Dictionary of Chemistry [6th Ed.]

*inhibition, binding of the inhibitor

to an allosteric site inhibits the activ-

ity of the enzyme. In an *allosteric

enzyme, the binding of a regulatory

molecule to the allosteric site

changes the overall shape of the en-

zyme, either enabling the substrate

to bind to the active site or prevent-

ing the binding of the substrate.

allotropy The existence of el-

ements in two or more different

forms (allotropes). In the case of oxy-

gen, there are two forms: ‘normal’

dioxygen (O 2 ) and ozone, or trioxy-

gen (O 3 ). These two allotropes have

different molecular conÜgurations.

More commonly, allotropy occurs be-

cause of different crystal structures

in the solid, and is particularly preva-

lent in groups 14, 15, and 16 of the

periodic table. In some cases, the al-

lotropes are stable over a tempera-

ture range, with a deÜnite transition

point at which one changes into the

other. For instance, tin has two al-

lotropes: white (metallic) tin stable

above 13.2°C and grey (nonmetallic)

tin stable below 13.2°C. This form of

allotropy is called enantiotropy. Car-

bon also has two allotropes – dia-

mond and graphite – although

graphite is the stable form at all tem-

peratures. This form of allotropy, in

which there is no transition tempera-

ture at which the two are in equilib-

rium, is called monotropy. See also

polymorphism.

allowed bandsSee energy bands.

allowed transitionA transition

between two electronic states al-

lowed according to *selection rules

associated with group theory. The

probability of a transition between

states m and n produced by the inter-

action of electromagnetic radiation

with an atomic system is propor-

tional to the square of the magnitude

of the matrix elements of the electric

allotropy 22

a

dipole moment. If this quantity is not

zero, the transition is an allowed

transition; if it is zero the transition

is a *forbidden transition as a dipole

transition. It may, however, be an al-

lowed transition for magnetic dipole

or quadrupole-moment transitions,

which have much smaller transition

probabilities and consequently give

much weaker lines in the spectrum.

alloyA material consisting of two

or more metals (e.g. brass is an alloy

of copper and zinc) or a metal and a

nonmetal (e.g. steel is an alloy of iron

and carbon, sometimes with other

metals included). Alloys may be com-

pounds, *solid solutions, or mixtures

of the components.

alloy steelsSee steel.

allyl alcoholSee propenol.

allyl group See propenyl group.

AlnicoA tradename for a series of

alloys, containing iron, aluminium,

nickel, cobalt, and copper, used to

make permanent magnets.

alpha helix The most common

form of secondary structure in *pro-

teins, in which the polypeptide chain

is coiled into a helix. The helical

structure is held in place by weak hy-

drogen bonds between the N–H and

C=O groups in successive turns of the

helix (see illustration). Compare beta

sheet.

alpha-iron See iron.

alphamethyltryptamine(AMT)A

synthetic derivative of *tyrptamine

NH

C

H 2

NH 2

CH 3

Alphamethyltryptamine