STRUCTURE OF IN-VITRO SYNTHESIZED RUBBER FROM FRESH BOTTOM FRACTION OF HEVEA LATEX

Bottom fraction (BF) separated from fresh Hevea brasiliensis latex by ultracentrifugation, after preincubation at 4 degrees overnight, produ ced new rubber on incubation at 37 degrees for 6 hr. The rubber yield was increased about 3.2- and 4.7-fold by the addition of isopentenyl d iphosphate (IDP) and farnesyl diphosphate (FDP), respectively. The rub ber formed on incubation in the presence of FDP showed a clearly bimod al molecular-weight distribution (MWD) with <(M)over bar (n)> of 9.6 x 10(4) and <(M)over bar (W)>, of 1.0 x 10(6); which were about 1/4 of that of the endogenous rubber. This indicates that the formation of ne w rubber in BF uses FDP as a direct initiator. The presence of the dim ethylallyl-group of the initiating species was observed in the new rub ber formed on incubation of fresh BF with FDP, although it was not det ected in in vivo rubber. The new rubber formed on incubation contained no long-chain fatty acid groups esterified to the rubber molecules, a s is the case in in vivo Hevea rubber. These facts show the difference s in the mechanism of initiation and termination between the in vitro and in vivo rubber formation reactions. (C) 1997 Elsevier Science Ltd.