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

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 2019, 20, 1394–1400]. 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 (ωPIαn + P, n = 1···6, "P" stands for PAP). By utilizing equilibrated systems, various physical quantities were estimated, including end-to-end distance (Ree), radius of gyration (Rg), end-to-end vector autocorrelation function (C(t)), average rotational relaxation time τrot, 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 τrot unveiled that PAP significantly hinders the dynamics of hydroxy-terminated and ester-terminated polyisoprene chains in the ωPIα1-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 HPIH, ωPIα1, ωPIα3, and ωPIα5 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 ωPIα2 + P, ωPIα4 + P, and ωPIα6 + 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 (PJPs) between PAP molecules and hydroxy- or ester-terminated polyisoprene chains through their respective α1, α2, α3, α4, α5, and α6 terminals. Notably, the formation of globular PJPs between PAP and ester terminal groups was observed, while networked PJPs were established between PAP molecules and hydroxy terminal groups. These PJPs are postulated to be responsible for the superior comprehensive properties exhibited by NR in comparison to synthetic polyisoprene.