d-orbital when viewed along the axis
of a molecule and has two units of
orbital angular momentum around
the internuclear axis. The formation
of δ bonds originates in the overlap
of d-orbitals on different atoms. Delta
orbitals contribute to the bonding of
cluster compounds of transition metals.
delta-brass A strong hard type of
*brass that contains, in addition to
copper and zinc, a small percentage
of iron. It is mainly used for making
cartridge cases.
deltahedron A polyhedron that
has triangular faces. See wade’s
rules.
delta-iron See iron.
delta orbital See delta bonding.
delta value A quantity that measures the shift in *nuclear magnetic
resonance (NMR).
denature 1. To add a poisonous or
unpleasant substance to ethanol to
make it unsuitable for human consumption (see methylated spirits).
2. To produce a structural change in
a protein or nucleic acid that results
in the reduction or loss of its biological properties. Denaturation is caused
by heat, chemicals, and extremes of
pH. The differences between raw and
boiled eggs are largely a result of denaturation. 3. To add another isotope to a Üssile material to make it
unsuitable for use in a nuclear
weapon.
dendrimer (dendritic polymer) A
type of macromolecule in which a
number of chains radiate out from a
central atom or cluster of atoms.
Dendritic polymers have a number
of possible applications. See also
supramolecular chemistry.
dendrite A crystal that has
branched in growth into two parts.
Crystals that grow in this way (dendritic growth) have a branching treelike appearance.
dendrochronology An absolute
*dating technique using the growth
rings of trees. It depends on the fact
that trees in the same locality show a
characteristic pattern of growth rings
resulting from climatic conditions.
Thus it is possible to assign a deÜnite
date for each growth ring in living
trees, and to use the ring patterns to
date fossil trees or specimens of
wood (e.g. used for buildings or objects on archaeological sites) with
lifespans that overlap those of living
trees. The bristlecone pine (Pinus aristata), which lives for up to 5000
years, has been used to date specimens over 8000 years old. Fossil
specimens accurately dated by
dendrochronology have been used
to make corrections to the *carbondating technique. Dendrochronology
is also helpful in studying past climatic conditions. Analysis of trace elements in sections of rings can also
provide information on past atmospheric pollution.
denitriÜcation A chemical process
in which nitrates in the soil are reduced to molecular nitrogen, which
is released into the atmosphere. This
process is effected by the bacterium
Pseudomonas denitriÜcans, which uses
nitrates as a source of energy for
other chemical reactions in a manner
similar to respiration in other organisms. Compare nitrification. See nitrogen cycle.
de novo pathway Any metabolic
pathway in which a *biomolecule is
synthesized from simple precursor
molecules. Nucleotide synthesis is an
example.
density The mass of a substance
per unit of volume. In *SI units it is
delta-brass
168
d
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of a molecule and has two units of
orbital angular momentum around
the internuclear axis. The formation
of δ bonds originates in the overlap
of d-orbitals on different atoms. Delta
orbitals contribute to the bonding of
cluster compounds of transition metals.
delta-brass A strong hard type of
*brass that contains, in addition to
copper and zinc, a small percentage
of iron. It is mainly used for making
cartridge cases.
deltahedron A polyhedron that
has triangular faces. See wade’s
rules.
delta-iron See iron.
delta orbital See delta bonding.
delta value A quantity that measures the shift in *nuclear magnetic
resonance (NMR).
denature 1. To add a poisonous or
unpleasant substance to ethanol to
make it unsuitable for human consumption (see methylated spirits).
2. To produce a structural change in
a protein or nucleic acid that results
in the reduction or loss of its biological properties. Denaturation is caused
by heat, chemicals, and extremes of
pH. The differences between raw and
boiled eggs are largely a result of denaturation. 3. To add another isotope to a Üssile material to make it
unsuitable for use in a nuclear
weapon.
dendrimer (dendritic polymer) A
type of macromolecule in which a
number of chains radiate out from a
central atom or cluster of atoms.
Dendritic polymers have a number
of possible applications. See also
supramolecular chemistry.
dendrite A crystal that has
branched in growth into two parts.
Crystals that grow in this way (dendritic growth) have a branching treelike appearance.
dendrochronology An absolute
*dating technique using the growth
rings of trees. It depends on the fact
that trees in the same locality show a
characteristic pattern of growth rings
resulting from climatic conditions.
Thus it is possible to assign a deÜnite
date for each growth ring in living
trees, and to use the ring patterns to
date fossil trees or specimens of
wood (e.g. used for buildings or objects on archaeological sites) with
lifespans that overlap those of living
trees. The bristlecone pine (Pinus aristata), which lives for up to 5000
years, has been used to date specimens over 8000 years old. Fossil
specimens accurately dated by
dendrochronology have been used
to make corrections to the *carbondating technique. Dendrochronology
is also helpful in studying past climatic conditions. Analysis of trace elements in sections of rings can also
provide information on past atmospheric pollution.
denitriÜcation A chemical process
in which nitrates in the soil are reduced to molecular nitrogen, which
is released into the atmosphere. This
process is effected by the bacterium
Pseudomonas denitriÜcans, which uses
nitrates as a source of energy for
other chemical reactions in a manner
similar to respiration in other organisms. Compare nitrification. See nitrogen cycle.
de novo pathway Any metabolic
pathway in which a *biomolecule is
synthesized from simple precursor
molecules. Nucleotide synthesis is an
example.
density The mass of a substance
per unit of volume. In *SI units it is
delta-brass
168
d
www.AzShimi.ir www.AzShimi.com