Question

As you might suspect, \(\beta\)-carotene, \(\mathrm{C}_{40} \mathrm{H}_{50}\) precursor to vitamin \(\mathrm{A}_{\text {, was first isolated from }}\) carrots. Dilute solutions of \(\beta\)-carotene are yellow, hence its use as a food coloring. In plants, it is almost always present in combination with chlorophyll to assist in the harvesting of the energy of sunlight and to protect the plant against reactive species produced in photosynthesis. As tree leaves die in the fall, the green of their chlorophyll molecules is replaced by the yellows and reds of carotene and carotene-related molecules. Compare the carbon skeletons of \(\beta\)-carotene and lycopene. What are the similarities? What are the differences?

Short answer

Answer: Similarities between β-carotene and lycopene include their molecular formula (C40H56), alternating single and double bonds with terminal CH3 groups, and being conjugated polyenes. Differences are that β-carotene has six alternating double and single bond pairs while lycopene has seven, β-carotene has 11 conjugated double bonds compared to lycopene's 13, and the increased number of conjugated double bonds in lycopene causes a stronger red color compared to the more yellowish color of β-carotene.

Step-by-step solution

Determine the structural formulas of β-carotene and lycopene

First, let's find the structural formulas for both β-carotene and lycopene. β-carotene: β-carotene has the molecular formula C40H56. Its structure can be represented as `-R1-(R2-R3)n-R1-` where R1 = `CH3`, R2 = `C(CH3)=CH-`, R3 = `CH=CH-`, n = 6. The complete structure of β-carotene is shown below (written in one line as requested): ``` CH3-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-CH3 ``` Lycopene: Lycopene has the molecular formula C40H56, the same as β-carotene. Its structure can be represented as `-R1-(R2-R3)n-R1-` where R1 = `CH3`, R2 = `C(CH3)=CH-`, R3 = `CH=CH-`, n = 7. The complete structure of lycopene is shown below (written in one line as requested): ``` CH3-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-C(CH3)=CH-CH=CH-CH3 ```

Compare the carbon skeletons

Now that we have the structural formulas for β-carotene and lycopene, let's compare their carbon skeletons and identify the similarities and differences. Similarities: 1. Both β-carotene and lycopene have a similar molecular formula: C40H56. 2. They both have alternating single and double bonds with terminal CH3 groups. 3. Both molecules are conjugated polyenes, meaning they have alternating single and double carbon-carbon bonds. Differences: 1. β-carotene has six alternating double and single bond pairs, while lycopene has seven such pairs. 2. β-carotene has a total of 11 conjugated double bonds, while lycopene has a total of 13 conjugated double bonds. 3. The increased number of conjugated double bonds in lycopene results in a stronger color (red) compared to the more yellowish color of β-carotene.

Key concepts

Beta-Carotene Structure

Beta-carotene (β-carotene) is an organic compound and a member of the carotenes, which are a type of photosynthetic pigment. Structurally, it's made up of 40 carbon atoms and 56 hydrogen atoms, with the molecular formula C₄₀H₅₆. This molecule is characterized by a long chain of alternating single and double bonds (conjugated system) that extends the length of the molecule. These conjugated bonds have considerable implications for its properties, including its deep orange color.

Each end of the β-carotene molecule caps off with a ring structure, which further contributes to its stability and ability to interact with other molecules. In plants, β-carotene plays a critical role in the absorption of light energy for use in photosynthesis. Additionally, once ingested by animals or humans, it can serve as a precursor to vitamin A, an essential nutrient for vision and immune function.

• Molecular formula: C₄₀H₅₆
• Characterized by 11 conjugated double bonds
• Adds to the colors seen in autumn foliage by its presence in decaying leaves

Lycopene Structure

Lycopene carries the same molecular formula as β-carotene, C₄₀H₅₆, but differs in the arrangement of its double bonds. Like β-carotene, lycopene is part of the carotene group, but because it possesses two additional conjugated double bonds in its structure (for a total of 13), it presents a stronger red color. These conjugated polyenes are responsible for its color and antioxidant properties.

Found in significant quantities in tomatoes and other red fruits and vegetables, lycopene is known for its health benefits, including its potential role in reducing the risk of certain types of cancers and heart disease. The extended conjugated system allows for the effective absorption of light, and this quality is leveraged in the role of lycopene as a pigment in the process of photosynthesis.

• Molecular formula: C₄₀H₅₆
• Contains 13 conjugated double bonds
• Imparts the red color to tomatoes and other red-hued plants

Conjugated Polyenes

Conjugated polyenes are a series of alternating single and double bonds within a molecule. This configuration allows for electrons to be more delocalized across the structure, increasing the molecule's ability to absorb light at various wavelengths. The length and extent of the conjugation determine the color of the compound, as seen in the variations between β-carotene and lycopene.

The conjugated systems in β-carotene and lycopene enable them to function as pigments that contribute to the vibrant colors in fruits and vegetables. Moreover, their electron-rich structures give them potent antioxidant properties, which help plants and the humans who consume these plants to combat oxidative stress.

• Electron delocalization leads to light absorption
• Conjugation length affects the color expression
• Essential for both plant coloration and protection

Photosynthesis Pigments

Photosynthesis pigments are crucial for capturing light energy to drive the photosynthetic process in plants. Chlorophyll is the most well-known of these pigments, but carotenes, including β-carotene and lycopene, play a supportive role. They not only contribute to light absorption over a broader range of the light spectrum but also protect chlorophyll from damage due to excessive light exposure.

While chlorophyll gives plants their green color by absorbing mainly blue and red light, carotenes absorb light in the blue and green parts of the spectrum and reflect yellow, orange, and red light, adding to the diversity of plant coloration. During autumn, as chlorophyll breaks down, the carotenes become more visible, leading to the beautiful display of fall foliage.

• Assist chlorophyll in light energy absorption
• Protect chlorophyll from photodamage
• Contribute to the colorful display of leaves in the fall