Just as certain objects are chiral, certain molecules are also chiral. Alanine and pro- pane provide a visual comparison between chiral and achiral molecules:
Mirror
O– O–
O
C
“Left-handed”
L-alanine H H3N+
CH3 C
O
C
“Right-handed”
D-alanine H NH3+
CH3 C
C
Propane
H H
CH3 CH3 Mirror
Alanine, a chiral molecule Propane, an achiral molecule
1 2 3
1 2 3
1 2 3
1 2 3
1 2 3 1
2 3
Alanine is a chiral molecule. Its mirror images cannot be superimposed. As a result, alanine exists in two forms that are mirror images of each other: a “right-handed” form known as d-alanine and a “left-handed” form known as l-alanine. Propane, by con- trast, is an achiral molecule. The molecule and its mirror image are identical, and it has no left- and right-handed isomers.
Why are some molecules chiral but others are not? Can we predict chirality from structural formulas? Recall that carbon forms four bonds oriented to the four corners of an imaginary tetrahedron. The formulas for alanine and propane are drawn next in a manner that emphasizes the four groups bonded to the central carbon atom. In
Chiral Having right- or left-handed- ness with two different mirror-image forms.
Achiral The opposite of chiral; hav- ing superimposable mirror images and thus no right- or left-handedness.
▲Figure 18.2
The meaning of superimposable.
It is easy to visualize the chair on top of its mirror image.
The D- and L-designations are derived from the relationship of the structures of the amino acids to the structure of glyceraldehyde, as we will see in Section 21.2.
Review the tetrahedral structure of carbon in Section 4.8.
alanine, this carbon is connected to four different groups: a iCOO- group, an iH atom, an iNH3+ group, and a iCH3 group:
C H 1.
2.
3.
4.
H
Alanine (chiral)
C H H
Propane (achiral) Different
1.
2.
3.
4.
H H
Identical
Identical CH3 CH3
H3N+ COO– COO– NH+3
CH3
CH3 CH3 CH3
Such a carbon atom is referred to as a chiral carbon atom, or a chiral center. The pres- ence of one chiral carbon atom always produces a chiral molecule that exists in two mirror-image forms. Thus, alanine is chiral. In propane, the central carbon atom is bonded to two pairs of identical groups, and the two other carbon atoms are each bonded to three hydrogen atoms. The propane molecule has no chiral carbon atoms and is therefore achiral. (If a molecule has two or more chiral carbon atoms, it may or may not be chiral, depending on its overall shape.)
The two mirror-image forms of a chiral molecule like alanine are called either enantiomers (pronounced en-an-ti-o-mers) or optical isomers (“optical” because of their effect on polarized light). The mirror-image relationship of the enantio- mers of a compound with four different groups on one carbon atom is illustrated in Figure 18.3.
Like other isomers, enantiomers have the same formula but different arrange- ments of their atoms. More specifically, enantiomers are one kind of stereoisomer, compounds that have the same formula and atoms with the same connections but different spatial arrangements. (Cis–trans isomers discussed in Section 13.3 are ste- reoisomers, too.) Pairs of enantiomers have many of the same physical properties.
Both enantiomers of alanine, for example, have the same melting point, the same sol- ubility in water, the same isoelectric point, and the same density. But pairs of enan- tiomers always differ in their effects on polarized light and in how they react with other molecules that are also chiral. Most importantly, pairs of enantiomers often differ in their biological activity, odors, tastes, or activity as drugs. For example, the very different natural flavors of spearmint and caraway seeds are attributed to these two enantiomers:
CH3
CH2
CH3
L-carvone (in spearmint)
C H O
CH3
CH2 CH3
D-carvone (in caraway)
C H O Chiral
carbon
What about the amino acids listed in Table 18.3; are any of them chiral? Of the 20 common amino acids, 19 are chiral because they have four different groups bonded to their a-carbons, iH, iNH2, iCOOH, and iR (the side chain). Only glycine, H2NCH2COOH, is achiral; its side chain is a hydrogen atom and thus its a-carbon is bonded to two hydrogen atoms. Even though the 19 chiral a@amino acids can exist either as d- or l-enantiomers, nature selectively uses only l-amino acids for making proteins.
The artificial sweetener aspartame (sold as Equal™ or NutraSweet™) provides another excellent illustration of the delicate nature of the structure–function rela- tionship and its role in biochemistry. Aspartame is the methyl ester of a dipeptide made from aspartate and phenylalanine in which both amino acids have the natu- rally occurring “left-handed,” or l, chirality. In contrast, if either of the two amino Chiral carbon atom A carbon atom
bonded to four different groups.
Mirror
▲Figure 18.3 A chiral molecule.
The central atom is bonded to four dif- ferent groups; the molecule is therefore chiral.
Enantiomers (optical isomers) The two mirror-image forms of a chiral molecule.
We will explore polarized light and how it is affected by enantiomers in Section 21.2.
▲ Spearmint leaves and caraway seeds. The very different flavors of these food seasonings are imparted by a pair of enantiomers, which interact in different ways with our taste buds.
Stereoisomers Isomers that have the same molecular and structural formu- las but different spatial arrangements of their atoms.
S E C T I O N 1 8 . 6 Molecular Handedness and Amino Acids 559 acids in this molecular structure were the d rather than the l isomer, the compound
would taste bitter.
Aspartame
(methyl ester of aspartylphenylalanine) H2N CH C NH
O
COOH
CH2 CH2
CH C O O
CH3
LOOKING AHEAD Amino acids, as you have seen, are chiral. Chirality is an important property of another major class of biomolecules. The individual sugar units in all carbohydrates are chiral, a topic addressed in Sections 21.2 and 21.3.
Worked Example 18.3 Determining Chirality
Lactic acid can be isolated from sour milk. Is lactic acid chiral?
Lactic acid CH3 CH C OH
OH O
1 2 3
ANALYSIS A molecule is chiral if it contains one C atom bonded to four different groups. Identify any C atoms that meet this condition.
SOLUTION
To find out if lactic acid is chiral, list the groups attached to each carbon atom:
Lactic acid CH3 CH C OH
OH O
1 2 3
Groups on carbon 1 Groups on carbon 2 Groups on carbon 3
1. iOH 1. iCOOH 1. iCH1OH2COOH
2. “O 2. iOH 2. iH
3. iCH1OH2CH3 3. iH 3. iH
4. iCH3 4. iH
Next, look at the lists to see if any carbon atom is attached to four different groups. Of the three carbons, carbon 2 has four different groups, and lactic acid is therefore chiral.
PROBLEM 18.11
2-Aminopropane is an achiral molecule, but 2-aminobutane is chiral. Explain.
PROBLEM 18.12
Which of the following molecules are chiral? (Hint: Draw each molecule and ana- lyze it as illustrated in Worked Example 18.3.)
(a) 3-Chloropentane (b) 2-Chloropentane (c) CH3CHCH2CHCH2CH3 CH3 CH3 PROBLEM 18.13
Two of the 20 common amino acids have two chiral carbon atoms in their structures.
Identify these amino acids and their chiral carbon atoms.
KEY CONCEPT PROBLEM 18.14
Two isomers have the formula C2H4BrCl. Draw both isomers and identify any chiral carbon atoms.