Problem of the Week

Chapter 17: Reaction Rates

Problem of the Week

It's Luminating
The first colonists on Mars probably will be plants. Professor Rob Ferl of the University of Florida is bioengineering mustard plants for a proposed mission that will put plants on Mars as soon as 2007. Instead of altering the plants so they can adapt more easily to the conditions on Mars, Professor Ferl is adding a gene from the jellyfish that allows the plant to bioluminesce. The added gene will act as a reporter gene so the mustard plants can send messages back to Earth about how they are surviving on the planet. These plants will be genetically wired to glow with a soft green aura when they encounter problems. By glowing, the mustard plants will report low oxygen levels, low water, or poor nutrients in the soil.

Bioluminescence and chemiluminescence comes from energy released from a chemical reaction in the form of cool light. Generally, chemiluminescence occurs when the product of an exothermic reaction is formed in the excited state. As the electron returns to the lower ground state, it releases energy that can be seen as a photon of light. Excited intermediates produced during a chemical reaction do not have favorable pathways to release this energy. When the intermediate species encounters a fluorescer it will transfer the energy. The fluorescer acts as a catalyst for the decomposition of the key intermediate, and this catalyst is an important factor in the efficiency of this chemical reaction. Luminol is a common fluorescer found in chemiluminescence reactions. One of the commonly known chemiluminescence phenomena that is visible to the human eye and occurs in living organisms is the bioluminescence of fireflies and photobacteria. In a luminescent reaction, two types of chemicals, luciferin and luciferase, combine together. The luciferase acts as an enzyme, allowing the luciferin to release energy as it is oxidized. The color of the light depends on the chemical structures of the chemicals. GFP, green fluorescent protein, is a fluorescent protein isolated from coelenterates, such as the pacific jellyfish. Aequorin, a bioluminescent photoprotein isolated from jellyfish is activated by calcium ions, and emits a blue light of 470nm. Due to its high sensitivity to Ca²⁺, aequorin has been widely used an indicator of intercellular calcium concentrations.

The following is the reaction mechanism used by many organisms that bioluminesce.

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a. Identify the intermediates and the catalyst in this reaction.
b. Determine the complex reaction.
c. Draw the reaction energy diagram for this exothermic reaction.

The following is the reaction mechanism for the chemiluminescence reaction of bis(2,4-dinitrophenyl) oxalate with hydrogen peroxide and 9, 10, 11, 12-tetraphenyl napthacene:

C₁₄H₆N₄O₁₂+ H₂O₂ → C₆H₅OC₂O₄H + C₆H₅OH + products

C₆H₅OC₂O₄H → C₂O₄ + C₆H₅OH

C₂O₄ + C₄₂H₂₈ → CO₂ + [CO₂] ^{· -} + [C₄₂H₂₈]· ⁺

[CO₂]^·-+ [C₄₂H₂₈]· ^{+ →} CO₂ + [C₄₂H₂₈]*

[C₄₂H₂₈]* → hv (light) + C₄₂H₂₈

From the above reaction mechanism, identify the intermediates, identify the catalyst, and determine the complex reaction.

A light stick is made of a flexible plastic tube containing a thin walled glass tube. The glass tube contains hydrogen peroxide dissolved in a phthalic ester. The plastic tube contains a phenyl oxalate ester and a fluorescent dye. The complex reaction and reaction mechanism for the light stick are given.

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oxalate ester + H₂O₂ → I + 2 phenol + 2 CO₂
I + fluorophor (F) → F* + products
F* → F + hv(light)

a. Using the above reaction mechanism, determine the intermediates and the catalyst in this chemiluminescent reaction.
b. Explain what a catalyst does in the reaction. Draw a reaction energy diagram to represent the addition of a catalyst.