Geoscience Reference
In-Depth Information
2.4.4 Phenylpropanes
Phenylpropanes are among the most widespread natural aromatic compounds (Robinson,
1991
). They are found as precursors, structural components (Higuchi,
1980
, Boerjan et al.,
2003
), and degradation products (Larsen and Rockwell,
1980
) of lignin. In addition to its
association with lignin,
p
-coumaric acid (
Figure 2.5a
) has also been found to play a role
in the photoreactivity of bacteria (Putschögl et al.,
2008
). Many phenylpropanes can be
described as hydroxylated cinnamic acids, but there are some whose stereochemistries are
unrelated to cinnamic acid (Robinson,
1991
). Cinnamic acids are also the aromatic centers
associated with the chlorogenic acids, a related group of compounds (Robinson,
1991
).
Caffeic (3,4-dihydroxycinnamic), ferulic (4-hydroxy-3-methoxycinnamic), and
p
-cou-
maric acids (also known as
p
-hydroxycinnamic acid; spectrum presented in Figure 5a) are
three common phenylpropanes.
The phenylpropanes have both carboxyl and phenolic functional groups, and fluores-
cence properties are pH dependent (Wolfbeis,
1985
; Putschögl et al.,
2008
). Wolfbeis
et al. (
1986
) identify the chemistries of the fluorophores active at different pH values for
o
-hydroxycinnamic acid. The fluorescence intensities of many compounds in this group
are weak, being strongest under mildly acidic or basic conditions (Wolfbeis,
1985
).
Data presented by Wolfbeis (
1985
) and Larsen and Rockwell (
1980
) indicate that simple
hydroxyl- and methoxycinnamic acids, including ferulic and caffeic acids, have similar
fluorescence properties (approximate excitation maxima between 295-350 nm and emis-
sion maxima between 390 and 445 nm). Larson and Rockwell (1980) noted the similarities
in fluorescence behavior between partially degraded lignin and ferulic and caffeic acid and
suggested that the fluorescence characteristics of partially degraded lignin results from oxi-
dative changes in phenylpropanoid side chains of lignin. In addition to potentially contrib-
uting directly to DOM fluorescence, phenylpropanes are reactive with other fluorophores,
such as proteins, in ways that influence their fluorescence behavior (Kang et al.,
2004
; Min
et al.,
2004
; Bian et al.,
2007
). Fluorescence methods have been developed for the analyses
of
p
-coumaric acid in beer (Garcia-Sanchez et al.,
1988
) and a suite of hydroxycinnamic
acids in wine (Bonerz et al.,
2008
).
2.4.5 Oxygen Ring Compounds
Molecules included in this group represent classes of biomolecules whose fluorescence
properties are influenced by an oxygen atom that is part of the ring structure of the mole-
cule. Included here are the coumarins and flavonoid groups of natural products. The optical
properties of compounds in this group have long been of interest because they are common
natural products, play important roles in the functioning of plants, and are often colored
(Wolfbeis,
1985
; Robinson,
1991
; Quina et al.,
2009
).
Coumarins (not to be confused with coumaric acid) are lactones of
o
-hydroxycinnamic
acids (
Figure 2.5b
; Robinson,
1991
). These aromatic compounds, many of which contain
