Geoscience Reference
In-Depth Information
(semiquinone, hydroquinone) forms, and are thought to play important roles in biogeo-
chemical electron transfer reactions (abiotic and biotic) associated with DOM (Nurmi and
Tratnyek,
2002
; Fimmen et al.,
2007
). As noted earlier, quinones are thought to play impor-
tant roles as acceptor molecules in charge transfer interactions that may influence lignin
fluorescence (Barsberg et al.,
2003
) and DOM absorbance properties (Del Vecchio and
Blough
2004
).
Oxidized quinones are relatively poor fluorophores; however, the reduced forms, hyd-
roquinones, are highly fluorescent, a property that has been utilized in developing photo-
chemical-reaction fluorescence detectors for liquid chromatography (Poulson and Birks,
1989
). Klapper et al. (
2002
) used the differences in fluorescence properties between oxi-
dized and reduced samples of humic substances isolated from a range of environments to
infer the presence of quinone-like moieties. These authors proposed that quinone moieties
contribute significantly to the fluorescence of humic substances and that fluorescence anal-
yses could be used to assess the redox status of humic substances.
Cory and McKnight (
2005
) analyzed fluorescence EEMs data for a large number of
DOM samples and, using PARAFAC analyses, concluded that approximately 50% of the
EEMs spectrum for a given sample could be accounted for by quinone-like fluorophores.
However, the presence of quinones in these samples was not quantified and the nature of
their influence on the EEMs spectra was not explored. Subsequently, Miller et al. (
2006
)
proposed that a ratio of PARAFAC regions thought to be related to the presence of quinone
moieties could be utilized as a proxy for DOM redox status (redox index).
Recently, a number of papers have provided data challenging the hypothesis that qui-
nones directly influence the fluorescence properties of DOM. Ma et al. (
2010
) demon-
strated that the fluorescence behavior of DOM on reduction with NaBH
4
was inconsistent
with the behavior of a large number of model quinones and their corresponding hydroqui-
nones. Further, based on the observation that subsequent reoxidation of the DOM samples
did not completely return the fluorescence spectra to the original state (i.e., the reduction
step was largely irreversible), Ma et al. (
2010
) suggested that aromatic ketones, known
to be structural components of terrestrially derived aquatic humic substances (Leenheer
et al.,
1987
), may play more important roles in DOM optical properties than quinones.
MacCalady and Walton (2010) and Mauer et al. (
2010
) observed no changes in fluores-
cence EEMs in regions previously assigned to quinone activity on either chemical (NaBH
4
)
or electrochemical reduction and reoxidation of their DOM samples. In the case of the
MacCalady paper, Miller et al. (
2010
) suggested that inner-filter effects may have obscured
changes in the redox sensitive regions of their spectra. The potential influences of charge-
transfer interactions involving quinone-like fluorophores on DOM fluorescence are still the
subject of study and debate.
2.4.8 Alkaloids
The alkaloids are a widely distributed class of natural products that contain nitrogen, often in
a heterocyclic ring (Robinson,
1991
). Almost all alkaloids are fluorescent (Wolfbeis,
1985
).
