Resistant Biopolymers from Outer Walls PRB

braunii outer walls are able to survive the successive degradations generally used for the isolation of sporopollenins which constitute a class of biopolymers occurring in the exine of spores and pollens. These latter biopolymers which would originate from the oxidative polymerization of carotenoids and/or carotenoid esters are insoluble in organic solvents and resistant to non-oxidative chemical attacks. 

After hexane extraction and potassium hydroxide and then phosphoric acid treatments (Fig. 1), B. braunii outer walls exhibit, on examination in an electron microscope, an unchanged organization (Plate 2). The complete degradation of the outer walls by the action of chromic acid, established that they do not contain any silica which is known to be sometimes associated with a resistant organic polymer in algal walls. 

Important structural information was also obtained from pyrolysis of PRB A at 400 °C carried out under an helium flow so as to minimize secondary reactions (67). GC-EIMS of the pyrolysate fractions obtained by CC and AgN03-Si02 TLC showed the predominance of hydrocarbons ca 55% of the total pyrolysate). Regular series of C13 to C31 n-alkanes and n-alk-l-enes, formed by cracking of C-C bonds, are the major constituents of this fraction. They are accompanied by minor series of n-alkylbenzenes, n-alkyl- and n-alkenylcyclohexanes and n-trans-alkenes. Pyrolysis also provided a complex mixture of unidentified ketones and a series of n-fatty acids dominated by palmitic and oleic acids. The recovery of fatty acids on pyrolysis of PRB A, although isolation of this resistant material required drastic basic and acid treatments, indicates that the corresponding esters are sterically protected in the polymeric network. 

In spite of all these results, the precise origin and structure of PRB A are still to be established. However the ether lipids (72)-(75) which were identified in the external lipids of the A race are very likely implicated as important building blocks in the formation of PRB A. Indeed, the structural features of these ether lipids (very long hydrocarbon chains, ether bridges and hydroxyl and ester groups) are consistent with those of PRB A and, as indicated in part 3.4, a polymeric network could be easily generated via cross-linking of such compounds. Moreover, a clear absorption at 1690 cm Mn the IR spectrum of PRB A probably reflects 

The question arises finally whether or not the aldehydic rubber also contributes to the formation of PRB A. Indeed, when the algal biomass is not extracted with CHCI3, all the rubber remains definitely included in the final residue and the PRB A content (ca 10% of dry biomass) is twice that found when rubber is removed by CHCI3 extraction prior to basic and acid hydrolysis (65). However, this observation does not entirely rule out the possible incorporation of some rubber, perhaps in reticulated form, into PRB A. 

---