relates to 0, p= 1, d= 2, and f= 3. The p(principal)
orbitals are said to be dumbbell shaped.
porins A class of proteins that create water-filled
channels across cell membranes.
porphyrin A macrocyclic molecule that contains four
pyrrole rings linked together by single carbon atom
bridges between the alpha positions of the pyrrole
rings. Porphyrins usually occur in their dianionic form
coordinated to a metal ion.
See alsoCOORDINATION.positron A particle equal in mass, but opposite in
charge, to the electron (a positive electron). Also
referred to as a β+particle when emitted in a nuclear
reaction.potency A comparative rather than an absolute
expression of drug activity. Drug potency depends on
both AFFINITYand EFFICACY. Thus, two AGONISTs can
be equipotent but have different intrinsic efficacies with
compensating differences in affinity. Potency is the dose
of DRUGrequired to produce a specific effect of given
intensity as compared with a standard reference.potential energy Stored energy that can be released
or harnessed to do work.potential-energy profile A curve describing the vari-
ation of the potential energy of the system of atoms
that make up the reactants and products of a reaction
as a function of one geometric coordinate, and corre-
sponding to the “energetically easiest passage” from
reactants to products (i.e., along the line produced by
joining the paths of steepest descent from the TRANSI-
TION STATEto the reactants and to the products). For
an ELEMENTARY REACTION, the relevant geometric
coordinate is the REACTION COORDINATE; for a STEP-
WISE REACTION, it is the succession of reaction coordi-
nates for the successive individual reaction steps. (The
reaction coordinate is sometimes approximated by a
quasichemical index of reaction progress, such as
“degree of atom transfer” or BOND ORDER of some
specified bond.)
See alsoPOTENTIAL-ENERGY(REACTION) SURFACE;
GIBBS ENERGY DIAGRAM.potential-energy (reaction) surface A geometric
hypersurface on which the potential energy of a set of
reactants is plotted as a function of the coordinates
representing the molecular geometries of the system.
For simple systems, two such coordinates (charac-
terizing two variables that change during the progress
from reactants to products) can be selected, and the
potential energy plotted as a contour map.218 porins
P orbital. An electron with one unit of angular momentum