Geoscience Reference
In-Depth Information
hydroxyl functional groups, are common in plants (Robinson,
1991
), are potential degrada-
tion products of lignin (Larson and Rockwell,
1980
), are reasonably water soluble, and are
highly fluorescent (Fink and Koehler,
1970
; Larsen and Rockwell,
1980
; Wolfbeis,
1985
).
As these compounds are lactones, opening of the ring structures (return to
o
-hydroxycin-
namic acids) is favored at high pH whereas the lactone form is favored at low pH. Not sur-
prisingly, the fluorescence properties of these molecules are strongly pH dependent (Fink
and Koehler,
1970
; Wolfbeis,
1985
). Larsen and Rockwell (
1980
) demonstrated that the
intensely fluorescent coumarin compound, esculetin, could be formed from caffeic acid
(3,4-dihydroxycinnamic acid) by a photooxidation reaction, whereas Fink and Koehler
(
1970
) suggest that cinnamic acid is a photolysis product of 7-hydroxycoumarin. Based on
similarities in the fluorescence spectra of coumarins and water soluble humic substances,
Larsen and Rockwell (
1980
) hypothesized that coumarins are important contributors to the
fluorescence properties of both DOM and humic substances.
Among natural products, the flavonoids represent the largest group of oxygen ring com-
pounds (Wolfbeis,
1985
). They are benzopyran derivatives having a carbon skeleton that
consists of two substituted benzene rings connected by a three-carbon aliphatic chain and
multiple phenolic hydroxyl groups (Wolfbeis,
1985
; Robinson,
1991
). The flavonoid cate-
gory includes flavanones, flavones, flavanols (e.g., quercitin, morin), and anthocyanins. The
distinctions between compound groups are determined by oxidation states and variations
of the three-carbon chain (Robinson,
1991
). The flavonoids are common throughout the
plant kingdom and include most of the common plant pigments (Robinson,
1991
). Many
flavonoids fluoresce strongly, especially in polar solvents, and exhibit large Stokes shifts
(Wolfbeis,
1985
). The fluorescence characteristics of individual flavonoids are influenced
by the number and positions of hydroxyl groups (Wigand et al.,
1992
; Ale et al.,
2002
).
These compounds and their fluorescence properties have long been of interest to natural
products chemists. Wolfbeis (
1985
) presents a good summary of earlier studies of flavonoid
fluorescence. Of particular interest, the anthocyanins, a major class of pigments in leaves,
flowers, and fruits, are an important compound class associated with red wines, a con-
nection that has resulted in the development of fluorescence approaches in their analyses
(Figueiredo et al.,
1990
; Bonerz et al.,
2008
). Whereas the flavonoid group of compounds
has not received the same attention as lignin or tannins as sources of DOM, it is possible
that they contribute to DOM fluorescence given their prevalence in the plant world and
aqueous solubility, although direct evidence for their presence is currently lacking.
The flavonoid group of compounds can exhibit complicated absorbance and fluores-
cence spectra (e.g., Figueiredo et al.,
1990
; Drabent et al.,
2007
). For example, multiple
absorbance and emission bands are evident in the aqueous spectra for naringin hydrate, a
flavanone (
Figure 2.5c
). Of the natural products reported on in this chapter, the flavonoids
are the only ones with fluorescence behavior in the “C” and “A” peak regions commonly
ascribed to humic materials (
Table 2.1
). Figueiredo et al. (
1990
) determined that fluores-
cence emission bands associated with malvidin 3,5-diglucoside, an anthocyanin, could be
assigned to six forms of the molecule - the flavylium cation (
λ
max
= 620 nm), hemiacetal
(
λ
max
= 370 nm), chalcone (
λ
max
= 435 nm), ionized chalcone (
λ
max
= 495 nm), quinonoidal
