Page 119 - Visions of the Future Chemistry and Life Science

108 A. R. HEMSLEY AND P. C. GRIFFITHS

aggregated particles and particle aggregates (‘raspberries’) are self-assem-
bling from polystyrene latex in a water/cyclohexane emulsion. These are
comparable to ‘raspberries’ and aggregated particles of sporopollenin
formed during the development of Selaginella spores (Figure 6.5(g)). Similar
structures occurring in water/rape seed oil emulsions (Figure 6.5(d)) closely
resemble some Selaginella spores in surface architecture and internal
organisation (Figure 6.5(e–f)).
The following hypothetical situation might arise, reflecting that found
in synthetic systems. An oil-in-water emulsion forms, comprising a
monomer such as a hydroxycinnamic acid (Figure 6.6) stabilised by fatty
acids. The polymerisation resulting in sporopollenin can occur through a
free radical mechanism involving the vinyl group, although the concentra-
tion of free radicals is likely to be low in natural systems, or through an
alcohol acid condensation to form an ester. The latter polymerisation,
certainly in a synthetic application, is very slow in the absence of any
added (acid) catalyst although a second molecule of acid could self-catalyse
the reaction. Nevertheless, the kinetics of this reaction are very sensitive
to concentration.
Furthermore, should free radicals be present, the vinyl groups would
much more rapidly polymerise depleting the emulsion droplets of
monomer, providing the control required for a particular particle size. The
composition of the solution thus determines not only the phase behaviour,
but the rate of polymerisation and the particle size. If, the organism has in
its genetic code, the ability to synthesise the monomer, it presumably has

Figure 6.6. Three hydroxycinnamic acids common in plants and of interest as
potential sporopollenin components.