Ellagitannin Oligomers from Eucalyptus camaldulensis Leaves and Their Role in the Detoxification of Aluminum

Abstract

Eucalyptus camaldulensis of the Myrtaceae family shows high resistance to aluminum (Al) ions and contains various compounds such as steroids, terpenoids, saponins, flavonoids, glycosides, alkaloids, and tannins. Although the ellagitannin oenothein B (12) isolated from E. camaldulensis exhibits remarkable properties for Al detoxification, likely contributing to its Al resistance, other ellagitannin oligomers present in E. camaldulensis have not been investigated in detail. In this study, novel dimeric and trimeric ellagitannin oligomers eucarpanin D₂ (1) and eucamalin A (2), together with known gallotannins (7, 8, and 10), monomeric ellagitannins (4-6, and 11), and dimeric ellagitannins (3, 9, and 12-14), were isolated from E. camaldulensis leaves. The structures of these novel compounds were elucidated based on their chemical and physicochemical properties, including the orientations of tergalloyl groups in compounds 1 and 2. Similar to compound 12, previously isolated from the roots of E. camaldulensis, the ellagitannins demonstrated good Al detoxification properties. Hence, these tannins may play a critical role in the high Al resistance of E. camaldulensis in acidic soils. This paper reports for the first time the isolation of ellagitannin oligomers from the leaves of E. camaldulensis.

Keywords: Al detoxification; Eucalyptus camaldulensis Dehnh.; ellagitannin oligomer; eucamalin A; eucarpanin D2; eurobustin C.

Figure 1

Chemical structures of gallotannins and ellagitannins isolated from E. camaldulensis leaves (compounds 1–14).

Figure 2

Smiles-type rearrangement, partial hydrolysis, and methylation of compound 1 followed by methanolysis.

Figure 3

Smiles-type rearrangement and partial hydrolysis of compound 2.

Figure 4

Partial hydrolysis and methylation of compound 3 followed by methanolysis.

Figure 5

Al detoxification properties of the extracts in the liquid–liquid distributions. Roots of A. thaliana were exposed to 500 mL of a 2% strength modified Molecular Genetics Research Laboratory (MGRL) medium (pH 5.0) containing 0 or 12 μM AlCl₃ and 3 mg of the extracts for 24 h. The relative root elongation is expressed as the percentage of the root elongation in the control sample (0 μM Al without extract). The values and error bars are expressed as means ± SE (n = 20).

Figure 6

Al detoxification properties of ellagitannins (compounds 1, 2, 9, 12, 13 and 14) isolated from E. camaldulensis. Roots of A. thaliana were exposed to a 2% strength modified MGRL medium (pH 5.0) containing 0 or 12 μM AlCl₃ and 0, 8, or 16 μM pyrogallol-equivalent ellagitannin for 24 h. The relative root elongation is expressed as the percentage of the root elongation in the control sample (0 μM Al, 0 μM ellagitannin). The values and error bars are expressed as means ± SE (n = 20). The bars marked with the same letter do not differ significantly at p < 0.05 (Tukey-Kramer test).

Figure 7

Effects of ellagitannins (compounds 1, 2, 9, 12, 13 and 14) on the root elongation. Roots of A. thaliana were exposed to 0 or 16 μM pyrogallol-equivalent ellagitannin in a 2% strength modified MGRL medium (pH 5.0) for 24 h. The relative root elongation is expressed as the percentage of the root elongation in the control sample (0 μM ellagitannin). The values and error bars are expressed as means ± SE (n = 20). The asterisks indicate a significant difference compared with the control at p < 0.001 (Student’s t-test).