Comparative study of the microstructure of solid rubber from Ficus carica and Hevea brasiliensis

Natural rubber from Hevea brasiliensis (Hb) is known to have a characteristic network structure, with branch points formed by minor nonrubber components such as proteins and phospholipids that connect the major rubber component, cis-1,4-polyisoprene chains. However, the structure of solid rubber from Ficus carica (Fc) is not well known. In this study, we examined the microstructure and protein composition of solid rubber from Fc. Transmission electron microscopy analysis of Fc solid rubber revealed that the nonrubber components formed a nonuniform framework in the rubber matrix, and that many rubber particles were fused together. We determined that ficin, a cysteine endoproteolytic protease, was a major protein in the Fc rubber. It appears that ficin degrades the proteins associated with rubber particles rather than acting as a protein connecting the cis-1,4-polyisoprene chains in Fc rubber. We also found that a macrogel fraction, which separated after the dissolution of Hb solid rubber in toluene, was absent in the Fc solid rubber. These results suggest that there are no proteinous branch points in Fc rubber. Furthermore, size exclusion chromatography with multiangle light scattering (SEC-MALS) analysis of the transesterified rubber demonstrated that no branch points composed of phospholipids were involved in Fc rubber. On the basis of our results, we propose a microstructure for the rubber particles in Fc solid rubber and discuss the relationship between the microstructure and the structure of the rubber chains within it.