RADICALs as intermediates by virtue of their ability to
act as SCAVENGERs toward these radicals.
If the rate of a reaction in the absence of inhibitor
is voand that in the presence of a certain amount of
inhibitor is v,the degree of inhibition (i)is given by
i= (vo–v)/vo
See alsoMECHANISM-BASED INHIBITION.inhibitor A substance that decreases the rate of catal-
ysis by an ENZYMEor of some other chemical reaction.
inhibitory catalyst One that decreases the rate of
reaction.
inhibitory postsynaptic potential(IPSP) A small
electrical charge of a few millivolts creating a local
hyperpolarization (increase in membrane potential on
the negativity of the inside of the neuron) in the mem-
brane of a postsynaptic neuron. Occurs when an
inhibitory neurotransmitter from a presynaptic cell
binds to a postsynaptic receptor; makes it difficult for a
postsynaptic neuron to generate an action potential.
initiation A reaction or process generating free RADI-
CALs (or some other REACTIVEreaction intermediates),
which then induce a CHAIN REACTION. For example, in
the chlorination of alkanes by a radical mechanism, the
initiation step is the DISSOCIATIONof molecular chlorine.
inner-sphere electron transfer Historically, an elec-
tron transfer between two metal centers sharing a lig-
and or atom in their respective coordination shells. The
definition has more recently been extended to any situ-
ation in which the interaction between the donor and
acceptor centers in the TRANSITION STATEis significant
(>20 kJ mol–1).
See alsoOUTER-SPHERE ELECTRON TRANSFER.
inorganic chemistry The study of inorganic com-
pounds and their structure, reactions, catalysis, and
mechanism of action.
insertion A CHEMICAL REACTIONor TRANSFORMA-
TIONof the general type
X–Z + Y →X–Y–Z
in which the connecting atom or GROUPY replaces the
bond joining the parts X and Z of the reactant XZ. An
example is the CARBENEinsertion reaction
R 3 C–H + H 2 C: →R 3 C–CH 3
The reverse of an insertion is called an EXTRUSION.
See alsoALPHA(α) ADDITION.insertion reaction A chemical reaction or transfor-
mation of the general type X–Z + Y →X–Y–Z in
which the connecting atom or group Y replaces the
bond joining the parts X and Z of the reactant XZ.insoluble compound One that does not dissolve to a
significant extent in the specified solvent.insulator A material that poorly conducts heat or
electricity.insulin A protein hormone produced in the pan-
creas by beta cells, located in the islets of Langerhans,
that stimulates cellular utilization of glucose by body
cells by converting glucose and other carbohydrates to
energy, and that helps control blood sugar levels by
acting antagonistically with glucagons, the chief
source of stored fuel in the liver. It is released by vari-
ous signals that are sensitive to the intake and diges-
tion of food. It also acts as an important regulator of
protein and lipid metabolism. Insulin is used as a drug
to control insulin-dependent diabetes mellitus, a disor-
der that is caused by the insufficient production of
insulin. Without insulin, cells do not absorb glucose.
Diabetic individuals may have type I diabetes (juve-
nile), comprising about 10 percent of the population,
or type II diabetes (adult). Presently some 16 million
Americans have diabetes, with 1,700 new cases being
diagnosed daily. Diabetes has been linked to the de-
velopment of a variety of diseases, including heart
disease, stroke, peripheral vascular disease, and neu-
rological disorders.140 inhibitor