Dictionary of Chemistry [6th Ed.]

CH 3 (CH 2 ) 9 CH:CH 2 , obtained from pe-
troleum and used in making *dode-
cylbenzene.

dodecylbenzene A hydrocarbon,
CH 3 (CH 2 ) 11 C 6 H 5 , manufactured by a
Friedel–Crafts reaction between do-
decene (CH 3 (CH 2 ) 9 CH:CH 2 ) and ben-
zene. It can be sulphonated, and the
sodium salt of the sulphonic acid is
the basis of common *detergents.

dolomite A carbonate mineral con-
sisting of a mixed calcium–magne-
sium carbonate, CaCO 3 .MgCO 3 ,
crystallizing in the rhombohedral
system. It is usually white or colour-
less. The term is also used to denote
a rock with a high ratio of magne-
sium to calcium carbonate. See lime-
stone.

domainA functional unit of the
tertiary structure of a protein. It
consists of chains of amino acids
folded into alpha helices and beta
sheets to form a globular structure.
Different domains are linked to-
gether by relatively straight sections
of polypeptide chain to form the pro-
tein molecule. Domains allow a de-
gree of movement in the protein
structure.

Donnan equilibriumThe equilib-
rium set up when two solutions are
separated by a membrane permeable
to some but not all of the ions in the
solutions. In practice, the membrane
is often permeable to the solvent and
small ions but not to charged entities
of colloidal size or such polyelec-
trolytes as proteins. An electrical po-
tential develops between the two
sides of the membrane with the two
solutions having varying osmotic
pressure. Donnan equilibrium is
named after the British chemist Fred-
erick George Donnan (1870–1956),
who developed the theory of mem-
brane equilibrium. Donnan equilib-
rium is important in biology.

donorAn atom, ion, or molecule

that provides a pair of electrons in

forming a coordinate bond.

dopa(dihydroxyphenylalanine) A

derivative of the amino acid tyrosine.

It is found in particularly high levels

in the adrenal glands and is a precur-

sor in the synthesis of *dopamine,

*noradrenaline, and *adrenaline.

The laevorotatory form, L-dopa, is ad-

ministered in the treatment of

Parkinson’s disease, in which brain

levels of dopamine are reduced.

185 dose

d

C

H 2

NH 2

C

H 2

OH

OH

OH

Dopamine

OH

OH

C

H 2

CH 2

NH 2

Dopa

dopamineA *catecholamine that

is a precursor in the synthesis of *no-

radrenaline and *adrenaline. It also

functions as a neurotransmitter in

the brain.

d-orbitalSee orbital.

doseA measure of the extent to

which matter has been exposed to

*ionizing radiation. The absorbed

doseis the energy per unit mass ab-

sorbed by matter as a result of such

exposure. The SI unit is the gray, al-

though it is often measured in rads (1

rad = 0.01 gray; see radiation units).

The maximum permissible dose is the

recommended upper limit of ab-

sorbed dose that a person or organ

should receive in a speciÜed period