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base (
λ
max
= 660 nm), and ionized quinonoidal base (
λ
max
= 665 nm). The presence of
any one of these forms is pH dependent; however, multiple forms exist at a given pH.
Further complicating interpretation of spectra for these compounds is their capacity for
interacting via H-bonding and charge transfer interactions with compounds such as gallic
acid, caffeine, chlorogenic acid (Wigand et al.,
1992
), and other flavonoids (Santhanam
et al.,
1983
). These interactions can result in increased absorption of light by anthocya-
nin chromophores and either fluorescence enhancement or quenching depending on the
anthocynanin (Santhanam et al.,
1983
; Wigand et al.,
1992
).
2.4.6 Lignin
According to Kirk et al. (
1980
), lignin is a generic term used to describe the complex
aromatic biopolymers that are major components of plant tissues. Lignin represents the
second most abundant terrestrial biopolymer after cellulose (Kirk et al.,
1980
; Boerjan
et al.,
2003
), and, given the absence of aromatic moieties in cellulose, the most abundant
class of aromatic biopolymers. Its significant annual production is roughly balanced by
its degradation by microorganisms (Kirk et al.,
1980
). It is a compound class that has
long been thought to contribute to the composition of terrestrially derived humic sub-
stances (Stevenson,
1985
). A large body of research has addressed lignin as a source of
aromatic compounds to DOM, and there is substantial direct evidence that lignin-derived
compounds are a major component of terrestrially derived DOM (e.g., Kujawinski et al.,
2009
; Sleighter et al.,
2010
). For example, Ertel et al. (
1986
) determined that 3-8% of the
humic carbon in the Amazon River was present as lignin components. The fluorescence
properties of lignin (
Figure 2.5d
) have long been the subject of study (e.g., Hartley,
1893
),
and assignment of structures responsible for lignin fluorescence has been elusive (Radotić
et al.,
2006
). As is the case for DOM, identification of lignin fluorophores and understand-
ing the dynamics responsible for lignin fluorescence are complicated by inherent structural
complexity and variability between different samples. Olmstead and Gray (
1997
) provide
a thorough review of the fundamentals of lignin fluorescence, applications of fluorescence
to the analyses of industrial lignins and wood pulp, and novel fluorescence approaches for
the study of nonfluorescent components in wood products. Phenylcoumarin, stilbene struc-
tures, coniferol alcohol,
p
-oxybenzaldehyde, biphenyl, and benzoquinone structures have
been proposed to be the primary fluorophores in lignin (Lundquist et al.,
1978
; Olmstead
and Gray,
1997
; Albinsson et al.,
1999
; Machado et al.,
2001
). The fluorescence properties
of these compound groups within lignin are thought to result from direct excitation of indi-
vidual fluorphores, energy transfer interactions, and the presence of charge-transfer com-
plexes. Lundquist et al. (
1978
) originally proposed that lignin fluorescence was the result of
the transfer of energy from excited structural elements in lignin to other structures that act
as “energy sinks” that subsequently emitted the energy as light. Phenylcoumarone-type and
stilbene-type components in lignin, neither of which exhibit efficient fluorescence of them-
selves, were proposed as potential acceptor groups. In another example of energy transfer, to
explain the observation that spruce lignin exhibited fluorescence at approximately 360 nm
