MCD SeeMAGNETIC CIRCULAR DICHROISM.
McMillan, Edwin Mattison (1907–1991) American
Physicist Edwin Mattison McMillan was born on
September 18, 1907, at Redondo Beach, California, the
son of Dr. Edwin Harbaugh McMillan, a physician,
and Anne Marie McMillan (néeMattison). He spent
his early years in Pasadena, California, obtaining his
education.
McMillan attended the California Institute of Tech-
nology, where he received a B.Sc. degree in 1928 and a
M.Sc. degree the following year. He went to Princeton
University for his Ph.D. in 1932.
He attended the University of California at Berke-
ley as a national research fellow working in the field of
molecular beams, in particular the measurement of the
magnetic moment of the proton by a molecular beam
method. He became a member of the team at the radia-
tion laboratory under Professor E.O. Lawrence, study-
ing nuclear reactions and their products and helping
design and construct cyclotrons.
He was a member of the faculty in the department
of physics at Berkeley as an instructor in 1935, an
assistant professor in 1936 and 1941, and a professor
in 1946. In 1940 the creation of element 93, neptu-
nium (symbol Np), was announced by Edwin M.
McMillan and Philip H. Abelson. It was the first ele-
ment heavier than uranium (known as a transuranium
element).
He worked on national defense matters from 1940
to 1945, and during 1945 he helped design the syn-
chrotron and synchrocyclotron. He returned to the
University of California Radiation Laboratory from
1954 to 1958.
In 1951 McMillan and Glenn T. Seaborg received
the Nobel Prize in chemistry “for their discoveries in
the chemistry of the transuranium elements.” He also
received the 1950 Research Corporation Scientific
Award and, in 1963, the Atoms for Peace Award along
with Professor V. I. Veksler. He retired in 1973.
He was married to Elsie Walford Blumer, a daugh-
ter of Dr. George Blumer, dean emeritus of the Yale
Medical School, and they had three children. He died
on September 7, 1991, in El Cerrito, California.
mean lifetime SeeLIFETIME.
mechanism A detailed description of the process
leading from the reactants to the products of a reac-
tion, including a characterization as complete as possi-
ble of the composition, structure, energy, and other
properties of REACTION INTERMEDIATEs, products, and
TRANSITION STATEs. An acceptable mechanism of a
specified reaction (and there may be a number of such
alternative mechanisms not excluded by the evidence)
must be consistent with the reaction stoichiometry, the
RATE LAW, and with all other available experimental
data, such as the stereochemical course of the reaction.
Inferences concerning the electronic motions that
dynamically interconvert successive species along the
REACTION PATH(as represented by curved arrows, for
example) are often included in the description of a
mechanism.
It should be noted that for many reactions, all this
information is not available, and the suggested mecha-
nism is based on incomplete experimental data. It is not
appropriate to use the term mechanismto describe a
statement of the probable sequence in a set of stepwise
reactions. That should be referred to as a reaction
sequence, and not a mechanism.
See alsoGIBBS ENERGY DIAGRAM.mechanism-based inhibition Irreversible INHIBI-
TIONof an enzyme due to its catalysis of the reaction of
an artificial substrate. Also called “suicide inhibition.”mechanoreceptor A specialized sensory receptor
that responds to mechanical stimuli, i.e., tension, pres-
sure, or displacement. Examples include the inner-ear
hair cells, carotid sinus receptors, and muscle spindles.mediator modulator(immune modulator; messen-
ger) An object or substance by which something is
mediated, such as:- A structure of the nervous system that transmits
impulses eliciting a specific response - A chemical substance (transmitter substance) that
induces activity in an excitable tissue, such as nerve
or muscle (e.g., hormones) - A substance released from cells as the result of an
antigen-antibody interaction or by the action of an
antigen with a sensitized lymphocyte (e.g., cytokine)
174 MCD