Free Radicals and Antioxidant Protocols
In recent years there has been a growing demand to achieve more rapid and accurate detection of RONs (reactive oxygen and nitrogen species), which affect normal physiological processes and pathological conditions, and also of redox stress. In Free Radical
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1. Introduction Vascular plants synthesize a diversified range of organic molecules – phytochemicals, referred to as “secondary metabolites.” Phenolic compounds are considered as secondary metabolites that are synthesized by plants during normal development (1) and under stress conditions such as infection, wounding, and UV radiation, among others (2). The term “phenolic compounds” or “phenolics” encompasses approximately 8,000 naturally occurring compounds, which share in common a phenol, i.e., an aromatic ring bearing at least one hydroxyl substituent. R.M. Uppu et al. (eds.), Free Radicals and Antioxidant Protocols, Methods in Molecular Biology 610, DOI 10.1007/978-1-60327-029-8 5, © Humana Press, a part of Springer Science+Business Media, LLC 1998, 2010
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A major class of phenolic compounds is phenolic acids. The name “phenolic acids,” in general, describes phenols that possess at least one carboxylic-acid functionality. However, when describing plant metabolites, it refers to a distinct group of organic acids which contain two distinguishing carbon frameworks: the hydroxybenzoic and the hydroxycinnamic structures (Fig. 5.1). Noteworthy is also the case of chlorogenic acid (5-O-caffeoylquinic acid) which is the caffeic acid esterified with quinic acid (Fig. 5.2). H
H
R1
COOH
R1
R2
H
R2
COOH
H
R3
R3
(I)
(II)
Fig. 5.1. General structures of naturally occurring phenolic acids. (I) Hydroxybenzoic and (II) hydroxycinnamic structures. R1 , R2 , and R3 denote one of the following atoms and chemical groups: H, OH, OCH3 .
O O COOH HO
OH
OH OH
OH
Fig. 5.2. Structure of chlorogenic acid.
Flavonoids belong to polyphenols and are usually glycosylated. They are planar molecules and their structural variation comes, in part, from the pattern of substitution: hydroxylation, methoxylation, prenylation, or glycosylation. Flavonoids have two aromatic rings enclosing a heterocyclic six-membered ring with oxygen (Fig. 5.3). They can be classified as anthocyanins, flavanols (catechins), flavones, flavanones, and flavonols. Isoflavones are structurally isomeric to flavonoids and along with
Occurrence and Analysis of Phenolic Compounds
3'
OH
2' HO
8
1 O
A
C
1' 2
B 5'
HO
8
1 O
A
C
6
2 2' 1'
6'
B OH
6
O
6'
R OH
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5'
3' OH
O
(I)
(II)
Fig. 5.3. Basic structure of (I) flavonoids (R = OH: flavonols, R = H: flavones) and (II) isoflavones.
their metabolites they have structures similar to the mammalian estradiol (3). The rapid and systematic measurement of phenolic acids and flavonoids is a serious challenge for analytical and food chemists, phytochemists, and biochemists because of the inherent structural diversity of the compounds, the dietary and health impact (4). This chapter focuses on the occurrence of phenolic acids and flavonoids in nature and their role in human health but most emphasis is placed on the different methodologies and techniques used in their analysis in a diversity of matrices. Steps in the sample treatment process are examined and chromatographic a
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