Abstract:
The initiating terminal group (ω-terminal) in natural rubber from Hevea brasiliensis (NR) was characterized by the use of low molecular-weight (MW) and polyprenol fractions from NR in connection with the biochemical study on the initiating species. The study on in vitro synthesis by fresh bottom fraction (BF) with various amounts of trans,trans-farnesyl diphosphate (FPP) showed low rubber yield as the FPP increased but high incorporation of labelled isopentenyl diphosphate into polyprenol fraction was observed. This suggests that the formation of new rubber chains was initiated by FPP molecules. The high-frequency 1H- and 13C-NMR with 2D-COSY techniques showed clearly the presence of the dimethylallyl group and two trans-isoprene units at the ω-terminal in polyprenol from Hevea shootings and fresh BF as well as the lowest MW fraction of rubber obtained by washing the cream fraction from fresh latex with surfactant (WRP). This finding suggested that the initiating species of rubber biosynthesis in H. brasiliensis is trans,trans-FPP as in the case of two-trans polyprenol. This ω-terminal was not detected in the high MW fractions of WRP and low MW fractions of the ordinary and deproteinized NR, suggesting the occurrence of a modification at the dimethylallyl residue. The linkage of oligopeptides at the modified ω-terminal was confirmed to be improbable based on the analysis of nitrogen atom per chain for fractionated NR from WRP, which was supported by new assignment of the FTIR band around 3320 cm-1. Low and high MW Jackfruit rubbers occurring as latex showed the ω-terminal composed of dimethylallyl-trans-trans sequence. The cDNA encoding FPP synthase was isolated and assay its function from a kind of Lactarius mushroom (L. chrysorrheus), which was the first time to elucidate the prenyl transferase in mushroom. Based on the present work, it can be concluded that the structure of dimethylallyl group at ω-terminal of the ordinary NR is modified by some enzymatic or chemical reaction on the surface of latex in the course of chain elongation to form high MW NR and coagulation process of latex."