Dictionary of Chemistry [6th Ed.]

adsorbed atom is removed as an ion

by a pulse of potential difference.

The atom then passes in the same di-

rection as the gas ions, through the

hole in the screen. This enables the

atom to be identiÜed by a mass spec-

trometer behind the screen. Atom-

probe FIM identiÜes both the type

and the position of the atom and can

be used to observe atomic processes,

such as evaporation, with the pulse

used for analysis lasting about 2

nanoseconds.

ATP (adenosine triphosphate)A nu-

cleotide that is of fundamental im-

portance as a carrier of chemical

energy in all living organisms. It con-

sists of adenine linked to d-ribose (i.e.

adenosine); the d-ribose component

bears three phosphate groups, lin-

early linked together by covalent

bonds (see formula). These bonds can

undergo hydrolysis to yield either a

molecule of ADP(adenosine diphos-

phate) and inorganic phosphate or a

molecule of AMP(adenosine mono-

phosphate) and pyrophosphate. Both

these reactions yield a large amount

of energy (about 30.6 kJ mol–1) that is

used to bring about such biological

processes as muscle contraction, the

active transport of ions and mol-

ecules across cell membranes, and

the synthesis of biomolecules. The

reactions bringing about these pro-

cesses often involve the enzyme-

catalysed transfer of the phosphate

group to intermediate substrates.

Most ATP-mediated reactions require

Mg2+ions as *cofactors.

ATP is regenerated by the rephos-

phorylation of AMP and ADP using

the chemical energy obtained from

the oxidation of food. This takes

place during *glycolysis and the

*Krebs cycle but, most signiÜcantly,

is also a result of the reduction–

oxidation reactions of the *electron

transport chain, which ultimately re-

duces molecular oxygen to water (ox-

idative phosphorylation).

atropine A poisonous crystalline al-

kaloid, C 17 H 23 NO 3 ; m.p. 118–119°C. It

can be extracted from deadly night-

shade and other solanaceous plants

and is used in medicine to treat colic,

to reduce secretions, and to dilate

the pupil of the eye.

atropisomers Conformers that

have highly restricted rotation about

a single bond and can consequently

be separated as distinct species. This

can occur, for example, in the case of

substituted biphenyls with large

groups at the orthopositions of the

rings.

ATRS See attenuated total re-

flectance spectroscopy.

attenuated total reÛectance

spectroscopy (ATRS) A variation of

infrared spectroscopy in which the IR

source is reÛected from the sample

ATP 50

a

• OO

O–

O

P

O–

O

P

O–

O

O P O CH 2

O

OH OH

ribose

N

N

N

N

NH 2

adenine

ATP