throughout a cyclic array of continuously bonded
atoms. It may be viewed as a reaction proceeding
through a fully conjugated cyclic transition state. The
number of atoms in the cyclic array is usually six, but
other numbers are also possible. The term embraces a
variety of processes, including CYCLOADDITION,
CHELETROPIC REACTION, ELECTROCYCLIC REACTION,
SIGMATROPIC REARRANGEMENT, etc. (provided they are
CONCERTED PROCESSes).
See alsoMULTICENTER REACTION.
period Elements in a horizontal row of the periodic
table.
periodicity With increasing atomic number, the elec-
tron configuration of the atoms displays a periodic
variation.
periodic law When the elements are arranged by
atomic number, their physical and chemical properties
vary periodically. The properties of the elements are
periodic functions of their atomic numbers.
periodic table of elements (periodic chart of ele-
ments) An arrangement of elements in an order of
increasing atomic numbers that also emphasizes period-
icity. SeeAppendix V, Periodic Table of Elements.
periselectivity The differentiation between two sym-
metry-allowed processes, for example the [2+4] vs.
[4+6] CYCLOADDITIONof cyclopentadiene to tropone.
peroxidase A HEMEprotein (donor:hydrogen-perox-
ide OXIDOREDUCTASE, EC class 1.11.1) that catalyzes
the one-electron oxidation of a SUBSTRATEby dihydro-
gen peroxide. Substrates for different peroxidases
include various organic compounds, CYTOCHROMEc,
halides, and Mn2+.
See alsoECNOMENCLATURE FOR ENZYMES.
peroxide A compound containing oxygen in the –1
oxidation state. An example is hydrogen peroxide H 2 O 2.
petroleum A generic name for naturally occurring
hydrocarbons, including crude oil, natural gas liquids,
natural gases, and their products.Pfeiffer’s rule States that in a series of chiral com-
pounds the EUDISMIC RATIOincreases with increasing
POTENCYof the EUTOMER.pharmacokinetics The study of absorption, distribu-
tion, METABOLISM, and excretion (ADME) of bioactive
compounds in a higher organism.pharmacophore (pharmacophoric pattern) The
ensemble of steric and electronic features that is neces-
sary to ensure the optimal supramolecular interactions
with a specific biological target structure and to trigger
(or to block) its biological response.
A pharmacophore does not represent a real
molecule or a real association of functional groups but
is a purely abstract concept that accounts for the com-
mon molecular interaction capacities of a group of
compounds toward their target structure. The pharma-
cophore can be considered to be the largest common
denominator shared by a set of active molecules. This
definition discards a misuse often found in the MEDICI-
NAL CHEMISTRYliterature, which consists of naming as
pharmacophores simple chemical functionalities such
as guanidines, sulfonamides, or dihydroimidazoles
(formerly imidazolines), or typical structural skeletons
such as flavones, phenothiazines, prostaglandins, or
steroids.pharmacophoric descriptors Are used to define a
PHARMACOPHORE, including H-bonding hydrophobic
and electrostatic interaction sites, defined by atoms,
ring centers, and virtual points.phase diagram A graphical representation of the
effects of conditions on the equilibria among the vari-
ous phases that may exist in a system. For a single sub-
stance, this usually represents the variation of melting
point, boiling point, or the temperature at which one
solid phase can be converted to another with pressure.210 period