Research Advances: Spinach Nutrient Levels; Steering Oil Droplets

Chemical Education Today

Research Advances: Spinach Nutrient Levels; Steering Oil Droplets; Possible Explanation of Left-Handed Preference? by Angela G. King Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109 [email protected]

Supermarket Lighting Enhances Nutrient Level of Fresh Spinach Far from being a food spoiler, the fluorescent lighting in supermarkets can actually boost the nutritional value of fresh spinach, scientists are reporting. The finding could lead to improved ways of preserving and enhancing the nutritional value of spinach and perhaps other veggies, scientists suggest in a recent study. Gene Lester, Donald J. Makus, and D. Mark Hodges, who have studied many aspects of spinach production and nutrient content (1), note that fresh spinach is a nutritional powerhouse, packed with vitamin C, vitamin E, folate (a B vitamin), and healthful carotenoid antioxidants. Supermarkets often display fresh spinach in clear plastic containers at around 39 °F in showcases that may be exposed to fluorescent light 24 h a day (Figure 1). Lester, Makus, and Hodges wondered how this continuous light exposure might affect spinach's nutritional value. The scientists exposed fresh spinach leaves to continuous light or darkness during simulated retail storage conditions for 3-9 days and measured nutrient levels with a combination of HPLC and spectrophotometry. Leaves were classified according to age, with top-canopy leaves being the youngest and bottomcanopy leaves being the oldest. All types of spinach leaves (top-, mid-, and bottom-canopy) stored in light for as little as 3 days had significantly higher levels of vitamins C, K, E, and folate (2). For example, after 9 days of continuous light exposure, folate levels increased between 84-100%. Levels of vitamin K increased between 50-100%, depending on the spinach variety tested. The stored spinach also had higher levels of the healthful carotenoids lutein and zeaxanthin (Figure 2). By contrast, say the scientists, spinach leaves stored under continuous darkness tended to have declining or unchanged levels of nutrients. Lester's team noted in their study that, although the leaves stored under light were more nutrient-dense than those stored in darkness, they also had increased wilting. While the increase in nutritional content would not be visually apparent to the consumer, the wilting might affect consumer willingness to buy. However, wilting varied between spinach cultivars and leaf maturity. Focusing on crinkle-leafed cultivars (such as “Samish”) and baby-leafed size might let the product be stored under continuous light to maximize nutrient content while minimizing wilting. Lester also recently published a study on the effect of irradiation of spinach nutrient levels (3). Because of student familiarity with spinach, it has previously been used in chemistry labs to introduce column chromatography and thin-layer chromatography, and also as the basis of a 766

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Figure 1. Spinach on display under 24-h light in supermarkets actually gains in content of some nutrients. Photo courtesy of Marc Villalobos, USDA-ARS.

stoichiometry learning activity regarding the bioavailability of calcium and iron (4-7). Oil Droplets Can Navigate Complex Maze Call them oil droplets with a brain or even “chemo-rats”. Scientists in Illinois have developed a way to make simple oil droplets “smart” enough to navigate through a complex maze almost like a trained lab rat (8). The finding could have a wide range of practical implications, including helping cancer drugs reach their target and controlling the movement of futuristic nanomachines, the scientists say. Bartosz Grzybowski and colleagues at Northwestern University note that the ability to solve a maze is a common scientific test of intelligence. Animals ranging from rats to humans can master the task. Scientists would like to pass along that same ability to anticancer drugs, for instance, to help these medications navigate complex mazes of blood vessels and reach tumors.

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Vol. 87 No. 8 August 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100562u Published on Web 06/14/2010


Figure 2. Top-canopy leaves held in continuous light (0) or darkness (9), mid-canopy leaves held in continuous light (4) or darkness (2), and bottomcanopy leaves held in continuous light (O) or darkness (b). Values are means (standard deviation of three field plots each determined as a mean of three replications). Reproduced with permission from ref 2. Copyright 2010 American Chemical Society.

The scientists developed postage-stamp-sized mazes and then powered oil droplets through the mazes with a combination of acid-base chemistry and surface-tension effects (Figure 3). The mazes were constructed with photolithography and contained channels 1.4 mm wide and filled with an alkaline solution and a surfactant to lower surface tension. The scientists then placed a gel containing a strong acid at the exit; this created a pH gradient. Nonpolar droplets, dyed to increase optical contrast and containing a weak acid such as 2-hexyldecanoic acid (HDA), were placed at the entrance of the mazes, giving rise to convective flows that propelled the droplets along the pH gradient toward the exit. Both the HDA and pH gradient were crucial to the droplet's “chemotaxis”. Because cancer cells are more acidic than other body cells, the experiment may serve as a model for designing new anticancer drugs that move along similar acid-base gradients to target diseased cells, the scientists suggest. In addition to this work, Grzybowski's research group focuses on understanding molecular self-assembly (SA) and selforganization (SO) and practical applications of SA and SO.

r 2010 American Chemical Society and Division of Chemical Education, Inc.

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Figure 3. Simple oil droplets (in red) can navigate a complex maze using a special chemical approach that could lead to improved delivery of anticancer drugs. Reproduced with permission from ref 8. Copyright 2010 American Chemical Society.

More information, including a video of the oil droplet navigating the maze, can be found on the research group's Web site (9). Educators interested in incorporating SA into their classes will find classroom activities modeling SA with macaroni and

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Figure 5. Molecules of L-aspartic acid in both crystal form (top) and as a computer image (bottom) could be the “ancestral Eve” of all amino acids;the building blocks of proteins;in life on Earth. Reproduced with permission from ref 14. Copyright 2010 American Chemical Society.

Figure 4. Crystals and crystallization of chiral molecules. Adapted from ref 14 and used with permission. Copyright 2010 American Chemical Society.

drinking straws helpful (10, 11). Additionally, this Journal has published undergraduate experiments involving either the synthesis or analysis of SA systems (12, 13). “Ancestral Eve” Crystal May Explain Origin of Life's LeftHandedness Scientists are reporting the discovery of what may be the “ancestral Eve” crystal that billions of years ago gave life on Earth its curious and exclusive preference for L-amino acids. Their study may help resolve one of the most perplexing mysteries about the origin of life. Tu Lee and Yu Kun Lin point out that conditions on the primordial Earth held an equal chance of forming the same amounts of L- and D-amino acids. Nevertheless, when the first forms of life emerged more than 3 billion years ago, all the amino acids in proteins had the L configuration. That pattern continued right up to modern plants and animals. The scientists used a solubility test, solution freezing point, and crystallization kinetics in laboratory experiments to see how temperature and other conditions affected formation of crystals grown from racemic aspartic acid solutions (14). In particular, they considered whether the liquid structure of racemic aspartic acid solution affects whether the racemic solution crystallizes in a racemic (a) or conglomerate (b) manner (Figure 4). Crystals (Figure 5) were analyzed with FT-IR and powder X-ray diffraction. Lee and Lin found that, under conditions that could have existed on the primitive Earth, left-handed aspartic acid crystals could have formed easily and on a large scale. “The [lefthanded] aspartic acid crystal would then truly become a single 768


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mother crystal: an ancestral Eve for the whole left-handed population”, the article notes. This Journal has previously published accounts of historical work on conglomerate crystallization (15, 16). Additional work in this area is described on Lee's Web site (17). Literature Cited 1. USDA ARS Research Project Home Page on Enhancement of Postharvest Quality of Fruits and Vegetables and Evaluation of Commodity Treatments of Quarantined Pests. http://www.ars. usda.gov/research/projects/projects.htm?ACCN_NO=418376 (accessed Jun 2010). 2. Lester, G.; Makus, D.; Hodges, D. Relationship between FreshPackaged Spinach Leaves Exposed to Continuous Light or Dark and Bioactive Contents: Effects of Cultivar, Leaf Size, and Storage Duration. J. Agric. Food Chem. 2010, 58, 2980–2987. 3. Lester, G..; Hallman, G.; Perez, J. γ-Irradiation Dose: Effects on Baby-Leaf Spinach Ascorbic Acid, Carotenoids, Folate, γ-Tocopherol, and Phylloquinone Concentrations. J. Agric. Food Chem. 2010, 58, 4901–4906. 4. Mewaldt, W.; Rodolph, D.; Sady, M. An Inexpensive and Quick Method for Demonstrating Column Chromatography of Plant Pigments of Spinach Extract. J. Chem. Educ. 1985, 62, 530. 5. Cousins, R.; Pierson, K. A Simplified Method for the Microscale Extraction of Pigments from Spinach. J. Chem. Educ. 1998, 75, 1268–1269. 6. Quach, H.; Steeper, R.; Griffin, G. An Improved Method for the Extraction and Thin-Layer Chromatography of Chlorophyll a and b from Spinach. J. Chem. Educ. 2004, 81, 385–387. 7. Walker, N. Oxalate Blockage of Calcium and Iron: A Student Learning Activity. J. Chem. Educ. 1988, 65, 533. 8. Lagzi, I.; Soh, S.; Wesson, P.; Browne, K.; Grzybowski, B. Maze Solving by Chemotactic Droplets. J. Am. Chem. Soc. 2010, 132, 1198–1199. 9. The Grzybowski Group: Self Assembly and Adaptive Systems. http://dysa.northwestern.edu/ (accessed Jun 2010).


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10. Burgan, D.; Baker, L. Investigating Self-Assembly with Macaroni. J. Chem. Educ. 2009, 86, 704A–704B. 11. Campbell, D.; Freidinger, E.; Hastings, J.; Quern, M. Spontaneous Assembly of Soda Straws. J. Chem. Educ. 2002, 79, 201–202. 12. Hof, F.; Palmer, L.; Rebek, J., Jr. Synthesis and Self-Assembly of the “Tennis Ball” Dimer and Subsequent Encapsulation of Methane. An Advanced Organic Chemistry Laboratory Experiment. J. Chem. Educ. 2001, 78, 1519–1521. 13. Maye, M.; Luo, J.; Han, L.; Zhong, C-J. Chemical Analysis Using Scanning Force Microscopy. An Undergraduate Laboratory Experiment. J. Chem. Educ. 2002, 79, 207–210.


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14. Lee, T.; Lin, K. The Origin of Life and the Crystallization of Aspartic Acid in Water. Cryst. Growth Des. 2010, 10, 1652– 1660. 15. Bernal, I. Conglomerate Crystallization and Chiral Discrimination Phenomena: In Vivo and In Vitro. J. Chem. Educ. 1992, 69, 468–469. 16. Suh, I.-H.; Park, K.; Jensen, W. P.; Lewis, D. E. Molecules, Crystals, and Chirality. J. Chem. Educ. 1997, 74, 800–805. 17. Tu Lee's Bio-Inspired Materials and High-Throughput Materials Screening Lab Web Site. http://www.ncu.edu.tw/~che/faculty/ Lee.htm (accessed Jun 2010).


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Research Advances: Spinach Nutrient Levels; Steering Oil Droplets

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