Amino Acid Chirality Explained
A chiral carbon atom is a carbon atom that is bonded to four different atoms or groups. In amino acids, the alpha-carbon (Cα) is typically chiral, connecting the amino group (-NH2), carboxyl group (-COOH), a hydrogen atom (-H), and a unique side chain (R-group).
Some amino acids have additional chiral centers within their side chains. Glycine is the exception, as its R-group is simply -H, making its alpha-carbon non-chiral.
Chiral Center Analysis of Amino Acids
We need to identify which of the given amino acids possess more than one chiral carbon.
- L-Leucine: The alpha-carbon is chiral. The side chain is -CH2-CH(CH3)2. Neither the -CH2- nor the -CH(CH3)2 carbons are chiral because the isopropyl group has two identical methyl (-CH3) groups attached to the branching carbon. L-Leucine has only one chiral carbon.
- L-Threonine: The alpha-carbon is chiral. The side chain is -CH(OH)-CH3. The beta-carbon (the carbon in the side chain) is bonded to four different groups: -H, -CH3, -OH, and the alpha-carbon group. Therefore, L-Threonine has two chiral carbons.
- L-Isoleucine: The alpha-carbon is chiral. The side chain is -CH(CH3)-CH2-CH3. The beta-carbon (the first carbon in the side chain) is bonded to four different groups: -CH3, -CH2CH3, -H, and the alpha-carbon group. Therefore, L-Isoleucine has two chiral carbons.
- L-Asparagine: The alpha-carbon is chiral. The side chain is -CH2-CH2-C(O)NH2. All carbons in the side chain are bonded to at least two hydrogen atoms, making them non-chiral. L-Asparagine has only one chiral carbon.
Conclusion on Chirality
From the analysis, L-Threonine and L-Isoleucine are the amino acids among the options that contain two chiral carbons.
Important Questions from Structure and Function of Biomolecules
- An element that is present in a nucleotide but not in a nucleoside is __________.
- Which one of the following coenzymes is utilised by alanine racemase for the conversion of L-Alanine to D-Alanine?
- Which of the following show(s) optical activity at 100 mM concentration in water?
An octapeptide composed of these L-amino acids – Lys, Thr, Ser, Met, Arg, Trp, Tyr, Glu was subjected to analyses with the following outcomes:
P. The N-terminal sequencing analysis by Sanger's method yielded 'Ser' at the N-terminus
Q. Chymotrypsin treatment gave a pentapeptide, a ‘Tyr' containing dipeptide and a free ‘Glu'
R. Cyanogen bromide treatment gave two tetrapeptides
S. Trypsin treatment gave two tripeptides and a dipeptide
Which one of the following is the correct octapeptide sequence?
- Terpenoids are made of ________ units
