From rubber particles of Hevea latex to Natural Rubber structure and properties: towards optimizing the performance of Natural Rubber – RUBBex

A multidisciplinary and integrated approach combining mainly chemistry (synthesis of biomimetic copolymers), physics (Langmuir films, Plasmon Waveguide Resonance (PWR), RX, PMIRRAS, …) and biochemistry (fractionation, latex composition, LC-MS, GC-MS, ...). This approach will allow to revisit some old concepts on natural rubber structuration and to validate the rubber particle model still under debate.

The four native latex fractions (cream, skim, serum and lutoid) were successfully separated via a 3-step centrifugation method. The integrity of lutoids was maintained with a suitable concentration of mannitol buffer adjusted according to the osmotic pressure of whole latex. Validation of the latex fractionation method was based on a satisfactory mass balance (>90%) and a correct separation of skim from cream, 2 rubber fractions made of small (? 200 nm) and large (500 nm – 1 ?m) particles, respectively. The biochemical characterization of the various fractions is in progress.
The synthesis of macromolecules miming the model of Tanaka comes true in three stages: i) obtaining a polyisoprène (PI) chain totally «1,4-cis”, ii) the grafting of a phospholipid on the ?-terminal of the PI chain and iii) the grafting of a protein on the ?-terminal of the PI chain. To obtain a PI totally «1,4-cis”, the controlled oxidative degradation of the PI extracted from natural rubber was performed. Different molar masses (from 10 000 to 70 000 g/mol) were obtained (polydispersity ~ 1,5). Two cysteinilated variants in C-terminal of the HbREF and HbSRPP were cloned, to couple them on functionalized PI with different chain ends (-OH, NH2, maléimide, …).
Other proteins, present in latexes other than that of hevea, were cloned and purified: IPP isomerase (HbIDI), hevein, prohevein, homologues of SRPP in Taraxacum (dandelion, TkSRPP1-5) and Arabidopsis (AtSRP3). These recombinant proteins were studied and tested on Langmuir's model membranes by ellipsometry and BAM (WP4). Protein HbREF and HbSRPP were also produced and tested on membranes reconstituted with native lipids directly extracted from fresh latex of hevea, in the form of phospholipids, neutral lipids and glycolipids.

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On the basis of the RUBBex project works, three articles (on June 18th, 2016) were published in scientific journals with impact factor. Two articles, centered on the prohévéine, allowed in particular to underline its amyloid properties. These works were also presented to the scientific community during international congresses.

Natural Rubber (NR), a biopolymer produced from the latex of Hevea brasiliensis, exhibits very specific properties never mimicked by synthetic rubbers. NR is thus a highly strategic material for the production of tires or others items. However, NR presents important drawbacks: i) the rather non consistency or variability of its properties, ii) a structuration dynamics or “storage hardening” that is still not fully understood.
The RUBBex project aims to study and identify the main biochemical components (poly(cis-1,4-isoprene), proteins and lipids) and the mechanisms involved in the structuration of NR in order to optimize the performances of raw NR. The main objective of the RUBBex project is to generate new knowledge that will allow targeting new treatments before processing for a better control of the variability of NR properties and of the NR structuration dynamics with time.
A multidisciplinary and biomimetic approach will allow the study of this material from fresh rubber particles (organization, composition) to the raw NR (composition, structures, properties). The achievement of this multidisciplinary project relies on the complementarity of 6 partners (3 French academic laboratories, Michelin Company and 2 Thai academic laboratories). They will share their expertise in analytical chemistry, macromolecular chemistry, structural characterization, properties of materials, biochemistry, biophysics and spectroscopies.