Geology Reference
In-Depth Information
Chemistry of Modern Leaves
The solvent extract of the modern leaves (total lipid extract) was saponifi ed to give
the overall composition of extractable fatty acyl components. The saponifi ed extract
consists of
n
-alkanoic acids with chain lengths ranging from C
10
to C
32
and with an
even-over-odd predominance (Fig.
3.6a
). The unsaturated C
18:1
fatty acid (oleic
acid) is the most abundant followed by C
16
fatty acid, as noted by Padley et al.
(
1986
). The TLE was further separated into neutral, 'free' fatty acid (FA) and phos-
pholipid fatty acid fraction (PLFA) fractions to assess the relative contribution of
each of these to the total fatty acid pool. The neutral fractions, as is commonly
observed, contain high-molecular-weight
n
-alkanes (
n
-C
27
to C
33
) characterized by
an odd-over-even predominance and high molecular weight n-alkanols characterised
by an even-over-odd predominance (Eglinton et al.
1962
; Eglinton and Hamilton
1967
), as well as a variety of triterpenoid and steroid alcohols. The PLFAs are
comprised primarily of C
16
and C
18
fatty acids, with C
16
being vastly more abundant;
longer chain acids are absent. Lesser abundances of C
14
fatty acid are also present,
while branched components were not detected.
The free fatty acid fraction (from esterifi ed components of lipid extract, analysed
as fatty acid methyl esters) contains primarily
n
-alkanoic acids with chain lengths
ranging from C
12
to C
30
and with an even-over-odd predominance as is typical for
higher plant functionalised leaf waxes (Eglinton and Hamilton
1967
). The C
16
followed by the C
18
saturated components are the most abundant fatty acids in this
fraction as well.
Following extraction, pyrolysis of the modern leaf residues (residue 1, Fig.
3.1
)
yielded predominantly carbohydrate, lignin, and protein moieties together with C
16
and C
18
fatty acids (Fig.
3.7a
), refl ecting the bulk composition of the leaf (see Gupta
and Pancost
2004
for detailed discussion). In addition to fatty acids, pyrolysis of
extracted modern leaves (residue 1, Fig.
3.1
) revealed a series of
n
-alkane/
n
-alk-1- ene
homologues. This was reported previously (Tegelaar et al.
1991
) and was thought to
represent pyrolysis of cutan. Extracted residues were then saponifi ed (hydrolysed
under basic conditions) and the extracts, analysed by GC-MS, consist mainly of C
16
and C
18
n
-alkanoic acids and mono, di and trihydroxy derivatives of these, typical
products of cutin hydrolysis (Fig.
3.6b
) (Kolattukudy
1980
; Nip et al.
1986
).
Pyrolysis of the post-saponifi cation residues (residue 2, Fig.
3.1
) released products
solely related to lignin and carbohydrates (Fig.
3.7b
), and no aliphatic components
(neither fatty acids nor
n
-alkane/
n
-alk-1-ene homologues) were detected, as noted by
van Bergen et al. (
1998
) for leaf litter. This is confi rmed clearly by the specifi c
summed mass chromatogram
m/z
83+85 (Fig.
3.7
, inset). The absence of fatty
acids (derived from cutin) is consistent with the removal of the cuticle. The suc-
cessful removal of the cuticle has been monitored and verifi ed by microscopic
investigations as well. Vinyl phenol is formed during pyrolysis from the
p
-hydroxy-
phenol unit in certain types of lignin, but in leaves it is related mainly to
p
-coumaric
acid. It is present both as ester and ether linked units in woody tissues (as part
of lignin), but also as part of decay resistant cuticle (Tegelaar et al.
1989b
). Thus
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