Structural Characterization of Naturally Occurring cis-Polyisoprenes

Department of Material Systems Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan 

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. 

Polyisoprenes are synthesized by thousands of plant species covering most generic families. Usually, the polyisoprenes are rubbery and have predominantly a cis-1,4 structure. Only a 

The biosynthesis of natural rubber has been studied from the viewpoint of an elucidation of initiation and propagation mechanisms mainly by tracer techniques. The steps in the formation of isopentenyl pyrophosphate from acetyl-coA via mevalonate are now well established in the in vitro synthesis of rubber. It has also been confirmed that chain extension occurs on the surface of existing rubber particles by successive additions of isopentenyl pyrophosphate to build up chains of 5000-7000 isoprene units (4,5). The initiation step of rubber formation, however, remains unknown due to the lack of detailed information concerning the direct precursor of the chain extension. 

This paper describes the structural analysis of naturally occurring ois -1,4 polyisoprenes using 130 NMR spectroscopy. First, the structural characterization of polyprenols, which are linear isoprenoid compounds containing 30 to 100 carbons, was carried out on the basis of information obtained from acyclic terpenes having various cis and trans isoprene units as model compounds. This method was also applied to the structural analysis of polyisoprenes. The elucidation of the structure of the end groups and the arrangement of isoprene units provides information on the mechanism of the biosynthesis of polyprenyl compounds in nature. 

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