Encyclopedia of Chemistry

Concerning mediators of immediate hypersensitiv-
ity, the most important include histamine, leukotriene
e.g., SRS-A (slow-reacting substance of anaphylaxis,
ECF-A (eosinophil chemotactic factor of anaphylaxis),
PAF (platelet-activating factor), and serotonin. There
are also three classes of lipid mediators that are syn-
thesized by activated mast cells through reactions ini-
tiated by the actions of phospholipase A2. These are
prostaglandins, leukotrienes, and platelet-activating
factors (PAF).

medicinal chemistry A chemistry-based discipline,
also involving aspects of biological, medical, and phar-
maceutical sciences. It is concerned with the invention,
discovery, design, identification, and preparation of
biologically active compounds; the study of their
METABOLISM; the interpretation of their mode of action
at the molecular level; and the construction of STRUC-
TURE-ACTIVITY RELATIONSHIPs.

medium The phase (and composition of the phase) in
which CHEMICAL SPECIESand their reactions are studied
in a particular investigation.

megapascal (MPa) A unit of pressure. 1 MPa =
1,000,000 Pa (pascals); 1 megapascal (MPa) = 10 bar; 1
bar is approximately equal to 1 atmosphere of pressure.

Meisenheimer adduct A cyclohexadienyl derivative
formed as LEWIS ADDUCTfrom a NUCLEOPHILE(LEWIS
BASE) and an AROMATICor heteroaromatic compound,
also called Jackson-Meisenheimer adduct. In earlier
usage the term Meisenheimer complexwas restricted to
the typical Meisenheimer alkoxide ADDUCTs of nitro-
substituted aromatic ethers, for example

Analogous cationic adducts, such as

are considered to be reaction INTERMEDIATESin ELEC-

TROPHILIC aromatic SUBSTITUTION REACTIONs, are

called “Wheland intermediates” and sometimes, inap-

propriately, σ-complexes.

See alsoCHEMICAL REACTION; SIGMA(σ) ADDUCT.

melting point The temperature when matter is con-

verted from solid to liquid.

melting point(corrected/uncorrected) The term

originally signified that a correction was made (not

made) for the emergent stem of the thermometer. In

current usage, it often means that the accuracy of the

thermometer was (was not) verified. This current usage

is inappropriate and should be abandoned.

membrane potential The difference in electrical

charge (voltage difference) across the cell membrane

due to a slight excess of positive ions on one side and

of negative ions on the other; the potential inside a

membrane minus the potential outside. A typical

membrane potential is –60 mV, where the inside is

negative relative to the surrounding fluid, and resting

membrane potentials are typically found between –40

and –100 mV.

meniscus The curvature of the surface of a liquid in

a vessel at the interface of the liquid with the con-

tainer wall. If the attractive forces between the

molecules of the liquid and the wall are greater than

those between the molecules of the liquid itself, the

meniscus curves up, and the surface is “wet” by the

liquid. The reverse causes the meniscus to curve down

(nonwetting).

See alsoVAN DERWAALS FORCES.

Menkes disease A sex-linked inherited disorder,

causing defective gastrointestinal absorption of copper

and resulting in copper deficiency early in infancy.

Menkes disease 175