An experimental example

Following identification of the colloidal crystal layer within our spore walls, an attempt was made to utilise a simple colloid consisting of polystyrene particles in water (a latex) to mimic the natural structure. To cause flocculation of the particles, carboxymethylcellulose (CMC) was introduced with the intention of initiating a depletion interaction as described above. Although different from sporopollenin, polystyrene shares some properties and is at least reasonably well understood with regard to its colloidal behaviour. CMC was chosen as a relatively 'natural' polysaccharide. These initial experiments proved successful and resulted in the formation of colloidal crystals like those within the spore walls, but more significantly, they were built by processes and components which we believe behave in a similar manner to those in the natural system. Similar particle flocculations, but of an amorphous nature and formed from particles of inconsistent size could be produced by either depletion or bridging floccu-lation. Subsequent experiments have utilised hydrocarbons and lipids (known from the natural system of wall production) to synthesise mimics resembling other types of spore wall with some success.

It is disconcerting how 'life-like' some structures built from synthetic colloidal particles can be (Figures 6.2(b) and 6.5(a-d). Hollow spheres of

Figure 6.5. Experiments involving mimics of sporopollenin (the principal component of spore walls) demonstrate that patterns very similar, if not identical to those of natural spores and pollen, can be produced from mixtures containing colloidal particles. All scales refer to bar in (a). (a) Spore-like structures of polystyrene particles and particle aggregates formed around a droplet of hydrocarbon. Scale = 10 ^m. (b) A broken structure like that shown in (a). Scale = 5 ^m. (c) Detail of the composition of the wall of the mimic spore shown in

(b). Scale = 2 ^m. (d) Large scale particle aggregates formed in the presence of lipids, again around a hydrocarbon droplet. Scale = 500 ^m. (e) A genuine spore of Selaginella selaginoides (club moss). Scale = 400^m. (f) The wall structure of a broken spore of Selaginella selaginoides. Scale = 3 ^m. (g) Natural sporopollenin particle aggregates and colloidal sporopollenin occurring during wall development in Selaginella laevigata. Scale = 10 ^m.

aggregated particles and particle aggregates ('raspberries') are self-assembling 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 concentration 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 caffeic acid p-OH-cinnamic acid (p-coumaric acid)

caffeic acid ferulic acid

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

ferulic acid

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

the information to degrade any excess. This natural equilibrium could also create the initiator species as a by-product of the reaction which breaks down the excess monomer.

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