Properties and Nano-observation of Natural Rubber with Nanomatrix Structure - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 316

and remaining proteins, peptides, or amino acid sequences. It is, thus, quite important
to establish a novel procedure to remove the proteins from NR rapidly and efficiently.
The structure of NR has been proposed to consist of α-terminal, two trans -1,4-
isoprene units, long sequence of cis -1,4-isoprene units, and ω-terminal, aligned in this
order (Tanaka, 1983, 1989a, b). The α-terminal was inferred to be a modified dimeth-
ylallyl group that can form hydrogen bonds between proteins, while the ω-terminal
was comprised of a phospholipid that may form chemical crosslinks with ionic link-
ages. According to the proposed structure of NR, it is expected that there is little pos-
sibility to form chemical linkages between NR and the proteins.
In previous work (Allen et al., 1963; Tangpakdee et al., 1997), NR coagulated from
fresh NR latex, just after tapping from Hevea brasiliensis , was found to be soluble
in toluene, cyclohexane, and tetrahydrofuran. In contrast, the rubber from latex pre-
served in the presence of ammonia contained about 30-70% gel fraction, which was
insoluble in the solvents. The formation of the insoluble fraction would be concerned
with the interactions of rubber and proteins, because the gel fractions are reported to
be soluble in the solvents after the enzymatic deproteinization (Eng et al., 1994; Tang-
pakdee et al., 1997). If the interactions are physical but not chemical, it is possible to
remove the proteins from the rubber after denaturation of the proteins with urea. Here,
the removal of the proteins from fresh NR latex and preserved high-ammonia latex
was investigated with urea in the presence of surfactant.
deproteinization of Nr with urea
Total nitrogen content, X , of both untreated and deproteinized rubbers, is shown in Ta-
ble 1. The total nitrogen content of HANR was reduced to 0.017 wt% after enzymatic
deproteinization (E-DPNR), as reported in the previous study (Tanaka et al., 1997). On
the other hand, it was reduced to 0.020 wt% after the treatment with urea, being similar
to the nitrogen content of E-DPNR. This implies that most proteins present in NR are
attached to the rubber with weak attractive forces. To remove further the proteins, the
treatment with urea was carried out after the enzymatic deproteinization of HANR la-
tex. The nitrogen content of the resulting rubber, EU-DPNR, was 0.008 wt%, less than
that of E-DPNR and U-DPNR. This suggests that the most of proteins are removed by
denaturation with urea, whereas the residue must be removed with proteolytic enzyme
in conjunction with urea.
To assure the difference in the role between the proteolytic enzyme and urea, the
treatment of fresh NR latex was made as well as HANR latex. The total nitrogen con-
tent of both untreated and deproteinized rubbers, which were prepared from fresh NR
latex, is also shown in Table 1. The total nitrogen content of fresh NR was reduced
to 0.014 wt% after enzymatic deproteinization (fresh E-DPNR), and 0.005 wt% after
enzymatic deproteinization followed by the treatment with urea (fresh EU-DPNR), as
in the case of HANR. On the other hand, after treatment with urea (fresh U-DPNR),
the total nitrogen content of fresh NR was 0.004 wt%, being the least among the de-
proteinized rubbers. This may be explained in that most proteins present in fresh NR
are attached to the rubber with weak attractive forces, which are able to be disturbed
with urea. Thus, it is possible to expect that almost all proteins present in fresh NR are
removed rapidly from the rubber by urea treatment.