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Solutions for Carbohydrates and Nucleic Acids
N
H
N
NH2
O
N
H
N
C
O
N
H
N
N
O
N
H O N O
N O
H2O
H+
N
H
N
N
O
N
N
H
N
O
O
H H
N
H
N
N
O
NH
H
O
H2O
H2O
H2O
N O
N
H
N
O
OH
N
H
N
N
O
N O
H
H
N
H
N
N
O
NH O
H
H2O
N
H
NH
O
O
N
H
N
N
O
NH O
H
N
H
N
N
O
N O
H
73 Recall from text section 19-16 that nitrous acid is unstable, generating nitrosonium ion.
two rapid proton transfers
– H2O
If this species exists, its 
lifetime is very short as 
water will attack quickly.
– N2 gas
uracil (U)enol tautomer
of uracil
(a)
(b) In base pairing, cytosine pairs with guanine. If cytosine is converted to uracil, however, each 
replication will not carry the complement of cytosine (guanine) but instead will carry the complement of 
uracil (adenine). This is the definition of a mutation, where the wrong base is inserted in a nucleic acid 
chain.
(c) In RNA, the transformation of cytosine (C) to uracil (U) is not detected as a problem because U is a 
base normally found in RNA so it goes unrepaired. In DNA, however, thymine (with an extra methyl 
group) is used instead of uracil. If cytosine is diazotized to uracil, the DNA repair enzymes detect it as a 
mutation and correct it.
cytosine (C)
No mechanism for substitution of diazonium ions 
was presented in Chapter 19. What is shown 
here is one possibility but not the only one.
H A
H A
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