Active food packaging has become attractive because of
the possibility
to provide a longer shelf-life by loading functional agents into the
packages to maintain the quality of food products. Herein, photoluminescent
and transparent polyvinyl alcohol (PVA)-based composites embedding
multicolor fluorescent carbon dots (CD/PVA) were prepared by the solvent
casting method. The prepared CDs emit a strong and stable fluorescence
in solution while the CD/PVA composite films were transparent, flexible,
and showed UV-blocking activity with a strong fluorescence emission.
Blue color-emitting CDs showed the highest UV blockage at UVA (87.04%),
UVB (87.04%), and UVC (92.22%) regions while PVA alone absorbed only
less than 25% of the light in all UV regions. UV blockage capacity
was shown to be decreased by half, in line with the emission color
shift from blue to red. Thermal properties of the PVA film were improved
by the addition of CDs to the polymer, and in vitro cell viability
tests showed that none of the CDs were cytotoxic against the human
lung fibroblast healthy cell line (MRC-F cells) when integrated into
the PVA. The antimicrobial activity of CD/PVA nanofilms was qualitatively
determined. The prepared films exhibited good antimicrobial activity
against both Gram-positive and Gram-negative bacteria with mild antioxidant
and metal chelating activity, and significant inhibition of biofilm
formation with a strong link with emitted color and the concentration
of the composites. Green- and red-emitting CD/PVA with the highest
antimicrobial activity were then analyzed and compared with the plane
PVA employing their effect on the shelf-life of strawberries as a
model for perishable foods. Fresh strawberries dip coated with CD/PVA
and PVA were monitored over time, and virtual evaluations showed that
CDs/PVA film coating resulted in reduced weight and moisture loss
and significantly inhibited the fungal growth and spoiling for over
6 days at RT and 12 days at fridge conditions maintaining the visual
appearance and natural color of the fruit. The findings in this work
indicated the potential of reported CD as non-cytotoxic, UV-blocking
antimicrobial additives for the development of edible coatings and
packages for their use in the food industry, as well as pharmaceutical
and healthcare applications.
Microbial biotransformation is an important tool in drug discovery and for metabolism studies. To expand our bioactive natural product library via modification and to identify possible mammalian metabolites, a cytotoxic cardenolide (gitoxigenin) was biotransformed using the endophytic fungus Alternaria eureka 1E1BL1. Initially, oleandrin was isolated from the dried leaves of Nerium oleander L. and subjected to an acid-catalysed hydrolysis to obtain the substrate gitoxigenin (yield; ~25%). After 21 days of incubation, five new cardenolides 1, 3, 4, 6, and 8 and three previously- identified compounds 2, 5 and 7 were isolated using chromatographic methods. Structural elucidations were accomplished through 1D/2D NMR, HR-ESI-MS and FT-IR analysis. A. eureka catalyzed oxygenation, oxidation, epimerization and dimethyl acetal formation reactions on the substrate. Cytotoxicity of the metabolites were evaluated using MTT cell viability method, whereas doxorubicin and oleandrin were used as positive controls. Biotransformation products displayed less cytotoxicity than the substrate. The new metabolite 8 exhibited the highest activity with IC50 values of 8.25, 1.95 and 3.4 µM against A549, PANC-1 and MIA PaCa-2 cells, respectively, without causing toxicity on healthy cell lines (MRC-5 and HEK-293) up to concentration of 10 µM. Our results suggest that A. eureka is an effective biocatalyst for modifying cardenolide-type secondary metabolites.
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