Tomato Juice Rainbow A Colorful and Instructive Demonstration Marie E. MacBeath and Andrew L. Richardson University of New Brunswick, Fredericton. NB. Canada E3B 6E2 The red color of tomatoes is mainly due to the carotenoid lycopene (1).
1 When saturated bromine water is gently stirred into canned tomato juice as shown in Figure 1, color changes occur, and the concentration gradient created causes the rainbow effect depicted in the figure and in full color on this month's cover.' The reaction takes place in less than a minute and the colors persist for hours, making this demonstration fairly dramatic yet safe and easy to do. The probable reasons for the results turn out to he nontrivial and, indeed, may give important new insights into reaction mechanisms in biological membranes. In satisfying the natural desire for an explanation for the unusual phenomenon, the instructor is afforded an excellent opportunity to focus attention on several important concepts: (1) the effect of the length of a conjugated system on the wavelength of the absorption maximum; (2) the manner in which bromine attacks and adds at a double
bond ,?I the existence of charge-tranrfer cumplexcr; II1 t h r rfl'wt uf the envrnmnent, specifically lipid aggregates, on reaction rated and merhanisms.
colors seen in the tomato juice rainbow phenomenon, it is necessary to look into the manner in which bromine adds a t a double bond. The generally accepted mechanism for bromination is shown below ( I ) .
CT complex
Br
hrominium ion First, the bromine sets up a charge-transfer (CT) complex with the olefin, which then dissociates into the bromonium and bromide ions. Attack of the bromide ion occurs trans, leading to the elimination of the double bond (2). These addition reactions are catalyzed by polar substances, light, and glass surfaces (3). CT interactions occur when a complex forms between a molecule which tends to donate an electron, in our case the olefin, and an acceptor molecule like bromine. When such a donor-acceptor complex absorbs light, it goes into an excited state where a T electron is transferred from the donor to the acceptor. The appearance of a new band a t longer wave-
I t is the length of the conjugated system in lycopene (1) and B-carotene (2) CH3
saturated brwnine wate CH,
2 that causes their absorption in the blue region of the visible spectrum (see Fig. 2) and gives them their characteristic red and orange colors. If a double bond is attacked and removed by an agent like bromine, the length of conjugation shortens, the wavelength becomes smaller, and the absorption shifts to the ultraviolet region of the spectrum causing the compound to become colorless. This indeed is what happens when bromine is added to a solution of lycopene in most solvents and is illustrated in the first demonstration suggested below. T o understand the unexpected blue and green
,unreacted red (tomato juice)
< r'The "rainbow" effect was discovered accidentally during the course of a workshop conducted by D. Bunbury of St. Francis Xavier University. 1092
Journal of Chemical Education
Figure
1. Saturated bromine water, produces a colorful "rainbow" which
7 />
when gently stirred into tomato juice. persists for hours.
shows the concentration to he about 6 X 10W M at room temperature. 6-Carotene-100% crystalline, synthetically prepared, obtained from Sigma. Experimental Bromination of Carotenoids in Methylene Chloride Place approximately equal volumes of CHZCIZand tomato juice into a test tube. stir well. and shake the CH9Clrto the bottom of the tuhe:l.he lgropme and &hermrotrnoidi & i r l c m intn rhe CII.(.I? layr giving i r n reddish eolnr. \ Y h m n few drops of a saturared xiueous hnmlne solurim are added to rhrs layr, fast color change.; are observed before it becomes colorless.
Wavelength in Figure 2. Molecular extinction curves hexane. All trans forms.
of
lycopene
nm (-)
and carotene
(- - - - -) in
length than that of the donor is characteristic of such absorotions (4). Because the e n t h a l ~ vof comolex formation is on& a fewkilocalories per mole infree solition, the rates of formation and decomoosition a t normal t e m ~ e r a t u r e sare high and the absorptibn hands of the components are also present. However, when molecules are immobilized and held in close proximity, as they are in lipid aggregates, the rate of decomposition of the complexes formed would be greatly reduced (4). This accounts for the overwhelming predominance of the C T absorption band seen in this experiment. To exolain the Dresence of the rainbow of colors in the bromination of tomato juice, the reaction was postulated to he takine d a c e in the lioids of the olant membranes of the crushed &atoes which>ompose thk thick juice. T o test this theorv. the artificial rainbow exoeriment. outlined below. was devised. I n it, the hromination is perfo&ed in amicellu: lar medium that simulates the vestiges - of bilaver memhranes present in tomato juice. Membranes (Fig. 3) and micelles (Fig. 4) are lipid aggregates made up of molecules that have long nonpolar hydrophobic alkane chains and polar hydrophilic headgroups. It is enthalpic considerations that explain why the long-chain carotenoids, whose structures are similar to the lipid molecules. transfer readilv from the aaueous medium into micelles or natural membranes (5).I n the case of the partitioning of bromine into the nonpolar micelles, thermodynamic contributions responsible for the lowering of free energy are smaller and entropic ( 5 , 6 ) .Thus, it was found that the bulk concentration of bromine must he much greater in order to have sufficient molecules present in the lipid to obtain the
Tomato Juice Rainbow Place a measured amount of tomato juice into any cylindrical container and, as shown in Figure 1, add an amount of saturated bromine water that is 10 to 15%of the volume of tomato juice used. Mix in the bromine water by gentle swirling with a stirring rod. Varying the amount of bromine water used and the method of stirring gives different effects. To show that the various colors produced in this experiment are due to increasing concentrations of bromine, 4 mL of saturated bromine water may be stirred into 20 mL of tomato juice in a heaker. A blue color is produced which, on successive additions of l-mL aliquots of the bromine water, turns first to hlue-green, then green, and finally yellow. The Artificial Rainbow To simulate the lipid aggregates in tomato juice, a potassium stearate micellular system is used. @-Caroteneis substituted for lvcooene Althoueh the . . since the latter is orohibitivelv. exoensive. . rolorr produwd are nut exartlv the came, the eftert is w r y similar. I'rclnrr an a q L w u s mixturr of polajiiutn slrarule wilh a conrrn-
Figure 3. Section of the bi-layer of a cell membrane. The black circles represent the polar head groups exposed to aqueous medium. and the kinked lines, the attached nonpolar alkane chains.
Cnnnvd lornolo juice any commercially available hrand. The ronrrnrration of lyropene i u the pice is approximaltlg 211 rng m L 170 ~.,.
Potossiumstearate-100 g maybe prepared by stirring 88.19 gof stearie acid (95%as obtained from Aldrich) into 310 mL of a 1 M solution of sodium or potassium hydroxide, More water may be added if necessary to aid in dissolution. Evaporate the solution to dryness to obtain the product. Saturated aqueous solution of bromine-spectroscopic analysis
Calculations show that the concentration of bromine:lycopene is approximately 400:l in this mix.
Figure 4. Crass section of a micelle made up of the salt of a fany acid, e.g., Potassium stearate. There are approximately 170 molecules per micelle. The stern layer indicates the average depths of water penetration. Volume 63
Number 12
December 1986
1093
tration of approximately 5 X lo-' M by adding the appropriate amounts of solid potassium stearate to water and applying heat with stirrinx to aid in dissolution.3The micellular svstem will be ooaoue and tr&4ucent. After cooline. add about 0.1 h ,. of solid 8-carotkne per i u !nl.,and stir. 1'0 make thesyrtem hmnogenroua, mixint: may rpqllirr U P 10 10 mi". Treat the mixture with bromine water as desrrihrd previmdg for the t~matuj u m rainhow and observe the lasting array of colors.
volume of lipid aggregates. As well, there would he low prohability for the formation of the bromonium and bromide ions, not only because they would not he stabilized in the nonpolar medium hut also because the high viscosity in the membrane would prevent their separation. In contrast, the fleeting color chances observed. when the bromination was carrit.(l out in free &htion. giveexcellent vihunl evidence of the speed with which these CT romnleues normallv form and disappear. Dlscusslon It is concluded that the tomato iuice rainbow vhenomenon The colors produced when bromine water is added to is caused by the stabilization of the CT compfex formed in tomato juice may be explained if one accepts that the first the first step of the hromination of carotenoids. Since even step in the bromin~tionreaction is the formation of a high trace amounts of water, or the glass vessel itself, catalyze the concentratiun of a CT complex which has an unusually long rapid breakdown of the CT complexes, i t appears that doing lifetime. The exact ahsorotion suectrum of the CT hand has - - . -----. ~-~ the reaction in the totally nonpolar interior of a lipid system not, as yet, been d e t e r ~ k n e d , ~ ' h uitt would be expected, is an ideal way to illustrate the existence of this transition characteristically, to fall in the red region of the spectrum species. ( 1 1 ) . The blue color of the lowest layer in the "rainbow" is thought to he due to the oredominance of this CT comolex. Acknowledgment In thk layer above, the c¢ratiou of the yellow bromine Andrew Richardson would like to thank the Challenge water is sufficiently high that its mixture with the blue gives Program 1985of the Canadian Government for fundine durthe green color observed. In the top layers, there would be a ing the development of this project. All drawings wereudone gradual breakup of lipid aggregates causing the release of the by Dena Correy. carotenoids into the aqueous medium where they would he rapidly hrominated and become colorless. Fortuitously, the Literature CHed thickness of the natural tomato juice is sufficient to main(1) Schmid, G. H.; Garratt, D. G. In Tho Chemiafry of Double-Bonded F u n c l o d Groups: Patei, S., Ed.; Wiley: London, 1977; pp 766785. tain the concentration gradient indefinitely. (2) Banth0rpe.D. V. Chem.Reu.1970.70.306. The ~ersistenceof the rainbow of colors indicates that the ~-~~~~ ~ - - ~ (3) Olsh. G. A.; Hmkswender, T. R., Jr. J. Am. Chem. Soc. 1974.96.3574 existence of the CT complex is favored in the low effective (4) Foster, R. Orgonic Choree-TransferComplere~;Academic: New York, 1969:p 33. (5) Muller. N. I" R
