INITIATION OF RUBBER BIOSYNTHESIS IN HEVEA-BRASILIENSIS .1. INITIATION OF RUBBER BIOSYNTHESIS IN HEVEA-BRASILIENSIS - CHARACTERIZATION OF INITIATING SPECIES BY STRUCTURAL-ANALYSIS

The initiating molecular species for rubber formation in Hevea brasili ensis has been investigated by structural characterization of the term inal group in the rubber by means of C-13 NMR and H-1 NMR spectroscopy . The presence of trans-isoprene units in trans-trans linkage was conf irmed by the characteristic C-1-methylene-carbon signal of the trans-i soprene unit. The number of trans-isoprene units was estimated to be a bout two from the relative intensity of this signal, and the degree of polymerization of various fractions from transesterified deproteinize d natural rubber. Two methyl-proton signals in the trans-trans sequenc e were detected by H-1 NMR spectroscopy. This spectral evidence shows that the trans-isoprene units are in the trans-trans-cis sequence at t he initiating terminal. However, the expected methyl- and olefinic-car bon signals and methyl-proton signal of the dimethylallyl group were n ot detected in natural rubber from regularly tapped trees and from a H evea seedling. Thus, the direct initiating species of rubber formation is either a C-15 allylic diphosphate consisting of two trans-isoprene unites as in the case of farnesyl diphosphate (FDP), but which is mod ified at the dimethylallyl group, or FDP, the dimethylallyl group of w hich is modified after polymerization.