right-handed circularly polarized light by a material as a
function of the wavelength. Most biological molecules,
including PROTEINs and NUCLEIC ACIDs, are chiral and
show circular dichroism in their ultraviolet absorption
bands, which can be used as an indication of SEC-
ONDARY STRUCTURE. Metal centers that are bound to
such molecules, even if they have no inherent CHIRALITY,
usually exhibit CD in absorption bands associated with
LIGAND-based or ligand-metal CHARGE- TRANSFER TRAN-
SITIONs. CD is frequently used in combination with
absorption and MAGNETIC CIRCULAR DICHROISM(MCD)
studies to assign electronic transitions.
cis In inorganic nomenclature, a structural prefix des-
ignating two groups occupying adjacent positions (not
generally recommended for precise nomenclature pur-
poses of complicated systems).
See alsoTRANS-.
cisplatin cis-Diamminedichloroplatinum(II). An anti-
tumor drug highly effective in the chemotherapy of
many forms of cancer. Of major importance in the anti-
tumor activity of this drug is its interaction with the
NUCLEIC ACIDbases of DNA.
cis-transisomerism Compounds with double bonds,
or alicyclic rings, may exhibit isomerism (a molecule
possessing the same molecular formula but with the
atoms arranged in a different way) due to the attached
groups lying above or below the plane of the double
bond or ring. The ciscompound has the groups on the
same side of the bond, while the transhas the groups on
the opposite sides. The different ISOMERs have different
physical and chemical properties.
class (a) metal ion A metal ion that combines pref-
erentially with LIGANDs containing ligating atoms that
are the lightest of their periodic group.
See alsoCLASS(B) METAL ION; HARD ACID.
class (b) metal ion A metal ion that combines pref-
erentially with LIGANDs containing ligating atoms other
than the lightest of their periodic group.
See alsoCLASS(A) METAL ION; HARD ACID.
clathrate SeeHOST; INCLUSION COMPOUND.Clausius-Clapeyron equation The differential
equation relating pressure of a substance to tempera-
ture in a system in which two phases of the substance
are in equilibrium. Also referred to as Clapeyron equa-
tion, or Clapeyron-Clausius equation.clay A very fine-grained soil that is plastic when wet
but hard when fired. Typical clays consists of silicate
and aluminosilicate minerals that are the products of
weathering reactions of other minerals, but the term is
also used to refer to any mineral of very small particle
size.clone A population of organisms, cells, viruses, or
DNA molecules that is derived from the replication of a
single genetic progenitor. In the case of B cells, each B
cell has a typical IMMUNOGLOBULIN(Ig), and so all the
cells that descend from one B cell (the clone) have the
same Ig. Typically, a cancer is a clone of cells. Some-
times the term clone is also used for a number of
recombinant DNA molecules that are all carrying the
same inserted SEQUENCE.close packing The structure of compounds based on
the stacking of spheres in arrangements, where the
occupied volume of the structure is maximized.cloud chamber An instrument or chamber filled
with a supersaturated vapor and designed for observing
the paths of speeding ionizing particles, which appear
as a trail of condensed liquid droplets as they pass
through. A related device is a bubble chamber, which
uses a liquid close to its boiling point and leaves a trail
of bubbles to reveal the path of the ionizing particle.cluster A number of metal centers grouped close
together that can have direct metal-bonding interactions
or interactions through a BRIDGING LIGAND, but that are
not necessarily held together by these interactions.
Examples can be found under the entries [2FE-2S],52 cis