Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/22/2006
Publication Date: 9/26/2006
Citation: Fish, W.W. 2006. Interaction of sodium dodecyl sulfate with watermelon chromoplasts and examination of the organization of lycopene within the chromoplasts. Journal of Agricultural and Food Chemistry. 54(21):8294-8300. Interpretive Summary: Lycopene, the carotenoid that imparts the red color to watermelons and tomatoes, is a strong antioxidant. Epidemiological studies suggest positive health benefits are to be derived from the consumption of diets high in lycopene. Red-fleshed watermelon is a rich source of lycopene, and plant breeders continue to strive to increase the levels of lycopene in the cultivars that they develop. This research demonstrates that lycopene is stored in watermelon in large membrane-surrounded aggregates, termed chromoplasts. The lycopene inside the chromoplast is aggregated in a head-to-tail fashion to form optically asymmetrical aggregates. The detergent, sodium dodecyl sulfate, binds to the chromoplast membrane and increases its solubility in water. This behavior was exploited to develop a simple, safe, environmentally friendly assay for lycopene in watermelon.
Technical Abstract: The properties of plant-derived precipitates of watermelon lycopene were examined in aqueous sodium dodecyl sulfate (SDS) as part of an ongoing effort to develop simpler, more economical ways to quantify carotenoids in melon fruit. Levels of SDS >0.2% were found to increase the water solubility of lycopene in the state in which it was isolated from watermelon. Electron microscopy and chemical analyses suggested that the watermelon lycopene as isolated is packaged inside a membrane to form a chromoplast. Spectral peaks in the visible region of the watermelon chromoplasts in SDS exhibited a bathochromic shift from those in organic solvent. Watermelon chromoplasts in SDS exhibited pronounced circular dichroic activity in the visible region. Binding measurements indicated that about 120 molecules of SDS were bound per molecule of lycopene inside the chromoplast; likely the detergent molecules are bound to the chromoplast membrane. Around 80% of the chromoplast-SDS complexes were retained on a 0.45 'm membrane filter. Together, these observations are consistent with lycopene in a J-type chiral arrangement inside a membrane to form a chromoplast. The binding of SDS molecules to the chromoplast membrane form a complex that is extensively more water-soluble than the chromoplast alone.