sustainability owing to the increased attention for ecological safety and utilization of renewable materials. For the scientific community, the challenge lies in the identification of greener synthetic approaches for reducing the environmental impact. In this context, we propose the preparation of novel biocomposites consisting of natural rubber latex (NRL) and sepiolite (Sep) fibers through the latex compounding technique (LCT), an ecofriendly approach where the filler is directly mixed with a stable elastomer colloid. This strategy favors a homogeneous dispersion of hydrophilic Sep fibers in the rubber matrix, allowing the production of high-loaded sepiolite/natural rubber (Sep/NR) without the use of surfactants. The main physicochemical parameters which control Sep aggregation processes in the aqueous medium were comprehensively investigated and a flocculation mechanism was proposed. The uniform Sep distribution in the rubber matrix, characteristic of the proposed LCT, and the percolative filler network improved the mechanical performances of Sep/NR biocomposites in comparison to those of analogous materials prepared by conventional melt-mixing. These outcomes indicate the suitability of the adopted sustainable procedure for the production of high-loaded clay⁻rubber nanocomposites with remarkable mechanical features.

Investigating the Mechanistic and Structural Role of Lipid Hydrolysis in the Stabilization of Ammonia-Preserved Hevea Rubber Latex

The stabilization mechanism of natural rubber (NR) latex from Hevea brasiliensis was studied to investigate the components involved in base-catalyzed ester hydrolysis, namely, hydrolyzable lipids, ammonia, and the products responsible for the desired phenomenon observed in ammonia-preserved NR latex. Latex stability is generally thought to come from a rubber particle (RP) dispersion in the serum, which is encouraged by negatively charged species distributed on the RP surface. The mechanical stability time (MST) and zeta potential were measured to monitor field latices preserved in high (FNR-HA) and low ammonia (FNR-LA) contents as well as that with the ester-containing components removed (saponified NR) at different storage times. Amounts of carboxylates of free fatty acids (FFAs), which were released by the transformation and also hypothesized to be responsible for the like-charge repulsion of RPs, were measured as the higher fatty acid (HFA) number and corroborated by confocal laser scanning microscopy (CLSM) both qualitatively and quantitatively. The lipids and their FFA products interact differently with Nile red, which is a lipid-selective and polarity-sensitive fluorophore, and consequently re-emit characteristically. The results were confirmed by conventional ester content determination utilizing different solvent extraction systems to reveal that the lipids hydrolyzed to provide negatively charged fatty acid species were mainly the polar lipids (glycolipids and phospholipids) at the RP membrane but not those directly linked to the rubber molecule and, to a certain extent, those suspended in the serum. From new findings disclosed herein together with those already reported, a new model for the Hevea rubber particle in the latex form is proposed.

The Hardy Rubber Tree Genome Provides Insights into the Evolution of Polyisoprene Biosynthesis

Eucommia ulmoides, also called hardy rubber tree, is an economically important tree; however, the lack of its genome sequence restricts the fundamental biological research and applied studies of this plant species. Here, we present a high-quality assembly of its ∼1.2-Gb genome (scaffold N50 = 1.88 Mb) with at least 26 723 predicted genes for E. ulmoides, the first sequenced genome of the order Garryales, which was obtained using an integrated strategy combining Illumina sequencing, PacBio sequencing, and BioNano mapping. As a sister taxon to lamiids and campanulids, E. ulmoides underwent an ancient genome triplication shared by core eudicots but no further whole-genome duplication in the last ∼125 million years. E. ulmoides exhibits high expression levels and/or gene number expansion for multiple genes involved in stress responses and the biosynthesis of secondary metabolites, which may account for its considerable environmental adaptability. In contrast to the rubber tree (Hevea brasiliensis), which produces cis-polyisoprene, E. ulmoides has evolved to synthesize long-chain trans-polyisoprene via farnesyl diphosphate synthases (FPSs). Moreover, FPS and rubber elongation factor/small rubber particle protein gene families were expanded independently from the H. brasiliensis lineage. These results provide new insights into the biology of E. ulmoides and the origin of polyisoprene biosynthesis.

Characteristics of mechanical properties of sulphur cross-linked guayule and dandelion natural rubbers

Roles of non-rubber components in guayule and dandelion natural rubbers on the mechanical properties are firstly revealed by analysing the Mullins effect, dynamic mechanical properties and strain-induced crystallization from a new viewpoint.

Control of interface interactions between natural rubber and solid surfaces through charge effects: an AFM study in force spectroscopic mode

Adhesion of nanoparticles (natural rubber) is monitored by slight changes in the surface charge state of the contacting solid surfaces.

Reinforcing biofiller “Lignin” for high performance green natural rubber nanocomposites

High performance eco-friendly natural rubber biocomposites with various contents up to 40 parts per one hundred rubber by weight of lignin were successfully prepared from sodium lignosulfonate and natural rubber latex using the soft process.

Graphene-philic surfactants for nanocomposites in latex technology

Graphene is the newest member of the carbon family, and has revolutionized materials science especially in the field of polymer nanocomposites. However, agglomeration and uniform dispersion remains an Achilles' heel (even an elephant in the room), hampering the optimization of this material for practical applications. Chemical functionalization of graphene can overcome these hurdles but is often rather disruptive to the extended pi-conjugation, altering the desired physical and electronic properties. Employing surfactants as stabilizing agents in latex technology circumvents the need for chemical modification allowing for the formation of nanocomposites with retained graphene properties. This article reviews the recent progress in the use of surfactants and polymers to prepare graphene/polymer nanocomposites via latex technology. Of special interest here are surfactant structure-performance relationships, as well as background on the roles surfactant-graphene interactions for promoting stabilization.

ADSORPTION OF WATER-EXTRACTABLE PROTEINS IN NATURAL RUBBER LATEX SERUMS BY POLY(METHYL METHACRYLATE)/POLYETHYLENEIMINE CORE-SHELL NANOPARTICLES

Poly(methyl methacrylate)/polyethyleneimine (PMMA/PEI) core-shell nanoparticles were prepared by emulsifier-free emulsion polymerization. Micrographs from a scanning electron microscope and transmission electron microscope displayed their spherical shape with core-shell morphology in which PMMA was a core and PEI was a shell. The PMMA/PEI nanoparticles' ability to adsorb proteins from the serum of commercial low-ammonia preserved fresh field natural rubber latex was illustrated. The driving force for adsorption was proposed to be mainly via electrostatic interaction between the protonated amino groups of PEI chains on the nanoparticles' surface and phospholipids or protein molecules on NR particles. The reduction percentage was about 50%, depending on the content of PMMA/PEI nanoparticles and mixing time. For comparison, the protein reduction performance by the nanoparticles with two additional extracted serums, high-ammonia preserved concentrated NRL and Thai advanced preservative system NRL, which have different initial protein contents and pH values, was also investigated. The preliminary evaluation of PMMA/PEI nanoparticles' performance in sulfur-prevulcanized high-ammonia preserved concentrated NRL was also studied. Its corresponding sheet had lower extractable proteins by 50% and had tensile strength and elongation at break of 25.5 MPa and 715%, respectively.

Diclofenac Potassium Transdermal Patches Using Natural Rubber Latex Biomembranes as Carrier

The aim of this study was to design a compound transdermal patch containing diclofenac potassium (Dic-K) using natural rubber latex (NRL) biomembrane. The NRL from Hevea brasiliensis is easily manipulated and low cost and presents high mechanical resistance. It is a biocompatible material which can stimulate natural angiogenesis and is capable of adhering cells on its surface. Recent researches have used the NRL for Transdermal Drug Delivery Systems (TDDSs). Dic-K is used for the treatment of rheumatoid arthritis and osteoarthritis and pain relief for postoperative and posttraumatic cases, as well as inflammation and edema. Results showed that the biomembrane can release Dic-K for up to 216 hours. The kinetics of the Dic-K release could be fitted with double exponential function. X-ray diffraction and Fourier Transform Infrared (FTIR) spectroscopy show some interaction by hydrogen bound. The results indicated the potential of the compound patch.

LIPID CONTENT IN SMALL AND LARGE NATURAL RUBBER PARTICLES

Knowledge of the surface composition of natural rubber (NR) latex is essential to manufacturers of latex goods. Films made from only small rubber particles (SRPs) and a mix of SRPs and large rubber particles (LRPs) differ in mechanical properties. The reason for this difference, which is still under debate, is hypothesized to be linked with biomolecules (proteins and lipids) present in the NR particle surface. In this study, we characterize the surface chemistry, particularly lipid content of the SRP and LRP, by performing investigations directly on these particles in aqueous conditions. Fluorescent probes were used to display protein and lipid affinity and analyze them in situ with steady-state fluorescence spectroscopy, fluorescence correlation spectroscopy, and fluorescence lifetime measurements. Results are atypical in showing that lipids are more abundant in LRPs than in SRPs, suggesting thicker and/or denser membranes in LRPs. The degree of membrane compacity affects rigidity, influences biomolecular interactions, and might impact natural rubber coagulation. These results provide additional insights into colloidal behavior of NR for more efficient industrial applications.

Identification of a Taraxacum brevicorniculatum rubber elongation factor protein that is localized on rubber particles and promotes rubber biosynthesis

Two protein families required for rubber biosynthesis in Taraxacum brevicorniculatum have recently been characterized, namely the cis-prenyltransferases (TbCPTs) and the small rubber particle proteins (TbSRPPs). The latter were shown to be the most abundant proteins on rubber particles, where rubber biosynthesis takes place. Here we identified a protein designated T. brevicorniculatum rubber elongation factor (TbREF) by using mass spectrometry to analyze rubber particle proteins. TbREF is homologous to the TbSRPPs but has a molecular mass that is atypical for the family. The promoter was shown to be active in laticifers, and the protein itself was localized on the rubber particle surface. In TbREF-silenced plants generated by RNA interference, the rubber content was significantly reduced, correlating with lower TbCPT protein levels and less TbCPT activity in the latex. However, the molecular mass of the rubber was not affected by TbREF silencing. The colloidal stability of rubber particles isolated from TbREF-silenced plants was also unchanged. This was not surprising because TbREF depletion did not affect the abundance of TbSRPPs, which are required for rubber particle stability. Our findings suggest that TbREF is an important component of the rubber biosynthesis machinery in T. brevicorniculatum, and may play a role in rubber particle biogenesis and influence rubber production.

New evidence disclosed for networking in natural rubber by dielectric relaxation spectroscopy

Resolving the structure of natural rubber (NR) has been an important issue for a long time and essential progress has been made. It is well established that non-rubber components have significant effects on the performance of NR. A detailed discussion on the effects of proteins and phospholipids on the chain dynamics of NR will be crucial for the in-depth understanding of the role of proteins and phospholipids in NR. However, to date, there is still a lack of elaborate studies on the dielectric spectroscopy of NR. In the present study, we performed detailed dielectric relaxation analysis, together with rheological measurements, to reveal the effects of proteins and phospholipids on the chain dynamics of NR. Distinctly different from the widely accepted segmental mode (SM) and normal mode (NM), a new relaxation mode in deproteinized NR (DPNR) was identified for the first time, which cannot be found either in NR or in transesterified DPNR (TE-DPNR). Because this new mode relaxation process behaves as a thermally activated process and it is about four orders of magnitude slower than NM, it could be rationally attributed to the relaxation of the phospholipids core of DPNR, named branch mode (BM) relaxation. When further conversion of DPNR to TE-DPNR was conducted, the phospholipids were removed and BM disappeared. In addition, a new relaxation mode, which occurs at considerably lower temperature than that for SM, was revealed in TE-DPNR, and may be related to the relaxation of free mono- or di-phosphate groups at the α ends in TE-DPNR. Hence, the identification of the new relaxation modes in DPNR and TE-DPNR provide new evidence for the natural networking structure linked by protein-based ω ends and phospholipids-based α ends.

In situ synthesis of natural rubber latex-supported gold nanoparticles for flexible SERS substrates

Natural rubber latex (NRL) from Hevea brasiliensis was used as a matrix to synthesize gold nanoparticles (AuNPs), leading to an organic–inorganic hybrid latex of NRL-supported AuNPs (AuNPs@NRL).

The characteristics and potential applications of structural lipid droplet proteins in plants

Plant cytosolic lipid droplets are storage organelles that accumulate hydrophobic molecules. They are found in many tissues and their general structure includes an outer lipid monolayer with integral and associated proteins surrounding a hydrophobic core. Two distinct types can be distinguished, which we define here as oleosin-based lipid droplets (OLDs) and non-oleosin-based lipid droplets (NOLDs). OLDs are the best characterized lipid droplets in plants. They are primarily restricted to seeds and other germinative tissues, their surface is covered with oleosin-family proteins to maintain stability, they store triacylglycerols (TAGs) and they are used as a source of energy (and possibly signaling molecules) during the germination of seeds and pollen. Less is known about NOLDs. They are more abundant than OLDs and are distributed in many tissues, they accumulate not only TAGs but also other hydrophobic molecules such as natural rubber, and the structural proteins that stabilize them are unrelated to oleosins. In many species these proteins are members of the rubber elongation factor superfamily. NOLDs are not typically used for energy storage but instead accumulate hydrophobic compounds required for environmental interactions such as pathogen defense. There are many potential applications of NOLDs including the engineering of lipid production in plants and the generation of artificial oil bodies.

Organic acids and protein compounds causing the photoluminescence properties of natural rubber membranes and the quenching phenomena from Au nanoparticle incorporation

Natural rubber membranes were fabricated using latex from Hevea brasiliensis trees (clone RRIM 600) by casting, and controlling the time and temperature of thermal treatment. Three temperatures were used: 65, 80 and 120 °C and the corresponding annealing times of 6, 8, 10 and 12 h. The centrifugation of the latex produces the constituent phases: solid rubber (F1), serum or protein components (F2) and bottom fraction (F3). The photoluminescence properties could be correlated with organic acid components of latex. Natural rubber membranes were used as the active substrate (reducing agent) for the incorporation of colloidal Au nanoparticles synthesized by in situ reduction at different times. The intensity of photoluminescence bands assigned to the natural rubber decreases with the increase in amount of nanoparticles present on the membrane surface. It can be assumed that Au nanoparticles may be formed by reduction of the Au cation reacting with functional groups that are directly related to photoluminescence properties. However, the quenching of fluorescence may be attributed to the formation of a large amount of metal nanostructures on the natural rubber surface.

Natural-rubber-based flexible microfluidic device

This paper reports a new method developed to fabricate natural-rubber-based microfluidic devices (NRMDs) for optical and electrochemical applications.

Shell structure of natural rubber particles: evidence of chemical stratification by electrokinetics and cryo-TEM

The interfacial structure of natural rubber (NR) colloids is investigated by means of cryogenic transmission electron microscopy (cryo-TEM) and electrokinetics over a broad range of KNO3 electrolyte concentrations (4-300 mM) and pH values (1-8). The asymptotic plateau value reached by NR electrophoretic mobility (μ) in the thin double layer limit supports the presence of a soft (ion- and water-permeable) polyelectrolytic type of layer located at the periphery of the NR particles. This property is confirmed by the analysis of the electron density profile obtained from cryo-TEM that evidences a ∼2-4 nm thick corona surrounding the NR polyisoprene core. The dependence of μ on pH and salt concentration is further marked by a dramatic decrease of the point of zero electrophoretic mobility (PZM) from 3.6 to 0.8 with increasing electrolyte concentration in the range 4-300 mM. Using a recent theory for electrohydrodynamics of soft multilayered particles, this "anomalous" dependence of the PZM on electrolyte concentration is shown to be consistent with a radial organization of anionic and cationic groups across the peripheral NR structure. The NR electrokinetic response in the pH range 1-8 is indeed found to be equivalent to that of particles surrounded by a positively charged ∼3.5 nm thick layer (mean dissociation pK ∼ 4.2) supporting a thin and negatively charged outermost layer (0.6 nm in thickness, pK ∼ 0.7). Altogether, the strong dependence of the PZM on electrolyte concentration suggests that the electrostatic properties of the outer peripheral region of the NR shell are mediated by lipidic residues protruding from a shell containing a significant amount of protein-like charges. This proposed NR shell interfacial structure questions previously reported NR representations according to which the shell consists of either a fully mixed lipid-protein layer, or a layer of phospholipids residing exclusively beneath an outer proteic film.

Histochemical study of trans-polyisoprene accumulation by spectral confocal laser scanning microscopy and a specific dye showing fluorescence solvatochromism in the rubber-producing plant, Eucommia ulmoides Oliver

A microscopic technique combining spectral confocal laser scanning microscopy with a lipophilic fluorescent dye, Nile red, which can emit trans-polyisoprene specific fluorescence, was developed, and unmixed images of synthesized trans-polyisoprene in situ in Eucommia ulmoides were successfully obtained. The images showed that trans-polyisoprene was initially synthesized as granules in non-articulated laticifers that changed shape to fibers during laticifer maturation. Non-articulated laticifers are developed from single laticiferous cells, which are differentiated from surrounding parenchyma cells in the cambium. Therefore, these observations suggested that trans-polyisoprene biosynthesis first started in laticifer cells as granules and then the granules accumulated and fused in the inner space of the laticifers over time. Finally, laticifers were filled with the synthesized trans-polyisoprene, which formed a fibrous structure fitting the laticifers shape. Both trans- and cis-polyisoprene are among the most important polymers naturally produced by plants, and this microscopic technique combined with histological study should provide useful information in the fields of plant histology, bioindustry and phytochemistry.

Bending of layer-by-layer films driven by an external magnetic field

We report on optimized architectures containing layer-by-layer (LbL) films of natural rubber latex (NRL), carboxymethyl-chitosan (CMC) and magnetite (Fe₃O₄) nanoparticles (MNPs) deposited on flexible substrates, which could be easily bent by an external magnetic field. The mechanical response depended on the number of deposited layers and was explained semi-quantitatively with a fully atomistic model, where the LbL film was represented as superposing layers of hexagonal graphene-like atomic arrangements deposited on a stiffer substrate. The bending with no direct current or voltage being applied to a supramolecular structure containing biocompatible and antimicrobial materials represents a proof-of-principle experiment that is promising for tissue engineering applications in biomedicine.