Polyacetals based on polymethylsqualene diols, precursors of algaenan in Botryococcus braunii race B

Aliphatic polyacetals comprising polymethylsqualene diol moieties were isolated from the heptane extracts of two strains of the green alga Botryococcus braunii race B. Size exclusion chromatography showed the isolates to have masses in the range 1.2 × 104 to 4 × 106 Daltons. Pulse-field gradient NMR analysis allowed the spectroscopic resolution of a crude algal extract into polyacetals, demonstrating that these polymers are not artifacts formed during the isolation process. The aliphatic polyaldehyde backbone of the polyacetals is likely derived from the condensation–polymerization of a n-C32 diunsaturated α,ω-dialdehyde, via an aldolization–dehydration mechanism. Acetalization of the polyaldehydes with some triterpene diols occurs for 40% up to 70% of the aldehyde functions, depending on the strain. These diols are closely related to methylated squalenes, including 3,7,18,22-tetramethylsqualene. Both acyclic and monocyclic polymethylated squalene derivatives, hydroxylated at positions 10 and 11, were characterized. Cross-linkages involving free aldehyde functions and diols derived from epoxy-polymethylated squalene moieties borne by the polymers are probably at the origin of the extension of the polymeric networks. The algaenan, i.e. the non-hydrolyzable, highly aliphatic and insoluble biomacromolecular material present in the cell walls of B. braunii, likely corresponds in race B to a high molecular weight form of these polyacetals, insoluble in organic solvents.