Chemistry Reference
In-Depth Information
O
O
Hg
II
N
N
N
N
O
O
R
R
Scheme 16.1
O
H
Hg
II
N
N
N
N
N
O
N
R
R
N
Scheme 16.2
nucleotides of the DNA] became 0.5; (vi) a possible Hg
II
binding site might be N3
of T/U.
Based on the above results, Katz reconsidered the stoichiometry of Hg
II
binding
to DNA molecules
6,7
(1962), and proposed a chemical structure of the Hg
II
mediated
T - T base pair (T - Hg
II
-T) as a possible interaction mode in DNA molecules
8
(1963)
(Scheme 16.1). Furthermore, he presented a possible mechanism for DNA strand
displacements based on the T-Hg
II
- T pair.
To obtain quantitative data, thermodynamic studies on Hg
II
- nucleoside com-
plexes were performed by Eichhorn and Clark
9
(1963), and Simpson
10
(1964). Simpson
determined association constants of Hg
II
to nucleosides under various pH values.
In 1965, Nandi
et al.
proposed a unique interaction mode for an A-Hg
II
- T base pair
that is different from the T-Hg
II
- T base pair
11
(Scheme 16.2 ).
In 1966, Gruenwedel and Davidson reported UV-monitored titration experi-
ments on natural DNA molecules (calf thymus DNA, bacteriophage T4 DNA and
M. lysodeikticus
DNA) with methymercuric hydroxide (CH
3
HgOH).
12
Based on the
Katz model of the T-Hg
II
-T pair, they porposed that methymercuric ions can dena-
ture DNA molecules by Hg
II
binding to N3 of T and N1 of guanosine (G).
These pioneering works stimulated many structural studies of the Hg
II
- mediated
T-T base pair in DNA duplexes, with crystallography, vibrational spectroscopy and
NMR spectroscopy being applied.
16.2 Crystallographic Studies on H g
II
- Nucleobase Complexes
There are two kinds of crystal structures for an Hg
II
-complex with uracil (U) or
thymine (T) (Figure 16.1). In 1971, Carrabine and Sundaralingam determined the
structure of a Hg
II
- uracil (U) complex
13
(Figure 16.1a). In this structure, two uracil
molecules are bound to Hg
II
at their O4-carbonyl oxygen [O4(U)]; this structure
was different, however, from the putative structure of the T-Hg
II
- T pair proposed
by Katz. In 1974, Kosturko, Folzer and Stewart reported a crystal structure of the
