nuclear decay Disintegration of atomic nuclei that
results in the emission of ALPHAor BETA PARTICLEs
(usually with gamma radiation).
nuclear fission The process of splitting nuclei with
high mass number by a variety of processes (usually
involving neutrons) into two nuclei of smaller mass (usu-
ally radioactive) and releasing energy and more neutrons.
nuclearity The number of CENTRAL ATOMs joined in
a single COORDINATIONentity by BRIDGING LIGANDs or
metal-metal bonds is indicated by dinuclear, trinuclear,
tetranuclear, polynuclear, etc.
nuclear magnetic resonance spectroscopy(NMR
spectroscopy) NMR spectroscopy makes it possible to
discriminate nuclei, typically protons, in different chemi-
cal environments. The electron distribution gives rise to
a chemical shift of the resonance frequency. The chemi-
cal shift, δ, of a nucleus is expressed in parts per million
(ppm) by its frequency, νn, relative to a standard, νref,
and is defined as δ= 10^6 (νn–νref)/νo, where νois the
operating frequency of the spectrometer. It is an indica-
tion of the chemical state of the group containing the
nucleus. More information is derived from the SPIN-SPIN
COUPLINGs between nuclei, which give rise to multiple
patterns. Greater detail can be derived from two- or
three-dimensional techniques. These use pulses of radia-
tion at different nuclear frequencies, after which the
response of the spin system is recorded as a free-induc-
tion decay (FID). Multidimensional techniques, such as
COSY (correlated spectroscopy) and NOESY (nuclear
overhauser effect [NOE] spectroscopy), make it possible
to deduce the structure of a relatively complex molecule
such as a small protein (molecular weight up to 25,000).
In proteins containing PARAMAGNETIC centers, nuclear
HYPERFINEinteractions can give rise to relatively large
shifts of resonant frequencies, known as contact and
pseudo-contact (dipolar) shifts, and considerable
increases in the nuclear spin relaxation rates. From this
type of measurement, structural information can be
obtained about the paramagnetic site.
nuclear radiation The radiation emitted during the
spontaneous decay of an unstable atomic nucleus.
nuclear reaction Any reaction involving a change in
the nucleus of an atom. For example, reaction between
neutron, proton, or nucleus from a reactor or particle
accelerator and a target nucleus resulting in the pro-
duction of product nuclides, gamma rays, particles, and
other radiations.nuclear reactor A system where a fission chain reac-
tion can be initiated, maintained, and controlled.nucleation The process by which nuclei are formed;
defined as the smallest solid-phase aggregate of atoms,
molecules, or ions that is formed during a precipitation
and that is capable of spontaneous growth.nucleic acids Macromolecules composed of SEQUENCEs
of NUCLEOTIDEs that perform several functions in living
cells, e.g., the storage of genetic information and its
transfer from one generation to the next (DNA), and the
EXPRESSION of this information in protein synthesis
(mRNA, tRNA). They may act as functional compo-
nents of subcellular units such as RIBOSOMEs (rRNA).
RNA contains D-ribose; DNA contains 2-deoxy-D-
ribose as the sugar component. Currently, synthetic
nucleic acids can be made consisting of hundreds of
NUCLEOTIDEs.nucleic acids 195Nuclear reactor. A system where a fission chain reaction can be
initiated, maintained, and controlled