Organic Chemistry

Section 21.11 Biologically Important Heterocycles 909

PROBLEM 24

Imidazole boils at 257°C, whereas N-methylimidazole boils at 199°C. Explain this differ-

ence in boiling points.

PROBLEM 25

What percent of imidazole will be protonated at physiological pH

Purine and Pyrimidine
Nucleic acids (DNA and RNA) contain substituted purines and substituted
pyrimidines(Section 27.1); DNA contains A, G, C, and T, and RNA contains A, G, C,
and U. (Why DNA contains T instead of U is explained in Section 27.14.) Unsubsti-
tuted purine and pyrimidine are not found in nature. Notice that hydroxypurines and
hydroxypyrimidines are more stable in the keto form. We will see that the preference
for the keto form is crucial for proper base pairing in DNA (Section 27.7).

Porphyrin
Substituted porphyrinsare important naturally occurring heterocyclic compounds. A
porphyrin ring systemconsists of four pyrrole rings joined by one-carbon bridges. Heme,
which is found in hemoglobin and myoglobin, contains an iron ion ligated by the
four nitrogens of a porphyrin ring system. Ligationis the sharing of nonbonding electrons
with a metal ion. The porphyrin ring system of heme is known as protoporphyrin IX; the
ring system plus the iron atom is called iron protoporphyrin IX.

Hemoglobin is responsible for transporting oxygen to cells and carbon dioxide
away from cells, whereas myoglobin is responsible for storing oxygen in cells. Hemo-
globin has four polypeptide chains and four heme groups; myoglobin has one
polypeptide chain and one heme group. The iron atoms in hemoglobin and myoglobin,
in addition to being ligated to the four nitrogens of the porphyrin ring, are also ligated
to a histidine of the protein component (globin), and the sixth ligand is oxygen or car-
bon dioxide. Carbon monoxide is about the same size and shape as O 2 ,but CO binds

C

H 3 C

−OOCCH 2 CH 2

H

C

NN

FeII

C

CH 3 HC

CH 2

C

C

CCH 3

CH

CH 2 CH 2 COO−

C

NC

HC

C N

C

H

C

C

C C CCH 3

C

H

C

H 2 C

a porphyrin ring system C C

iron protoporphyrin IX

heme

N

NHN

NH

(Fe^2 +)

N N

H

N

N

purine

O

N NH O O

N

HN

H 2 N

guanine

NH 2

N NH

N

N

adenine

N

N

pyrimidine

O

N

H

HN

uracil

O

O

CH 3

N

H

HN

thymine

NH 2

N

H

N

cytosine

1

4

49

8

7

5

6

1

2

3

26

3 5

1 7.3 2?

3-D Molecule:

Heme