3. Which one of the following amino acids is not chiral?
a. Cysteine
b. Alanine
c. Glycine
d. Leucine
The amino acid that is not chiral among the given options is glycine. All amino acids except glycine have a chiral alpha carbon, which means they exist as non-superimposable mirror images (enantiomers) and can rotate plane-polarized light. Glycine is unique because its side chain is simply a hydrogen atom, so its alpha carbon is bonded to two hydrogen atoms, making it achiral and optically inactive.
Detailed Explanation of Each Option
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Cysteine: Cysteine has a chiral alpha carbon because it is bonded to four different groups: an amino group, a carboxyl group, a hydrogen, and a side chain containing sulfur. Thus, cysteine is chiral.
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Alanine: Alanine also has a chiral alpha carbon, bonded to an amino group, a carboxyl group, a hydrogen, and a methyl group. Alanine is chiral.
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Glycine: Glycine’s alpha carbon is bonded to two hydrogen atoms, an amino group, and a carboxyl group. Because two of the groups are identical (hydrogen), glycine does not have a chiral center and is achiral.
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Leucine: Leucine’s alpha carbon is bonded to four different groups: an amino group, a carboxyl group, a hydrogen, and a branched hydrocarbon side chain. Leucine is chiral.
Introduction
Chirality is a fundamental concept in biochemistry, especially when studying amino acids. Most amino acids are chiral, meaning they have a central carbon atom bonded to four different groups. However, one amino acid stands out for being achiral: glycine. This article explains why glycine is not chiral and examines the chiral properties of other amino acids like cysteine, alanine, and leucine.
What Makes an Amino Acid Chiral?
A chiral amino acid has an alpha carbon bonded to four different substituents: an amino group, a carboxyl group, a hydrogen atom, and a side chain. This asymmetry means the molecule cannot be superimposed on its mirror image, making it optically active.
Why is Glycine Not Chiral?
Glycine is the only common amino acid without a chiral center. Its alpha carbon is bonded to two hydrogen atoms, an amino group, and a carboxyl group. Since two substituents are identical (hydrogen), glycine lacks chirality and is optically inactive.
Chirality in Other Amino Acids
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Cysteine: Chiral due to its sulfur-containing side chain.
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Alanine: Chiral with a methyl side chain.
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Leucine: Chiral with a branched hydrocarbon side chain.
Conclusion
Glycine is the only amino acid among the 20 standard amino acids that is not chiral. Its simple structure, with two hydrogen atoms on the alpha carbon, makes it unique and achiral. Understanding this property is essential for biochemistry and molecular biology studies.
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