Structure of Naturally Occurring Cis-Polyisoprenes

The chemical shifts of the characteristic carbon signals in acyclic terpenes, polyprenols, and cis-trans isomerized poly-isoprenes are plotted in Fig. 3. Here, the chemical shifts are correlated using the C-5 methyl carbon signal at 17.66 ppm as an internal standard (except for isomerized polyisoprenes) in order to compensate for the effect of solution concentration. It is clear that these chemical shifts are independent of the chain length of the compounds and can be used for the determination of the arrangement of isoprene units as well as the terminal units in various isoprenoid compounds (8). 

The number of the internal cis units determined from the intensity ratio of either the cis C-1 or C-4 methylene signal to the a C-4 methylene carbon signal, was approximately 1000 and 2200 for the polymers S-1 and S-2, respectively. Similar NOE and shorter T values for these carbon atoms indicate that an almost quantitative evaluation is possible for the intensity ratios between these methylene carbon signals. The degree of polymerization determined from the a-terminal unit agrees with that from GPC for these samples, i.e., 1100 for S-1 and 1800 for S-2. This 

These facts clearly indicate that the -polyisoprene 

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The chemical structure of naturally occurring cis polyisoprenes was determined by 13C NMR spectroscopy using acyclic terpenes and polyprenols as model compounds. The arrangement of the isoprene units along the polymer chain was estimated to be in the order dimethylallyl terminal unit, three trans units, a long block of cis units, and cis isoprenyl terminal unit. This result demonstrates that the biosynthesis of cts polyisoprenes in higher plants starts from trans,transytrans-geranylgeranyl pyrophosphate. 

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