251
lipid kinase signaling pathways (Williams et al. 2004). Inhibition of xanthine oxidase and elevation of endogenous antioxidants have been also mentioned as important mechanisms (Tsao 2010). Polyphenols can induce antioxidant enzymes such as
glutathione peroxidase, catalase, and superoxide dismutase that decompose hydroperoxides, hydrogen peroxide, and superoxide anions, respectively, and inhibit the
expression of enzymes such as xanthine oxidase (Du et al. 2007). These emerging
views of polyphenols’ actions question the actual role of polyphenols in vivo, which
is based on the idea of a low cellular level of concentration of polyphenols and
metabolites. The low in vivo availability should indeed preclude any direct radical
scavenging action.
Polyphenols are also known as metal chelators, able to make complexes with
trace metals such as Al
3+
, Fe
3+
, and Cu
+
. These metals play an important role in
oxygen metabolism and free radical formation. Chelation of metals like Fe
2+
or Cu
+
can limit Fenton reaction, thus preventing oxidation caused by highly reactive
hydroxyl radicals (OH
•
) (Pietta 2000; Perron and Brumaghim 2009).
7.3 Deep Eutectic Solvent and Natural Deep Eutectic
Solvent, Definition and General Properties
7.3.1 What Are Deep Eutectic Solvents?
The term eutectic is used to design a mixture whose melting temperature is lower
than these of its pure components. As a result, two solids can liquefy upon mixing.
Although this observation might seem intriguing, it is a quite general phenomenon,
since a compound with an impurity solidifies with more difficulty than the pure
compound. A well-known example of eutectic mixture is the mixture of NaCl and
iced water, freezing below 0°C at atmospheric pressure. Deep eutectics are characterized by a “large” temperature depression, which is obviously a subjective feature,
but can be defined as a temperature depression which is significantly deeper than
that of the ideal liquid mixture (Martins et al. 2019). The gap to ideality generally
results from strong intermolecular interactions between the different components of
the mixture. The different types of interaction will be discussed in more detail
below. There is a single composition, called the “eutectic composition,” where the
pure liquid state reaches the lowest temperature. It is important to mention that
contrary to what is often reported in the literature, the eutectic composition is not
related to the formation of a stoichiometric complex, but rather to the intersection of
two independent melting curves. An emblematic preparation of deep eutectics is the
mixture of hydroxyethyltrimethyl ammonium (i.e., choline chloride) (melting point
T m = 302 °C) with urea (T m = 133 °C), which results in a eutectic of very low melting point (T m = 12 °C) (Abbott et al. 2003). The recent discovery of many room
temperature eutectics and of their ability to solubilize both organic and inorganic
compounds has launched an intense research activity about their uses as solvents,
and the term deep eutectic solvents (DES) is now widely used in this field.
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
lipid kinase signaling pathways (Williams et al. 2004). Inhibition of xanthine oxidase and elevation of endogenous antioxidants have been also mentioned as important mechanisms (Tsao 2010). Polyphenols can induce antioxidant enzymes such as
glutathione peroxidase, catalase, and superoxide dismutase that decompose hydroperoxides, hydrogen peroxide, and superoxide anions, respectively, and inhibit the
expression of enzymes such as xanthine oxidase (Du et al. 2007). These emerging
views of polyphenols’ actions question the actual role of polyphenols in vivo, which
is based on the idea of a low cellular level of concentration of polyphenols and
metabolites. The low in vivo availability should indeed preclude any direct radical
scavenging action.
Polyphenols are also known as metal chelators, able to make complexes with
trace metals such as Al
3+
, Fe
3+
, and Cu
+
. These metals play an important role in
oxygen metabolism and free radical formation. Chelation of metals like Fe
2+
or Cu
+
can limit Fenton reaction, thus preventing oxidation caused by highly reactive
hydroxyl radicals (OH
•
) (Pietta 2000; Perron and Brumaghim 2009).
7.3 Deep Eutectic Solvent and Natural Deep Eutectic
Solvent, Definition and General Properties
7.3.1 What Are Deep Eutectic Solvents?
The term eutectic is used to design a mixture whose melting temperature is lower
than these of its pure components. As a result, two solids can liquefy upon mixing.
Although this observation might seem intriguing, it is a quite general phenomenon,
since a compound with an impurity solidifies with more difficulty than the pure
compound. A well-known example of eutectic mixture is the mixture of NaCl and
iced water, freezing below 0°C at atmospheric pressure. Deep eutectics are characterized by a “large” temperature depression, which is obviously a subjective feature,
but can be defined as a temperature depression which is significantly deeper than
that of the ideal liquid mixture (Martins et al. 2019). The gap to ideality generally
results from strong intermolecular interactions between the different components of
the mixture. The different types of interaction will be discussed in more detail
below. There is a single composition, called the “eutectic composition,” where the
pure liquid state reaches the lowest temperature. It is important to mention that
contrary to what is often reported in the literature, the eutectic composition is not
related to the formation of a stoichiometric complex, but rather to the intersection of
two independent melting curves. An emblematic preparation of deep eutectics is the
mixture of hydroxyethyltrimethyl ammonium (i.e., choline chloride) (melting point
T m = 302 °C) with urea (T m = 133 °C), which results in a eutectic of very low melting point (T m = 12 °C) (Abbott et al. 2003). The recent discovery of many room
temperature eutectics and of their ability to solubilize both organic and inorganic
compounds has launched an intense research activity about their uses as solvents,
and the term deep eutectic solvents (DES) is now widely used in this field.
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents