Accelerated shelf-life test of alkamides in Echinacea purpurea root aqueous ethanol Soxhlet extracts
Echinacea species are purported to have anti-inflammatory, anti-viral and other medicinal values. Echinacea alkamides, which are unsaturated fatty acyl isobutylamides/methylbutylamides, are thought to be the characteristic compounds responsible for some of these medicinal values. Echinacea species are rich in anti-oxidant phenolic acids, thus may protect the alkamides from oxidation. Our hypothesis is antioxidant phenolic acids protect degradation of alkamides in Echinacea purpurea . Alkamide stability was evaluated in ethanol extracts of E. purpurea: phenolics-rich and phenolics-poor extracts. The stability of the alkamides in extracts was evaluated in dry form at 70, 80 and 90°C and in DMSO solution at 90, 100, and 110°C. Analytical HPLC with a photodiode array detector was used to measure alkamide concentrations. The relative abundance of the alkamides are as follows: Alkamide 2 > 9 > 8 > 3 > 4 > 1 > 7 > 5 > 10 > 11 (See Figure 2-2 for structures of alkamides). Alkamides in phenolic acid-rich extracts were more stable than in phenolics acid-poor extracts in dry form. However, the alkamide were more stable in phenolics-poor extracts than in phenolics-rich extracts when dissolved in DMSO. In addition, the alkamides, with or without phenolic acids, were more stable in DMSO than in dry form. The degradation of the alkamides followed apparent first order kinetics. The order of the degradation of the alkamides in dry was: 1 ≈ 2 > 9 ≈ 8 > 3 ≈ 4 ≈ 5 ≈ 7 ≈ 10 ≥ 11, which appeared to have followed singlet oxygen oxidation mechanism. The order of the degradation of the alkamides in DMSO was: 9 ≈ 8 > 2 > 1 > 5 ≈ 7 ≈ 4 ≈ 10 ≥ 11 ≈ 3, which appeared to have followed free radical oxidation mechanism. Alkamides are very stable compounds. There is a difference in the energy of activation (Ea) of degradation of the alkamides in dry form and in DMSO. Ea (kJ/K-mol) for alkamides 1, 3, 9 and 8 in phenolics-rich dry form were 83, 74, 66 and 70 respectively and in phenolics-rich DMSO solution were 163, 120, 111 and 120, respectively, suggesting a difference in degradation mechanism. However, the change in the Gibb's free energy of activation (DeltaG ‡) of the alkamides are the same in all treatments except for phenolics-rich DMSO solution, which suggests a similar degradation mechanism of the alkamides in all treatments, except for phenolics-rich DMSO solution. The main difference between the DMSO and the dry form is probably O2 availability.