Organic Chemistry

974 CHAPTER 23 Amino Acids, Peptides, and Proteins

cis configuration, so the of adjacent amino acids are trans to each other

(Section 4.11).

Free rotation about the peptide bond is not possible because of its partial double-bond

character. The carbon and nitrogen atoms of the peptide bond and the two atoms to

which each is attached are held rigidly in a plane (Figure 23.6). This regional planari-

ty affects the way a chain of amino acids can fold, so it has important implications for

the three-dimensional shapes of peptides and proteins (Section 23.13).

CH N

C

R H

R

CH

O

CH N

C +

R H

R

CH

O−

trans configuration

-carbon

-carbon

a-carbons

O

C

CH

H

N

N

R H

R

CH

C

O

O

C

CH

H

N

N

R H

R

CH

C

O

O

C

CH

H

N

N

R H

R

CH

C

O

C

CH

H

N

O R

Figure 23.6N

A segment of a polypeptide chain.

The plane defined by each peptide

bond is indicated. Notice that the

R groups bonded to the

are on alternate sides of the

peptide backbone.

a-carbons

PROBLEM 19

Draw a peptide bond in a cis configuration.

Disulfide Bonds

When thiols are oxidized under mild conditions, they form disulfides. A disulfideis a

compound with an bond.

An oxidizing agent commonly used for this reaction is (or ) in a basic solution.

Because thiols can be oxidized to disulfides, disulfides can be reduced to thiols.

Cysteine is an amino acid that contains a thiol group. Two cysteine molecules there-

fore can be oxidized to a disulfide. This disulfide is called cystine.

Two cysteine residues in a protein can be oxidized to a disulfide. This is known as a

disulfide bridge. Disulfide bridges are the only covalent bonds that can form between

nonadjacent amino acids. They contribute to the overall shape of a protein by holding

the cysteine residues in close proximity, as shown in Figure 23.7.

cysteine cystine

mild oxidation

2 HSCH 2 CHCO−

O

−OCCHCH

2 S

O

SCH 2 CHCO−

O

+NH

3

+NH

3

+NH

3

reduction

a disulfide

RS SR

a thiol

2 R SH

HO−

H 2 O

RSH

Br Br

RS− RSBr RSSR+ Br−

+ Br−

RS−

mechanism for oxidation of a thiol to a disulfide

Br 2 I 2

mild oxidation

a disulfide

RS SR

a thiol

2 R SH

S¬S

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