Biotransformation of Underutilized Natural Resource to Valuable Compounds in Ionic Liquid: Enzymatic Synthesis of Caffeic Acid Phenethyl Ester Analogues from Immature Coffee Beans

Abstract

Caffeic acid esters are widely distributed in plants and propolis (Tagashira & Ohtake, 1998; Ysrael & Nonato, 1999). Caffeic acid phenethyl ester (CAPE) especially has been found in propolis and has a broad spectrum of biological activities, including antimicrobial, antiinflammatory, antioxidant, and antitumor activities (Bankova, 2005); it also has an inhibitory effect on HIV-1 integrase, cyclooxygenase, and lipooxygenase (Fesen et al., 1993; Michaluart et al., 1999; Nicklaus et al., 1997; Sud'ina et al., 1993). It has been reported that the ester part of CAPE is important for its antiproliferative effect on various human tumor cells (Nagaoka et al., 2003). Additionally, it was suggested that conversion of the phenyl group to a cyclohexyl group in a CAPE analogue enhanced the antiproliferative effect (Kadota & Tezuka, 2004). Immature green coffee beans are not marketed as coffee because contamination of these beans negatively affects the flavor. However, they contain appreciable amounts of various caffeoylquinic acids, for example, 4.8–5.8 g of 5-caffeoylquinic acid/100 g of immature green coffee beans (Kishimoto et al., 2005a). These immature beans are notable among unused agricultural resources, and we are therefore currently investigating the enzymatic conversion of their caffeoylquinic acids to valuable products. In recent work, we synthesized CAPE using 5-caffeoylquinic acid and 2-phenylethanol as substrates with chlorogenate hydrolase by a transesterification reaction in a biphasic aqueous-alcohol state and elucidated the antibacterial, antimutagenic, and anti-influenza virus activities of CAPE (Kishimoto et al., 2005b). The procedure using chlorogenate hydrolase provided various CAPE analogues, but the maximum conversion yield of CAPE was 50%. The insufficient yield was probably due to the hydrolysis of 5-caffeoylquinic acid by the enzyme to caffeic acid in the aqueous phase. Therefore, a new procedure for the synthesis of CAPE analogues superior to that method in terms of the conversion yield remained to be developed. Ionic liquids (ILs), which are composed of a bulky asymmetric cation and a small anion, are easily modified with respect to the combination of cation and anion, and therefore,