Mono- and dicyclic unsaturated triterpenoid hydrocarbons in ...

880 R. de Mesmay et al. / Organic Geochemistry 39 (2008) 879–893
not always easy to perform due to the common co-occurrence
of complex mixtures of analogues difficult to purify
using HPLC. To date, among the fifty botryococcenes tentatively
identified using gas chromatography-mass spectrometry
(GC-MS) analysis of lipids of isolated strains of B.
braunii or oils extracted from natural samples (e.g. Wake
and Hillen, 1981; Metzger et al., 1985a,b, 1988; Okada
et al., 1995; Metzger and Largeau, 1999 and references
therein), only twenty structures have been fully characterized
(Okada et al., 1997; Metzger and Largeau, 1999 and references
therein). Likewise, a few botryococcenes and
botryococcanes of fossil origin have been identified (Huang
and Murray, 1995; Huang et al., 1995, 1996; Grice et al.,
1998; Smittenberg et al., 2005). The reduced C34 botryococcane
was first discovered in a Sumatran crude oil (Moldowan
and Seifert, 1980) and then in Australian coastal
bitumens (McKirdy et al., 1986). Some C31 and C33 botryococcanes
were found in Maoming Oil Shale, China (Brassell
et al., 1986) and sulfurized cyclobotryococcanes in hypersaline
sediments from the Dead Sea Basin, Israel (Grice et al.,
1998); both cyclic and acyclic botryococcenes, together
with partially reduced counterparts, were discovered in
sediments from crater lakes in Kenya (Huang et al., 1995,
1996, 1999; Huang and Murray, 1995) and China (Fuhrmann
et al., 2003) and several C34 botryococcenes were
present in large amounts in recent sediments from a crater
lake in Galápagos (Zhang et al., 2007).
The C 30 botryococcene (m; letters in bold refer to the
structures in the Appendix), the precursor of all botryococcenes
and derivatives, is synthesized in the chloroplast
via the non-mevalonate pathway (Sato et al., 2003; Okada
et al., 2004). It arises from condensation of two farnesyl
units via presqualene pyrophosphate, which is also a precursor
for squalene. From this intermediate, rearrangement
of the cyclopropyl cation (path a, Fig. 1) or ring
opening of the cyclopropyl ring (path b, Fig. 1) and subsequent
reaction with NADPH, lead to the formation of squalene
or C 30 botryococcene, respectively (Huang and
Poulter, 1989; Okada et al., 2004). Higher homologues of
botryococcene and squalene are derived from their respective
C30 precursors by successive methylation with S-adenosylmethionine,
as indicated by bold arrows in Fig. 1 for
C30 botryococcene (Metzger et al., 1987; Achitouv et al.,
2004). Afterwards, botryococcenes are excreted to the outer
walls, forming a dense oily matrix (Metzger et al., 1987),
whose structural element is a chemically resistant biopolymer,
the algaenan derived from a polyacetal network bearing
polymethylsqualene derivatives (Metzger et al., 2007).
Cyclization of the terminal moieties of botryococcenes may
also occur (Metzger et al., 1985b; David et al., 1988; Huang
et al., 1988; Huang and Poulter, 1988; Huang et al., 1995,
1996). Accordingly, the same central pattern exhibiting 1)
the quaternary C-10 bearing a methyl and the D 26 exomethylene
unsaturation, 2) the trans D 11 double bond
and 3) the methyl group at C-13, is present in all botryococcenes.
Moreover, while C31–C34 methylated squalenes
are implicated via their diol derivatives in the synthesis
of the structural element of the outer walls, the biological
role of botryococcenes is not obvious. However, as their
accumulation (accounting generally for 30–40% of the dry
wt; Metzger et Largeau, 1999) ‘‘results in the colonies predominating
in the upper regions of the water column,
where little incident radiation is lost to the water mass”
(Wake and Hillen, 1980), they may allow occupation of
some ecological niches suitable for the algal growth.
In the course of our study of the lipid biomarkers from a
30 metre sediment core from Lake Masoko, southern Tanzania,
preliminary analysis of a few samples revealed the
presence of a wide variety of botryococcenes, most of unknown
structure. We report here the structural identification
of three dicyclic C 34–C 36 and seven monocyclic C 34–C 37
botryococcenes and partially reduced counterparts isolated
from a ca. 32,000 year old sediment layer. The possible
influence of some physicochemical and environmental
factors on the distribution and abundance of these algal
biomarkers in Lake Masoko is currently being studied on
a core section corresponding to the last 32,000 years (de
Mesmay et al., 2007b).
OP P
b
a
path a path b
+ +
3 7
26
10
11
16 20
Squalene C 30 Botryococcene
Fig. 1. Simplified scheme of squalene and C 30 botryococcene biosynthesis from presqualene diphosphate (adapted from Huang and Poulter, 1989) and sites
of methylation (bold arrows).