**3.2 Esters with aliphatic acids**

Biological activities of aliphatic acids are not of a big importance in comparison with aromatic acids. These compounds are mainly accepted as energy storage and components of several compartments of cells, such as membranes, enzymes, surfactants, etc. In the literature, more studies can be found describing changes in biological activities of flavonoids after their acylation with aliphatic acids.

The aliphatic acylation of anthocyanins with malonic acid is important for enhancing the pigment solubility in water, protecting glycosides from enzymatic degradation and stabilizing anthocyanin structures (Nakayama et al., 2003). Several *in vitro* observations suggest that acylation with malonic acid or sinapic acid is crucial for efficient flavonoid accumulation in plants.

Fatty acid esters of catechins were reported to display antitumor, antibacterial and 5-α reductase inhibiting activity (Fukami et al., 2007) as well as antioxidant properties (Sakai et al., 1994). Lee et al. (2003) reported anti-atherogenic activity of two naringenin derivatives, 7-*O*-oleic ester and 7-*O*-cetyl ether.

Acylation of the flavonoid molecule with polyunsaturated fatty acids introduces potential antitumor and antiangiogenic properties (Mellou et al., 2006). Anticarcinogenic effects were observed also in silybin esters acylated with butyric and lauric acid (Xanthakis et al., 2010). Recently, we found that acylation of rutin with unsaturated fatty acids, such as oleic, αlinoleic and linolenic, increased the antioxidant potential of the initial compound (Viskupicova et al., 2010). This observation is in accordance with the results of Mellou et al. (2006) and Katsoura et al. (2006).

In the field of fatty acid ester synthesis, information on the photoprotective effectiveness of new quercetin derivatives acylated with acetic, propionic and palmitic acids, has been reported. The authors found that esterification with a short side-chain (such as acetate or propionate) may improve migration through the aqueous environment and interaction with or penetration into phospholipid membranes (Saija et al., 2003).

Recent experimental findings indicate that acylation of flavonoid may increase enzyme inhibitory activity. Lin et al. (2010) observed increased 5α-reductase inhibition after acylation of (-)-epigallocatechins. Salem et al. (2011) showed that the acylation of isorhamnetin-3-*O*-glucoside with different aliphatic acids enhanced its capacity to inhibit xanthine oxidase. Our recent investigations showed that lipophilic rutin and naringin esters were strong inhibitors of transport enzymes such as sarcoplasmic reticulum Ca2+- ATPase and plasma membrane Ca2+-ATPase (Augustyniak et al., 2010; Viskupicova et al., 2009), and thus might be useful in calcium regulation. We presume that there might be a general mechanism involved in the enhanced inhibitory activity of the acylated flavonoids on structurally diverse classes of enzymes which seems to be donated by the medium to long fatty acid chains.
