mercury battery No longer used or manufactured in
the United States due to pollution potential.
mesolytic cleavage Cleavage of a bond in a RADICAL
IONwhereby a RADICALand an ion are formed. The term
reflects the mechanistic duality of the process, which can
be viewed as homolytic or heterolytic, depending on how
the electrons are attributed to the fragments.
See alsoHETEROLYSIS; HOMOLYSIS.
mesomeric effect The effect (on reaction rates, ion-
ization equilibria, etc.) attributed to a substituent due
to overlap of its por pi orbitals with the por pi
orbitals of the rest of the MOLECULAR ENTITY. DELO-
CALIZATION is thereby introduced or extended, and
electronic charge may flow to or from the substituent.
The effect is symbolized by M.
Strictly understood, the mesomeric effect operates
in the ground electronic state of the molecule. When
the molecule undergoes electronic excitation or its
energy is increased on the way to the TRANSITION STATE
of a CHEMICAL REACTION, the mesomeric effect may be
enhanced by the ELECTROMERIC EFFECT, but this term
is not much used, and the mesomeric and electromeric
effects tend to be subsumed in the term RESONANCE
EFFECTof a SUBSTITUENT.
See alsoELECTRONIC EFFECT; FIELD EFFECT; INDUC-
TIVE EFFECT.
mesomerism Essentially synonymous with RESO-
NANCE. The term is particularly associated with the pic-
ture of pi electrons as less localized in an actual molecule
than in a LEWIS FORMULA. The term is intended to imply
that the correct representation of a structure is interme-
diate between two or more Lewis formulae.
See alsoAROMATIC(2); DELOCALIZATION.
mesophase The phase of a liquid crystalline compound
between the crystalline and the isotropic liquid phase.
messenger RNA(mRNA) An RNA molecule that
transfers the coding information for protein synthesis
from the chromosomes to the ribosomes. Fragments of
ribonucleic acid serve as templates for protein synthesis
by carrying genetic information from a strand of DNA
to ribosomes for translation into a protein. The infor-
mation from a particular gene or group of genes is
transferred from a strand of DNA by constructing a
complementary strand of RNA through transcription.
Transfer RNA (tRNA), composed of three nucleotide
segments attached to specific amino acids, correctly
match with a template strand of mRNA, lining up the
correct order of amino acids and bonding them, via
translation in the ribosome with rRNA (ribosomal
RNA), to form a protein.met- A qualifying prefix indicating the oxidized form
of the parent protein, e.g., methemoglobin.metabolism The entire physical and chemical pro-
cesses involved in the maintenance and reproduction of
life in which nutrients are broken down to generate
energy and to give simpler molecules (CATABOLISM) that
can be used to form more complex molecules
(ANABOLISM).
In the case of HETEROTROPHIC ORGANISMs, the
energy evolving from catabolic processes is made avail-
able for use by the organism.
In medicinal chemistry the term metabolismrefers
to the BIOTRANSFORMATIONof XENOBIOTICs and par-
ticularly DRUGS.metabolite Any intermediate or product resulting
from METABOLISM.metal Metals comprise 80 percent of known ele-
ments. Any element below and to the left of the step-
wise division (metalloids) in the upper right corner of
the PERIODIC TABLEof elements.metallic bonding The bonding in metallic elements
and a few other compounds in which the valence elec-
trons are delocalized over a large number of atoms to
produce a large number of molecular orbitals whose
energies are close enough together to be considered to
make up a continuous band rather than discrete energy176 mercury battery