state (usually judged relative to kBT,the product of the
Boltzmann constant kB, and the absolute temperature
T). The states of those biradicals whose radical centers
interact particularly weakly are most easily understood
in terms of a pair of local doublets.
Theoretical descriptions of low-energy states of
biradicals display the presence of two unsaturated
valences (biradicals contain one fewer bond than per-
mitted by the rules of valence): the dominant valence
bond structures have two dots; the low-energy molecu-
lar orbital CONFIGURATIONs have only two electrons in
two approximately nonbonding molecular orbitals;
two of the natural orbitals have occupancies close to
one; etc.
The term is synonymous with “diradical.”
black smoker Chimneylike accumulations of mineral
deposits found at hydrothermal vents.
bleomycin(BLM) A glycopeptide molecule that can
serve as a metal chelating ligand. The Fe(III) complex
of bleomycin is an antitumor agent, and its activity is
associated with DNA cleavage.
BLM SeeBLEOMYCIN.
blood An animal fluid that transports oxygen from
the lungs to body tissues and returns carbon dioxide
from body tissues to the lungs through a network of
vessels such as veins, arteries, and capillaries. It trans-
ports nourishment from digestion, hormones from
glands, disease-fighting substances to tissues, as well as
wastes to the kidneys. Blood contains red and white
blood cells and platelets that are responsible for a vari-
ety of functions from transporting substances to fight-
ing invasion from foreign substances. Some 55 percent
of blood is a clear liquid called plasma. The average
adult has about five liters of blood.
blotting A technique used for transferring DNA,
RNA, or protein from gels to a suitable binding matrix,
such as nitrocellulose or nylon paper, while maintain-
ing the same physical separation.
blue copper protein An ELECTRON-TRANSFER PRO-
TEINcontaining a TYPE 1 COPPERsite. Characterized
by a strong absorption in the visible region and an
EPR signal with an unusually small hyperfine cou-
pling to the copper nucleus. Both characteristics are
attributed to COORDINATIONof the copper by a cys-
teine sulfur.
See also ELECTRON PARAMAGNETIC RESONANCE
SPECTROSCOPY.Bodenstein approximation SeeSTEADY STATE.body-centered unit cell A structure in which every
atom is surrounded by eight adjacent atoms, regardless
of whether the atom is located at a corner or at the cen-
ter of a unit cell.30 black smoker
Computer artwork of the lattice of a body-centered cubic crystal
over water. This structure is adopted by the metals lithium,
sodium, potassium, and iron below 906°C. The lattice has a single
atom at the center and another eight on the corners of a cube, and
the pattern is repeated again and again to form a crystal. It is a
relatively open structure, utilizing about 68 percent of the avail-
able space. Metals that form this structure cannot use a lattice
with less space because the thermal vibrations of the atoms are
able to overcome the relatively weak binding forces between
them.(Courtesy of Laguna Design/Science Photo Library)