talline solid, H 2 S 2 O 7 ; r.d. 1.9; m.p.
35°C. It is commonly encountered
mixed with sulphuric acid as it is
formed by dissolving sulphur trioxide in concentrated sulphuric acid.
The resulting fuming liquid, called
oleum or Nordhausen sulphuric acid,
is produced during the *contact
process and is also widely used in the
*sulphonation of organic compounds. See also sulphuric acid.
dithionate A salt of dithionic acid,
containing the ion S 2 O 6
2– , usually
formed by the oxidation of a sulphite
using manganese(IV) oxide. The ion
has neither pronounced oxidizing
nor reducing properties.
dithionic acid An acid, H 2 S 2 O 6 ,
known in the form of its salts
(dithionates).
dithionite See sulphinate.
dithionous acid See sulphinic
acid.
divalent (bivalent) Having a valency of two.
DLA See diffusion limited aggregation.
D-lines Two close lines in the yellow region of the visible spectrum of
sodium, having wavelengths 589.0
and 589.6 nm. As they are prominent
and easily recognized they are used
as a standard in spectroscopy.
dl-isomer See optical activity;
racemic mixture.
DLVO theory A theory of colloid
stability proposed in the 1940s by the
Soviet scientists Boris Derjaguin and
Lev Landau and independently by the
Dutch scientists Evert Verwey and
Theo Overbeek. The DLVO theory
takes account of two types of force in
a stable colloid: the van der Waals’
force, which is attractive and binds
particles together, and electrostatic
repulsion. The total interaction potential can be calculated as a function of distance, with colloid stability
being attained when the two forces
balance each other. The DLVO theory
is the basis for understanding colloid
stability and has a considerable
amount of experimental support.
However, it is inadequate for the
properties of colloids in the aggregated state, which depend on shortrange interactions taking into
account the speciÜc properties of
ions, rather than regarding them as
point particles.
p-DMAB See p-dimethylaminobenzadehyde.
DMF See dimethylformamide.
DMG See dimethylglyoxime.
DMSO See dimethyl sulphoxide.
DNA (deoxyribonucleic acid) The genetic material of most living organisms, which is a major constituent of
the chromosomes within the cell nucleus and plays a central role in the
determination of hereditary characteristics by controlling protein synthesis in cells. DNA is a nucleic acid
composed of two chains of *nucleotides in which the sugar is deoxyribose and the bases are
*adenine, *cytosine, *guanine, and
*thymine (compare rna). The two
chains are wound round each other
and linked together by hydrogen
bonds between speciÜc complementary bases to form a spiral laddershaped molecule (double helix: see
illustration).
When the cell divides, its DNA also
replicates in such a way that each of
the two daughter molecules is identical to the parent molecule. The
hydrogen bonds between the complementary bases on the two strands
of the parent molecule break and the
strands unwind. Using as building
bricks nucleotides present in the
nucleus, each strand directs the
183
DNA
d
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35°C. It is commonly encountered
mixed with sulphuric acid as it is
formed by dissolving sulphur trioxide in concentrated sulphuric acid.
The resulting fuming liquid, called
oleum or Nordhausen sulphuric acid,
is produced during the *contact
process and is also widely used in the
*sulphonation of organic compounds. See also sulphuric acid.
dithionate A salt of dithionic acid,
containing the ion S 2 O 6
2– , usually
formed by the oxidation of a sulphite
using manganese(IV) oxide. The ion
has neither pronounced oxidizing
nor reducing properties.
dithionic acid An acid, H 2 S 2 O 6 ,
known in the form of its salts
(dithionates).
dithionite See sulphinate.
dithionous acid See sulphinic
acid.
divalent (bivalent) Having a valency of two.
DLA See diffusion limited aggregation.
D-lines Two close lines in the yellow region of the visible spectrum of
sodium, having wavelengths 589.0
and 589.6 nm. As they are prominent
and easily recognized they are used
as a standard in spectroscopy.
dl-isomer See optical activity;
racemic mixture.
DLVO theory A theory of colloid
stability proposed in the 1940s by the
Soviet scientists Boris Derjaguin and
Lev Landau and independently by the
Dutch scientists Evert Verwey and
Theo Overbeek. The DLVO theory
takes account of two types of force in
a stable colloid: the van der Waals’
force, which is attractive and binds
particles together, and electrostatic
repulsion. The total interaction potential can be calculated as a function of distance, with colloid stability
being attained when the two forces
balance each other. The DLVO theory
is the basis for understanding colloid
stability and has a considerable
amount of experimental support.
However, it is inadequate for the
properties of colloids in the aggregated state, which depend on shortrange interactions taking into
account the speciÜc properties of
ions, rather than regarding them as
point particles.
p-DMAB See p-dimethylaminobenzadehyde.
DMF See dimethylformamide.
DMG See dimethylglyoxime.
DMSO See dimethyl sulphoxide.
DNA (deoxyribonucleic acid) The genetic material of most living organisms, which is a major constituent of
the chromosomes within the cell nucleus and plays a central role in the
determination of hereditary characteristics by controlling protein synthesis in cells. DNA is a nucleic acid
composed of two chains of *nucleotides in which the sugar is deoxyribose and the bases are
*adenine, *cytosine, *guanine, and
*thymine (compare rna). The two
chains are wound round each other
and linked together by hydrogen
bonds between speciÜc complementary bases to form a spiral laddershaped molecule (double helix: see
illustration).
When the cell divides, its DNA also
replicates in such a way that each of
the two daughter molecules is identical to the parent molecule. The
hydrogen bonds between the complementary bases on the two strands
of the parent molecule break and the
strands unwind. Using as building
bricks nucleotides present in the
nucleus, each strand directs the
183
DNA
d
www.AzShimi.ir www.AzShimi.com