Effect of Impurities on the Formation of End-Group Clusters in Natural Rubber: Phenylalanine Dipeptide as an Impurity Protein

Abstract

Natural rubber (NR), containing nonrubber constituents such as proteins, exhibits exceptional characteristics including high toughness, tear resistance, and wet skid resistance. Gaining a thorough understanding of the interplay between proteins and the terminal groups of the cis-1,4-polyisoprene chains in NR is vital for comprehending the superior properties of NR in comparison to synthetic polyisoprene rubber. The terminal ends of the cis-1,4-polyisoprene chains in NR encompass two distinct types of terminal groups: ω terminals and α terminals. Extensive investigations employing solid-state NMR analysis have revealed the structures of the ω and α terminals in NR, identifying ω as dimethyl allyl-(trans-1,4-isoprene) 2 (DMA), while the α terminals have been categorized into six types [Oouchi, M.; Ukawa, J.; Ishii, Y.; Maeda, H. Structural analysis of the terminal groups in Commercial Hevea Natural Rubber by 2D-NMR with DOSY filters and multiple-WET methods using ultrahigh-field NMR. Biomacromolecules 2019Biomacromolecules , 20, 1394Biomacromolecules −1400. In this study, our primary focus is to explore the interaction between phenylalanine dipeptide (PAP) and the terminal groups within six types of melt systems ( ω PI αn + P, n = 1•••6, "P" stands for PAP). By utilizing equilibrated systems, various physical quantities were estimated, including end-to-end distance (R ee ), radius of gyration (R g ), end-to-end vector autocorrelation function (C(t)), average rotational relaxation time τ rot , self-diffusion coefficients of polymer chains, radial distribution functions (RDFs) of terminal groups around PAP, and the survival probability P(τ) for terminal groups surrounding PAP. Analysis of C(t) and τ rot unveiled that PAP significantly hinders the dynamics of hydroxy-terminated and ester-terminated polyisoprene chains in the ω PI α1-to-α6 melt systems. Examination of RDFs demonstrated a robust association between PAP molecules and α terminals compared to ω terminal groups. Moreover, the local density of α terminal groups around other α terminal groups was notably reduced in the presence of PAP. The association between PAP and DMA was found to be weaker than that of DMA and DMA, indicating a weak correlation between PAP molecules and ω terminal groups. By employing the potentials of mean force, we conducted an investigation to calculate the cluster formation fraction of terminal groups associated with PAP as well as terminal groups forming clusters of various sizes in the 13 melt systems. Our findings revealed that in the H PI H , ω PI α1 , ω PI α3 , and ω PI α5 systems, firm cluster formation was not observed without PAP. However, in the presence of PAP, stable clusters comprising PAP − α1, PAP − α3, and PAP − α5 were formed. Conversely, in the ω PI α2 + P, ω PI α4 + P, and ω PI α6 + P systems, stable clusters involving α2 and α2, PAP and α2, α4 and α4, PAP and α4, α6 and α6, and PAP and α6 with sizes ranging from 2 to 9 were observed. These findings provide evidence for the formation of physical junction points (PJ...

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 disparity between experimental findings necessitates a thorough investigation into the molecular characteristics of these α-terminal groups in NR. It is also reported in the literature ,,, that, in a melt of natural cis -1,4-PI, a physical network structure is formed through hydrogen bonds (HBs) in α-terminal and through hydrogen and ionic bonds with phospholipids, while ω-terminal bonds are formed with protein molecules. These branch points in the network play a vital role in polymer chain elongation during deformation.…”

“…The ω-terminal consists of a dimethylallyl group and 2–3 trans isoprene. The α-terminals are considered to be categorized into those containing hydroxy, ester, and phosphate groups. ,,,− Previous investigations have conducted structural analysis using solution NMR on the terminal units of NR subsequent to chemical treatment involving alterations such as deproteinization with enzymes and other chemicals. ,, These studies revealed the presence of monophosphate, diphosphate, and phospholipids at the α-terminus. ,− In contrast, Oouchi et al devised a NMR-based approach to analyze terminal unit structures in commercial NR, either without chemical treatments or with mild treatments like acetone extraction to remove impurities. Their novel methods, integrating 2D-NMR, effectively suppressed signals from low-molecular-weight impurities.…”

“…In this work, we used the terminology α′-terminal instead of α-terminal because the terminal-containing PO 4 has not been confirmed yet by an experiment to be a terminal group of cis -1,4-PI taken from a rubber tree. Our multiscale approach is based on our recent work, − where we developed a model for cis -1,4-PI chains with six types of terminal groups (i.e., from α1 to α6, according to their notation) . The α6-terminal group represents hydroxy-terminated 1,4-cis-isoprene [−CH 2 –C(CH 3 )CH– CH 2 –OH] and IP–PO 4 ] terminal group represents phosphate-terminated 1,4-cis-isoprene [−CH 2 –C(CH 3 )CH–CH 2 –H 2 PO 4 ].…”

“…Table provides details of the three distinct melt systems with different terminal groups (see Figure ). Molecular modeling and MD simulations have been invaluable in investigating the mechanical and chemical properties of polymers, complementing experimental studies. ,− In our recent research, − we employed a multiscale approach to explore the interactions of terminal groups in the cis -1,4-PI chain, which was recently generated as reported by Choi et al In the current study, we focus on cis -1,4-PI with ω-terminal and two distinct types of α′ terminals, namely, the α6-terminal group and the PO 4 terminal group. In addition, as a reference melt system, we also consider a melt system of hydrogen-terminated cis -1,4-PI ( H PI H ).…”

“…Namely, we investigate three different melt systems ( H PI H ), ω PI α6 , and ω PI PO 4 , each characterized by unique combinations of ω-terminal and α′-terminal groups (illustrated in Figure ). Our approach builds upon the same multiscale model utilized in our previous work. − Detailed information about each melt system is provided in Table .…”