Chirality in Biology
581
Isomer
Compounds having the same molecular formula but differing in the nature or
sequence of bonding of their atoms, or in the spatial arrangement of their atoms.
Optical Activity
The property, possessed by some substances (or solutions of some substances) of
rotating the plane of plane-polarized light. For a chiral substance, one enantiomer will
produce dextrorotation, (
+
), and the other, levorotation (
).
Prochiral (Prostereoisomerism)
Describing a molecule that yields a chiral structure by replacement of one of two
chemically like groups by a different group. Thus prochiral Caabc yields chiral Cabcd
by replacement of one ‘‘a’’ by ‘‘d’’.
Racemic
A mixture of equal amounts of two enantiomers of a compound; this mixture is
without optical activity. Hence, racemization, the conversion of an enantiomer to a
racemic mixture, is characterized by the time-dependent loss of optical activity.
R/S Notation
An unambiguous method to specify con±guration of chiral compounds on the basis of
the ordering of atoms or groups of atoms (by a sequence rule) when the molecular
model is viewed from a de±ned direction.
Stereoisomer
Isomers differing only in the spatial arrangement of their atoms.
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An object, structure, or molecule for which the image in a plane mirror cannot
be superposed on the original is chiral; for example, the two human hands.
Chiral structures or growth forms are common in nature and in manufactured
objects. A typical chiral molecule contains a carbon atom linked to four different
groups, Cabcd. Such molecules exist in two forms of opposite handedness that are
termed
enantiomers
. Most metabolites are chiral, and enzymes usually exhibit a high
degree of speci±city toward enantiomers. A molecule containing two chemically
like ‘‘a’’ groups, Caabc, is described as prochiral; replacement of one ‘‘a’’ group
by ‘‘d’’ leads to a chiral molecule, Cabcd. Enzymes usually discriminate between
the two chemically like groups in a prochiral molecule. Enantiomers can have very
different physiological actions, for example, taste or odor. With chiral pharmaceutical
agents, the enantiomers may also have different physiological actions. Physiological
differences arise from the phenomenon of chiral recognition – the ability of biological
receptors, enzymes, and some catalysts and reagents to discriminate between the
enantiomers of a given compound. Although many hypotheses account for the
homochirality observed on planet Earth, the original process or processes leading to
this situation remain unclear.
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