excited state State of a system with energy higher
than that of the GROUND STATE. This term is most com-
monly used to characterize a molecule in one of its elec-
tronically excited states, but can also refer to
vibrational and/or rotational excitation in the elec-
tronic ground state.
exclusion principle No two electrons existing in an
atom can have the same four quantum numbers.
exegetic reaction A spontaneous reaction in which
energy flows out of the system; a decrease in free
energy; a reaction that liberates heat.
exogenous Originating externally. In the context of
metalloprotein LIGANDs, exogenousdescribes ligands
added from an external source, such as CO or O 2.
exon A section of DNA that carries the coding
SEQUENCEfor a protein or part of it. Exons are sepa-
rated by intervening, noncoding sequences (called
INTRONs). In EUKARYOTEs, most GENEs consist of a
number of exons.
exosphere The uppermost layer of the atmosphere,
its lower boundary is estimated at 500 km to 1,000 km
above the Earth’s surface. It is only from the exosphere
that atmospheric gases can, to any appreciable extent,
escape into outer space.
exothermic A reaction that produces heat and
absorbs heat from the surroundings.
See alsoENDOTHERMIC.
exotoxin A toxic substance produced by bacteria
and then released outside its cell into its environment.
expression The cellular production of the protein
encoded by a particular GENE. The process includes
TRANSCRIPTION of DNA, processing of the resulting
mRNA product and its TRANSLATIONinto an active
protein. A recombinant gene inserted into a host cell by
means of a vector is said to be expressed if the synthe-
sis of the encoded polypeptide can be demonstrated.
For the expression of metalloproteins, other gene prod-
ucts will usually be required.extended Hammett equation This term applies in a
general way to any multiparametric extension of the
HAMMETT EQUATION. It is sometimes used specifically
for a form of DUAL SUBSTITUENT-PARAMETER EQUATION
in which the actual value of the correlated property P
under the influence of the substituent Xis used, rather
than the value relative to that for X= H.An intercept
term hcorresponding to the value of Pfor X= His
introduced, for example
P= ασI+ βσR+ h
The equation can be applied to systems for which the
inclusion of further terms to represent other effects,
e.g., steric, is appropriate.extended X-ray absorption fine structure(EXAFS)
EXAFS effects arise because of electron scattering by
atoms surrounding a particular atom of interest as that
special atom absorbs X rays and emits electrons. The
atom of interest absorbs photons at a characteristic
wavelength, and the emitted electrons, undergoing con-
structive or destructive interference as they are scat-
tered by the surrounding atoms, modulate the102 excited state
Exothermic. A reaction that produces heat and absorbs heat from
the surroundings