Encyclopedia of Chemistry

chloroplast The double-membrane organelle of
eukaryotic photosynthesis; contains enzymes and pig-
ments that perform photosynthesis.
See alsoEUKARYOTE.

cholesterol A soft, waxy, fat-soluble steroid formed by
the liver and a natural component of fats in the blood-
stream (as LIPOPROTEINs); the most common steroid in
the human body and used by all cells in permeability of
their membranes. It is used in the formation of many
products such as bile acids, vitamin D, progesterone,
estrogens, and androgens. In relation to human health,
there is “good,” high-density cholesterol (HDL), which
protects the heart, and “bad,” low-density cholesterol
(LDL), which causes heart disease and other problems.

chromatin The combination of DNA and proteins
that make up the chromosomes of EUKARYOTEs. Exists
as long, thin fibers when cells are not dividing; not visi-
ble until cell division takes place.

chromatography A method of chemical analysis
where compounds are separated by passing a mixture
in a suitable carrier over an absorbent material. Com-
pounds with different absorption coefficients move at
different rates and are separated.
See also GAS CHROMATOGRAPHY; HIGH-PERFOR-
MANCE LIQUID CHROMATOGRAPHY.

chromophore That part of a molecular entity con-
sisting of an atom or group of atoms in which the elec-
tronic transition responsible for a given spectral band is
approximately localized.

chromosome The self-replicating GENE-carrying
member found in the CELLnucleus and composed of a
DNA molecule and proteins (chromatin). Prokaryote
organisms contain only one chromosome (circular
DNA), while EUKARYOTEs contain numerous chromo-
somes that comprise a genome. Chromosomes are
divided into functional units called genes, each of
which contains the genetic code (instructions) for mak-
ing a specific protein.
See alsoNUCLEUS.

CIDNP(chemically induced dynamic nuclear polar-

ization) Non-Boltzmann nuclear spin state distribu-

tion produced in thermal or photochemical reactions,

usually from COLLIGATIONand DIFFUSION, or DISPRO-

PORTIONATION of RADICAL PAIRs, and detected by

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY by

enhanced absorption or emission signals.

cine-substitution A SUBSTITUTION REACTION(gener-

ally AROMATIC) in which the entering group takes up a

position adjacent to that occupied by the LEAVING

GROUP. For example,

See alsoTELE-SUBSTITUTION.

circular dichroism(CD) A spectroscopic method

that measures the difference in absorbance of left- and

circular dichroism 51

Series of chromatograms showing the separation of black ink.

Chromatography is an analytical process, which separates a com-

pound into its constituent chemicals. Chromatography paper is

dipped vertically in a solvent with the ink painted on it (left). Capil-

lary action draws the solvent up through the paper (center) and

dissolves the ink. As the solvent travels up the paper, it takes the

various chemicals in the ink with it, separating them into a series

of colored bands.(Courtesy of Andrew Lambert Photography/

Science Photo Library)