# Phenolic Compounds in Trees and Shrubs of Central Europe
## 3 Results
### Phenolic Compounds
#### 3.1.3 Tannins  
Tannins are a group of organic chemical compounds, derivatives of phenols, which are naturally produced by plants. Tannins are usually divided into two basic groups: hydrolysable and non-hydrolysable (condensed) tannins [115; 116; 117]. In the centre of the hydrolysable tannin molecule is a monosaccharide (glucose or other polyols, e.g., branched sugar-hamamelose, shikimic, quinic acid, and even pectin), whose hydroxyl groups, partially or completely, are esterified with gallic acid residues, e.g., m-digalus acid. These tannins are easily hydrolysed by weak acids and bases or enzymes to monomeric products. Depending on the type of resulting products, gallotannins and ellagitannins are distinguished. Gallotanins are the simplest  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Flavonoids**} & \multicolumn{2}{c}{**Fir**} & \multicolumn{2}{c}{**Larch**} & \multicolumn{2}{c}{**Pine**} & \multicolumn{2}{c}{**Spruce**} \\ \cline{2-9}  & \multicolumn{2}{c}{**Range**} & \multicolumn{2}{c}{**Range**} & \multicolumn{2}{c}{**Range**} & \multicolumn{2}{c}{**Range**} \\ \hline Apigenin & 0.0 & 0.5 & 0.2 & 0.6 & 0.1 & 0.4 & Nd & 0.2 \\ \hline Katechin & 248.3 & 264.2 & 24.6 & 28.9 & 23.7 & 32.5 & 3.6 & 4.9 \\ \hline Kempfer  
tannins, containing, in their molecule, glucose and ester in association with gallic acid. Another example of such compounds is tannic acid (C\({}_{76}\)H\({}_{52}\)O\({}_{46}\)); see Figures 3 and 4. Proanthocyanidins (PA) (the condensed tannins-non-hydrolysable tannins) are oligomers or polymers of flavonoid structures, mainly flavan-3-ols, (-)--epicatechin and (+)--catechin (Figure 1), flavan-3,4-diols or a mixture of both. In contrast to hydrolysis tannins, they do not contain sugar units. Some authors distinguish an additional third group--catechin tannins--examples of which are the polyphenols found in green tea leaves. They have a characteristic carbon-condensed tannin and do not contain sugar residues. These include monomeric flavan-3-ols, e.g., (+)--catechin, (-)--epicatechin, (+)--gallocatechin and (-)--epigallocatechin or their ester derivatives, e.g., 3-gallates (-) -epicatechin and (-)--epigallocatechin. The latter group of compounds is easily enzymatically hydrolysed under the influence of tannase (4). The flavan-3-ol units are linked mainly through the C4\(\rightarrow\)C8 bond (Figure 4), but the C4\(\rightarrow\)C6 bond also exists (both called B-type). The flavan-3-ol units can also be doubly linked by an additional ether bond at C2\(\rightarrow\)O7 (A-type). The size of PA molecules can be described by their degree of polymerisation (114). Three common flavan-3-ols, which differ in their hydroxylation patterns, are found in PAs. Proanthocyanidins, consisting exclusively of (epi)catechin, are called procyanidins (PCs). Proanthocyanidins, containing (epi) afzelechin or (epi)gallocatechin as subunits, are named propelargonidins (PPs) or prodelphinidins (PDs), respectively. Propelargonidins and PDs are less common in nature than procyanidins [115]. The basic route for the synthesis of all tannins is the pathway associated with sugar catabolism, leading to shikimic acid. Gallic acid is formed from shikimic or quinic acid, which is a substrate in various types of condensation, resulting in the synthesis of tannins [115]. Proanthocyanidins (also called condensed or non-hydrolysable tannins) are also found in leaves, lignified parts of plants, as well as flowers and fruits [118]. Inflorescences of hawthorn (_Crataegi inflorescentia_, _Crataegus_ sp. _Rosaceae_) are well-known sources of proanthocyanidin that have been used in herbal medicine for years [119; 120]. Recently, intensive studies have been conducted on an extract from the bark of maritime pine _Pinus pinaster (Pinaceae)_, patented as Pycnogenol, whose proanthocyanidin content is 85%. This preparation exhibits, e.g., strong antioxidant properties (Table 9) [121; 122; 123]. Figure 3: Tannic acid.