Super-Resolution Fluorescence Imaging of Spatial Organization of Proteins and Lipids in Natural Rubber

Exploring the Role of Hydroxy- and Phosphate-Terminated cis-1,4-Polyisoprene Chains in the Formation of Physical Junction Points in Natural Rubber: Insights from Molecular Dynamics Simulations

This study elucidates the pivotal role of terminal structures in cis-1,4-polyisoprene (PI) chains, contributing to the exceptional mechanical properties of Hevea natural rubber (NR). NR's unique networking structure, crucial for crack resistance, elasticity, and strain-induced crystallization, involves two terminal groups, ω and α. The proposed ω terminal structure is dimethyl allyl-(trans-1,4-isoprene)2, and α terminals exist in various forms, including hydroxy, ester, and phosphate groups. Among others, we investigated three types of cis-1,4-PI with different terminal combinations: HPIH (pure PI with H terminal), ωPIα6 (PI with ω and α6 terminals), and ωPIPO4 (PI with ω and PO4 terminals) and revealed significant dynamics variations. Hydrogen bonds between α6 and α6 and PO4 and PO4 residues in ωPIα6 and ωPIPO4 systems induce slower dynamics of hydroxy- and phosphate-terminated PI chains. Associations between α6 and α6 and PO4 and PO4 terminals are markedly stronger than ω and ω, and hydrogen terminals in HPIH and ω PIα6,PO4 systems. Phosphate terminals exhibit a stronger mutual association than hydroxy terminals. Potentials of mean force analysis and cluster-formation-fraction computations reveal stable clusters in ωPIα6 and ωPIPO4 , supporting the formation of polar aggregates (physical junction points). Notably, phosphate terminal groups facilitate large and highly stable phosphate polar aggregates, crucial for the natural networking structure responsible for NR's outstanding mechanical properties compared to synthetic PI rubber. This comprehensive investigation provides valuable insights into the role of terminal groups in cis-1,4-PI melt systems and their profound impact on the mechanical properties of NR.

The Interplay of Protein Hydrolysis and Ammonia in the Stability of Hevea Rubber Latex during Storage

Natural rubber (NR) latex derived from Hevea brasiliensis is a complex colloid comprising mainly rubber hydrocarbons (latex particles) and a multitude of minor non-rubber constituents such as non-rubber particles, proteins, lipids, carbohydrates, and soluble organic and inorganic substances. NR latex is susceptible to enzymatic attack after it leaves the trees. It is usually preserved with ammonia and, to a lesser extent, with other preservatives to enhance its colloidal stability during storage. Despite numerous studies in the literature on the influence of rubber proteins on NR latex stability, issues regarding the effect of protein hydrolysis in the presence of ammonia on latex stability during storage are still far from resolved. The present work aims to elucidate the interplay between protein hydrolysis and ammoniation in NR latex stability. Both high- and low-ammonia (with a secondary preservative) NR latexes were used to monitor the changes in their protein compositions during storage. High-ammonia (FNR-A) latex preserved with 0.6% (v/v) ammonia, a low 0.1% ammonia/TMTD/ZnO (FNR-TZ) latex, and a deproteinized NR (PDNR) latex were labeled with fluorescence agents and observed using confocal laser scanning microscopy to determine their protein composition. Protein hydrolysis was confirmed via sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The results revealed that protein hydrolysis increased with the storage duration. The change in protein composition accompanying hydrolysis also allows the spatial distribution of allergenic proteins to be estimated in the latex. Concurrently, the latex stability increased with the storage duration, as measured by the latex's mechanical stability time (MST) and the zeta potential of the latex particles. As monitored by AFM, the surface roughness of the NR latex film increased markedly during extended storage compared with that of the DPNR latex, which remained smooth. These results underscore the pivotal role of ammonia in bolstering NR latex stability brought on by protein hydrolysis, which greatly impacts latex film's formation behavior. NR latex stability underpins the quality of latex-dipped goods during manufacturing, particularly those for medical gloves.

Interactions of REF1 and SRPP1 rubber particle proteins from Hevea brasiliensis with synthetic phospholipids: Effect of charge and size of lipid headgroup

According to the fatty acid and headgroup compositions of the phospholipids (PL) from Hevea brasiliensis latex, three synthetic PL were selected (i.e. POPA: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate POPC: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and POPG: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol) to investigate the effect of PL headgroup on the interactions with two major proteins of Hevea latex, i.e. Rubber Elongation Factor (REF1) and Small Rubber Particle Protein (SRPP1). Protein/lipid interactions were screened using two models (lipid vesicles in solution or lipid monolayers at air/liquid interface). Calcein leakage, surface pressure, ellipsometry, microscopy and spectroscopy revealed that both REF1 and SRPP1 displayed stronger interactions with anionic POPA and POPG, as compared to zwitterionic POPC. A particular behavior of REF1 was observed when interacting with POPA monolayers (i.e. aggregation + modification of secondary structure from α-helices to β-sheets, characteristic of its amyloid aggregated form), which might be involved in the irreversible coagulation mechanism of Hevea rubber particles.

Role of Terminal Groups of cis-1,4-Polyisoprene Chains in the Formation of Physical Junction Points in Natural Rubber

The terminal structures of cis-1,4-polyisoprene (PI) chains play a vital role in the excellent comprehensive performance of Hevea natural rubber (NR) with properties such as high toughness, tear-resistance, and wet skid resistance. The cis-1,4-polyisoprene chain constituting NR exhibits a distinct composition of terminal groups comprising two distinct types, namely, the ω and α terminal groups. The structures of the ω terminal [dimethyl allyl (DMA)-(trans-1,4-isoprene)2] and six kinds of α end groups of the polymer chain of NR have been explored by utilizing a newly developed 2D NMR method. In the present work, we examine different kinds of PI melt systems, and we choose various combinations of terminal groups: Hydrogen, one DMA unit with two trans isoprene units as ω end groups and ester-terminated isopentene (α1), hydroxy-terminated isopentene (α2), ester-terminated isobutane (α3), hydroxy-terminated isobutane (α4), ester-terminated 1,4-cis-isoprene (α5), and hydroxy-terminated 1,4-cis-isoprene (α6), i.e., HPIH (PI0)-pure PI (Hydrogen terminal), ωPIα1 (PII), ωPIα2 (PIII), ωPIα3 (PIIII), ωPIα4 (PIIV), ωPIα5 (PIV), and ωPIα6 (PIVI). We evaluated dynamic and static properties of PI chains such as the end-to-end vector autocorrelation function (C(t)), its average relaxation time (τ), end-to-end distance (Ree), and radius of gyration (Rg). We also estimated the diffusion coefficients of polyisoprene chains and pair correlation functions [radial distribution functions (RDFs)], potentials of mean force (PMFs) in between end residues, and survival probability (P(τ)) of end groups around the end group by analyzing the equilibrated trajectories of full-atom MD simulations. As per the examination of C(t), rotational relaxation time τ, and RDFs, we discovered that the existence of a strong hydrogen bond in α2-α2, α4-α4, and α6-α6 residues makes the dynamics of hydroxy-terminated polyisoprene chains in ωPIα2,α4,α6 melt systems slower. From the analyses of RDFs and PMFs (W(r)), the association between [α2]-[α2], [α4]-[α4], and [α6]-[α6] terminals in ωPIα2,α4,α6 melt systems is significantly stronger than in [ISO]-[ISO] [Hydrogen terminated 1,4-cis-isoprene:(ISO)] in HPIH and ω-ω, [α1]-[α1], [α3]-[α3], and [α5]-[α5] in ωPIα1,α3,α5 systems. We quantified the fraction of cluster formation of terminal groups of a given size in the seven PI melt systems by employing the criteria of PMFs. It is revealed that no stable cluster exists in the HPIH, ωPIα1, ωPIα3, and ωPIα5 melt systems. Conversely, in the ωPIα2, ωPIα4, and ωPIα6 systems, we perceived stable clusters of [(α2)p] [(α4)p] and [(α6)p] end groups where p (2 ≤ x ≤ 6). These stable clusters validate the presence of physical junction points in between hydroxy-terminated polyisoprene chains through their α2, α4, and α6 terminals. These physical junction points might be crucial for superior properties of NR such as high toughness, crack growth resistance, and strain-induced crystallization.

Influence of Clones on Relationship between Natural Rubber and Size of Rubber Particles in Latex

IAN873, Dongfang93114 and Reyan73397, created through vegetative propagation for their high yield and excellent cold resistance, are major clones planted in China. In this work, latexes with rubber particles of the same size from these clones are separated from fresh natural rubber latex, and corresponding rubber films are prepared from each latex. The structure and components of each film are measured. This indicates that the characteristics of the rubbers obtained from latexes with similar particle sizes show some resembling trends among different clones, while for specific samples, those characteristics vary depending on the clone. The molecular weight is generally highest in IAN873 and lowest in Reyan73397. Rubber chains in small rubber particles are longer, and large rubber particles show a wider molecular weight distribution. The gel content of every sample from Reyan73397 is lower than the other two clones. The nitrogen content increases with the size of rubber particles in all clones. The ester content of small rubber particles in IAN873 and Reyan73397 is almost zero. Large rubber particles have more branching points formed via esters. This study provides a new perspective on the influence of clones on the relationship between characteristics of natural rubber and the size of rubber particles in natural rubber latex.

The Role of Non-Rubber Components on Molecular Network of Natural Rubber during Accelerated Storage

Though the non-rubber components have long been recognized to be a vital factor affecting the network of natural rubber (NR), the authentic role of non-rubber components on the network during accelerated storage has not been fully illuminated. This work attempts to clarify the impact of non-rubber components on the network for NR during accelerated storage. A natural network model for NR was proposed based on the gel content, crosslinking density, and the non-rubber components distribution for NR before and after centrifugation. Furthermore, the effect of non-rubber components on the network was investigated during accelerated storage. The results show that terminal crosslinking induced by non-rubber components and entanglements are primary factors affecting the network formation during accelerated storage. By applying the tube model to analyze the stress-strain curves of NR, we found that the contribution of the entanglements to the network formation is larger than that of terminal crosslinking during accelerated storage. The work highlights the role of non-rubber components on the network during accelerated storage, which is essential for understanding the storage hardening mechanism of NR.

Effect of protein on the thermogenesis performance of natural rubber matrix

Under high-speed strain, the thermogenesis performance of natural rubber products is unstable, leading to aging and early failure of the material. The quality of rubber latex and eventually that of the final products depends among others on the protein content. We found that when the protein is almost removed, the heat generated by the vulcanized rubber increases rapidly. After adding soy protein isolate to the secondary purification rubber, the heat generation of the vulcanized rubber is reduced, and the heat generation is the lowest when the added amount is 2.5–3.0 phr, which on account of protein promotes the construction of a vulcanization network and increases the rigidity of the rubber chain, resulting in a decrease in the potential frictional behavior of the rubber chain during the curl up-extension process.

Mimicking the Mechanical Robustness of Natural Rubber Based on a Sacrificial Network Constructed by Phospholipids

Mechanical strength and toughness are usually mutually exclusive, but they can both appear in natural rubber (NR). Previous studies ascribe such excellent properties to highly cis stereoregularity of NR. To our surprise, after the removal of non-rubber components (NRC) by centrifugation, the strength and toughness of NR decrease dramatically. It is still a challenge for us to make out for the problem of how NRC affect the properties of NR. Our group ascribes the superior mechanical robustness of NR to NRC. To further verify such a viewpoint, we add phospholipids (phosphatidylcholines) into NR without NRC. Phosphatidylcholines construct a sacrificial network, which ruptures preferentially upon deformation to dissipate energy. Moreover, some of phosphatidylcholines participate in the vulcanization reaction, which further improves the mechanical strength and energy dissipation. As a result, the mechanical strength and toughness of samples are as high as 21.1 MPa and 49.6 kJ/m², respectively, which have reached the same level as that of NR. Therefore, this work not only imitates the excellent mechanical robustness of NR but also further provides a rational design for elastomers with excellent mechanical robustness.