ion-selective electrode An electrode, the potential of
which depends on the concentration of a specific ion
with which it is in contact. An example is the glass elec-
trode that is used to measure hydrogen ion concentra-
tions (pH). An ion-selective electrode is used in a wide
variety of applications for determining the concentra-
tions of various ions in aqueous solutions.
ipso attack The attachment of an entering group to
a position in an aromatic compound already carrying
a SUBSTITUENT group (other than hydrogen). The
entering group may displace that substituent group
but may also itself be expelled or migrate to a differ-
ent position in a subsequent step. Ipso is a position on
a phenyl ring. The term ipso substitutionis not used,
since it is synonymous with substitution.
For example:
where E+is an ELECTROPHILEand Z is a substituent
(other than hydrogen).
See alsoCINE-SUBSTITUTION; TELE-SUBSTITUTION.
iron-responsive element A specific base SEQUENCE
in certain MESSENGER RNAs that code for various pro-
teins of iron METABOLISM, which allows REGULATIONat
translational level by the IRON-RESPONSIVE PROTEIN.
iron-responsive protein (IRP) A protein that
responds to the level of iron in the cell and regulates
the biosynthesis of proteins of iron METABOLISM by
binding to the IRON-RESPONSIVE ELEMENTon MESSEN-
GER RNA.
See alsoREGULATION.
iron-sulfur cluster A unit comprising two or more
iron atoms and BRIDGINGsulfide LIGANDs in an IRON-
SULFUR PROTEIN. The recommended designation of a
CLUSTER consists of the iron and sulfide content in
square brackets, for example [2Fe-2S], [3Fe-4S]. The
possible oxidation levels are indicated by the net charge
excluding the ligands, for example a [4Fe-4S]2+; [4Fe-
4S]1+(or [4Fe-4S]2+;1+) cluster.iron-sulfur protein Proteins in which non-HEME
iron is coordinated with cysteine sulfur and, usually,
with inorganic sulfur. Divided into three major cate-
gories: RUBREDOXINs; simple iron-sulfur proteins, con-
taining only IRON-SULFUR CLUSTERs; and complex
iron-sulfur proteins, containing additional active redox
centers such as FLAVIN, molybdenum, or heme. In most
iron-sulfur proteins, the clusters function as ELECTRON
TRANSFERgroups, but in others they have other func-
tions such as catalysis of hydratase/dehydratase reac-
tions, maintenance of protein structure, or REGULATION
of activity.
See alsoCOORDINATION.IRP SeeIRON-RESPONSIVE PROTEIN.ischemia Local deficiency of blood supply and dioxy-
gen to an organ or tissue owing to constriction of the
blood vessels or to an obstruction.isobacteriochlorin (2,3,7,8-Tetrahydroporphyrin)
A reduced PORPHYRINwith two pairs of confused satu-
rated carbon atoms (C-2, C-3 and C-7, C-8) in two of
the pyrrole rings.
See alsoCHLOROPHYLL.isochore A line or surface of constant volume on a
graphical representation of a physical system. A con-
tour line that corresponds to values measured at identi-
cal volumes.isodesmic reaction A reaction (actual or hypotheti-
cal) in which the types of bonds that are made in form-
ing the products are the same as those that are broken
in the reactants, e.g.,
PhCOOH + p-ClC 6 H 4 COO–→PhCOO–+ p-
ClC 6 H 4 COOH
ClCHCH 2 + ClCH 2 CH 2 Cl →CH 2 CH 2 +
Cl 2 CHCH 2 Clisodesmic reaction 145