Fig. 9
Group III, samples from the Botanic Gardens Archives, Singapore, dated 1892–1901, with inset at the top showing stereochemistry of cis-polyisoprene of the model compound. The identification labels on the left follow the samples’ archival inventory (51 med, 51 small, etc.) and year inscribed by the botanists-collectors (1892, 1900, etc.). The.1H NMR spectra of the archival specimens indicated an abundance of most likely oxidation products, in addition to the main, indicative peaks in the 2.01–1.75 ppm region. Only one sample showed the characteristic peaks of cis (1892, 51 small) pointing to natural rubber, while all other are trans-polyisoprenes, characterizing gutta percha. This agreed with the information recorded on the collector’s label included with the specimen Additional file 1: (Appendix I, Table 2, Complete Set)