a BRONSTED ACID). The term is preferred to the syn-
onym proticor the more ambiguous expression acidic
by itself. Also called HBD (hydrogen-bond donor)
solvent.
See alsoPROTOPHILIC SOLVENT.
protolysis This term has been used synonymously
with PROTON(HYDRON)-TRANSFER REACTION. Because
of its misleading similarity to HYDROLYSIS, PHOTOLYSIS,
etc., its use is discouraged.
See alsoAUTOPROTOLYSIS.
proton A subatomic particle having a mass of 1.0073
amu and a charge of +1, found in the nuclei of atoms.
The nucleus of normal hydrogen is made up of a pro-
ton; thus the ionized form of normal hydrogen is often
called a proton.
proton affinity The negative of the enthalpy change
in the gas-phase reaction (real or hypothetical) between
a PROTON (more appropriately HYDRON) and the
CHEMICAL SPECIES concerned, usually an electrically
neutral species to give the CONJUGATE ACIDof that
species. Proton affinity is often, but unofficially, abbre-
viated as PA.
See alsoGAS-PHASE BASICITY.
protonation The addition of a proton to an atom,
molecule, or ion.
proton motive force Energy or force created by the
transfer of protons (hydrogen ions) on one side only of
a cell membrane and across the membrane during
chemiosmosis; an electrochemical gradient that has
potential energy. This force can be channeled to oper-
ate rotating flagella, generate ATP, and other needed
activities.
proton pump Proton pumps are a type of active
transport and use the energy of ATPhydrolysis to force
the transport of protons out of the cell and, in the pro-
cess, create a membrane potential.
proton-transfer reaction A CHEMICAL REACTION,
the main feature of which is the INTERMOLECULARor
INTRAMOLECULARtransfer of a proton (HYDRON) from
one BINDING SITEto another. For example,
CH 3 CO 2 H + (CH 3 ) 2 CO →CH 3 CO 2 – +
(CH 3 ) 2 CO+H
In the detailed description of proton-transfer reactions,
especially of rapid proton transfers between electroneg-
ative atoms, it should always be specified whether the
term is used to refer to the overall process (including
the more-or-less ENCOUNTER-CONTROLLED formation
of a hydrogen-bonded complex and the separation of
the products [seeMICROSCOPIC DIFFUSION CONTROL])
or just to the proton-transfer event (including solvent
rearrangement) by itself.
See also AUTOPROTOLYSIS; PROTOLYSIS; TAU-
TOMERISM.protophilic solvent Capable of acting as proton
acceptor, strongly or weakly basic (as a BRONSTED
BASE). Also called HBA (hydrogen-bond acceptor)
solvent.
See alsoPROTOGENIC SOLVENT.protoplasm The living material within cells.protoporphyrin IX The PORPHYRIN LIGAND of
HEMEb. Heme b is a Fe(II) porphyrin complex readily
isolated from the hemoglobin of beef blood, but it is
also found in other proteins, including other
HEMOGLOBINs, MYOGLOBINs, CYTOCHROMEs P-450,
CATALASEs, and PEROXIDASEs, as well as b-type
cytochromes. Protoporphyrin IX contains four methyl
groups in positions 2, 7, 12, and 18; two vinyl groups
in positions 3 and 8; and two propionic acid groups in
positions 13 and 17.prototropic rearrangement(prototropy) See TAU-
TOMERISM.pseudobinary ionic compounds Compounds con-
taining more than two elements but that are named like222 protolysis