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 δbonds originates in the overlap
of d-orbitals on different atoms. Delta
orbitals contribute to the bonding of
cluster compounds of transition met-
als.
delta-brassA 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 orbitalSee delta bonding.delta valueA quantity that meas-
ures the shift in *nuclear magnetic
resonance (NMR).denature 1.To add a poisonous or
unpleasant substance to ethanol to
make it unsuitable for human con-
sumption (see methylated spirits).
2.To produce a structural change in
a protein or nucleic acid that results
in the reduction or loss of its biologi-
cal properties. Denaturation is caused
by heat, chemicals, and extremes of
pH. The differences between raw and
boiled eggs are largely a result of de-
naturation. 3.To add another iso-
tope to aÜssile 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.
dendriteA crystal that has
branched in growth into two parts.Crystals that grow in this way (den-
dritic growth) have a branching tree-
like 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 ob-
jects on archaeological sites) with
lifespans that overlap those of living
trees. The bristlecone pine (Pinus aris-
tata), which lives for up to 5000
years, has been used to date speci-
mens over 8000 years old. Fossil
specimens accurately dated by
dendrochronology have been used
to make corrections to the *carbon-
dating technique. Dendrochronology
is also helpful in studying past cli-
matic conditions. Analysis of trace el-
ements in sections of rings can also
provide information on past atmos-
pheric pollution.denitriÜcation A chemical process
in which nitrates in the soil are re-
duced 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 organ-
isms. Compare nitrification. See ni-
trogen cycle.de novo pathwayAny metabolic
pathway in which a *biomolecule is
synthesized from simple precursor
molecules. Nucleotide synthesis is an
example.densityThe mass of a substance
per unit of volume. In *SI units it isdelta-brass 168d