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Chemistry and Biochemistry of Dietary Polyphenols

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Chemistry and Biochemistry of Dietary Polyphenols ( chemistry-and-biochemistry-dietary-polyphenols )

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Nutrients 2010, 2 1232 has been somewhat confusing and its implied chemical structures are often vague even to researchers. Studies have also shown that different polyphenol subgroups may differ significantly in stability, bioavailability and physiological functions related to human health. This review intends to clarify some confusion related to the classification of polyphenols, to identify problems and examine methods in the characterization of polyphenols. Discussions about other aspects of polyphenol chemistry and biochemistry, i.e., biosynthesis in plants, and possible biological roles and implications to human health, can help establish new directions for future research. 2. Classification of Polyphenols Dietary phenolics or polyphenols constitute one of the most numerous and widely distributed groups of natural products in the plant kingdom. More than 8000 phenolic structures are currently known, and among them over 4000 flavonoids have been identified [5-7]. Although polyphenols are chemically characterized as compounds with phenolic structural features, this group of natural products is highly diverse and contains several sub-groups of phenolic compounds. Fruits, vegetables, whole grains and other types of foods and beverages such as tea, chocolate and wine are rich sources of polyphenols. The diversity and wide distribution of polyphenols in plants have led to different ways of categorizing these naturally occurring compounds. Polyphenols have been classified by their source of origin, biological function, and chemical structure. Also, the majority of polyphenols in plants exist as glycosides with different sugar units and acylated sugars at different positions of the polyphenol skeletons. To simplify the discussions, classification of polyphenols in this review will be done according to the chemical structures of the aglycones. 2.1. Phenolic Acids Phenolic acids are non-flavonoid polyphenolic compounds which can be further divided into two main types, benzoic acid and cinnamic acid derivatives based on C1–C6 and C3–C6 backbones (Figure1). While fruits and vegetables contain many free phenolic acids, in grains and seeds—particularly in the bran or hull—phenolic acids are often in the bound form [8-10]. These phenolic acids can only be freed or hydrolyzed upon acid or alkaline hydrolysis, or by enzymes. Figure 1. Typical phenolic acids in food: Left, Benzoic acids; right, Cinnamic acids. OO HO R O RO HO R O O OH Protocatechuic acid, R = H; Vanillic acid, R = OCH3 HO OH HO p-coumaric acid HO OR Caffeic acid, R = H; Chlorogenic acid, R = 5-quinoyl; Cryptochlorogenic acid, R = 4-quinoyl; Neochlorogenic acid, R = 3-quinoyl; O OH3COO HO Syringic acid, R = OCH3 H3CO OH HO ferulic acid H3CO OH sinapic acid Gallic acid, R = H; Benzoic acids Cinnamic acids OH

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