Carotenoids in mature (green) and senescing leaves of invasive alien plant species
Japanese (Fallopia japonica Houtt) and Bohemian (Fallopia x bohemica) knotweed were
identified and quantified. Both plants showed similar pigment profiles.The amount of free
xanthophylls and carotenes was significantly lower in senescing leaves in comparison to
green leaves. On the other hand new xanthophyll compounds appeared in senescing
leaves – xanthophyll fatty acid esters. Particularly green leaves of both studied knotweeds
represent a rich and sustainable natural source of bioactive carotenoids, mainly β-carotene
(69–97 mg/100g D.W.) and xanthophyll lutein (97–144 mg/100g D.W.), thus exploitation
of these invaders can be used for the production of high added-value products. However,
due to carotenoids chemical structure, they are very unstable outside of the plant matrix
and they degrade fast in the presence of elevated temperature, light, oxidants, acids and
metals. We found an inspiration to solve this problem in nature itself, where in autumn
xanthophylls are converted into more chemically stable xanthophyll esters. To this end,
we developed the first environmentally friendly and economically viable synthetic
platform, where overall green, renewable, GRAS and recyclable materials were used. The
synthesis was carried out in β-pinene as the reaction solvent and at ambient conditions
(room temperaure, Atmospheric pressure), where 55 different xanthophyll esters
(combining 5 model xanthophylls with 11 structurally and electronically distinct acid
anhydrides) were prepared. We have shown that esterification can be performed also
without use of a solvent and by exploitation of a waste plant material, e.g. extract of green
leaves of Japanese knotweed or avocado peels, which represent rich renewable source of
lutein and anteraxanthin, respectively. The synthesized compounds were purified and
characterized by means of HPLC-PDA-MS and offline HRMS. Out of 55 compounds, 22
were successfully synthesized for the first time. The efficiency of our approach to achieve
higher chemical stability of xanthophylls via esterification was tested on 13 different
lutein diesters, which were synthesized either from pure lutein or from green leaves
extract of Japanese knotweed. Each individual lutein diester was separately examined at
each individual stress conditions: elevated temperature (60 °C), light (UV-A), oxidant
(H2O2) and acidic environment. Based on systematic stability studies, we were first to
identify a relationship between the chemical stability, chemical structure and the
environment by which lutein esters were surrounded. Of all the prepared lutein esters,
lutein di(2,2-dimethylpropanoate), lutein di(2-methylpropanoate) and lutein di(3-
methylbutanoate) proved to be the most stable. They showed to be 1,4–20 times more
stable than free lutein, regardless of the chosen stress condition. Depending on the stress
condition the plant matrix can strongly (positively or negatively) affect the stability of an
individual xanthophyll. The green leaves of Japanese knotweed, therefore, represent a
suitable plant waste material that enables the preparation of potential high value-added
products – xanthophyll esters with increased chemical stability.
|
