Nucleic Acids in Chemistry and Biology

8-azidopurine nucleosides for use as photoaffinity labels).^15 It is much more difficult to control the use of ele-
mental fluorine, though fluorine gas has been used in anhydrous acetic acid (care!) to prepare 5-fluorouracil
and 5-fluorouridine.^16 5-Iodouridines are best made by the method described in Section 3.1.4.

Covalent Interactions of Nucleic Acids with Small Molecules and Their Repair 299

N

N

pentose

O

NH 2

NH 2 OH

N

N

pentose

O

NH 2

O 3 S

H N

NH

pentose

O

O

O 3 S

H

H 2 O

-NH 3

N

NH

pentose

O

-HSO 3

pH8

O

a

N

N

dR

O

NH 2

N

N

dR

O

NH 2

b

S

Enz

Met

S

Me

Ado

S

Enz

H

Me

N

N

dR

O

NH 2

S

Enz

Me

H

H

H

H

Figure 8.6 (a) Mechanism of chemicaldeamination of cytidine and deoxycytidine catalysed by bisulfite. (b) Schematic
mechanism for the restriction methylation of deoxycytidine by S-adenosyl L-methionine catalysed by
M Hhal

N

N

dR

O

NH 2

NH 2 OH

N

N

dR

O

NH 2

HONH

H N

N

dR

O

NHOH

HONH

H

NH 2 OH

N

N

dR

O

NHOH

–NH 2 OH

N

NH

dR

O

N

HO

H

H

H

H

Figure 8.5 Reaction of hydroxylamine with deoxycytidine leading to tautomerization of N^4 -hydroxy-2
-deoxycytidine

N

NH

Me

dR

O

O

N

N

dR

O

NH 2

N

N

Me

O

N

H

N

NH 2

O

HO

OH

NH

O NH 2

O

HO

OH

NHNH 2

N 2 H 4

N 2 H 4

N 2 H 4

+

Figure 8.4 Hydrazinolysis of pyrimidine nucleosides