macromolecule A large molecule of high molecular
mass composed of more than 100 repeated monomers
(single chemical units of lower relative mass); a poly-
mer. DNA, proteins, and polysaccharides are examples
of macromolecules in living systems; a large complex
molecule formed from many simpler molecules.
macrophage A type of blood cell that is able to
ingest a wide variety of particulate materials. They are
a type of PHAGOCYTE.
macroscopic diffusion control See MIXING CON-
TROL.
Madelung constant A term that accounts for the
particular structure of an ionic crystal when the lattice
energy is evaluated from the coulombic interactions.
The value is different for each crystalline structure.
magic acid SeeSUPERACID.
magnetic circular dichroism (MCD) A measure-
ment of CIRCULAR DICHROISMof a material that is
induced by a magnetic field applied parallel to the
direction of the measuring light beam. Materials that
are achiral still exhibit MCD (the Faraday effect),
since the magnetic field leads to the lifting of the
degeneracy of electronic orbital and spin states and to
the mixing of electronic states. MCD is frequently used
in combination with absorption and CD studies to
effect electronic assignments. The three contributions
to the MCD spectrum are the A-term, due to Zeeman
splitting of the ground and/or excited degenerate
states; the B-term, due to field-induced mixing of
states; and the C-term, due to a change in the popula-
tion of molecules over the Zeeman sublevels of a PARA-
MAGNETICground state. The C-term is observed only
for molecules with ground-state paramagnetism and
becomes intense at low temperatures; its variation with
field and temperature can be analyzed to provide mag-
netic parameters of the ground state, such as spin, g-
factor, and zero-field splitting. Variable-temperature
MCD is particularly effective in identifying and assign-
ing electronic transitions originating from paramag-
netic CHROMOPHOREs.magnetic equivalence Nuclei having the same reso-
nance frequency in NUCLEAR MAGNETIC RESONANCE
SPECTROSCOPY; also, identical spin-spin interactions
with the nuclei of a neighboring group are magneti-
cally equivalent. The spin-spin interaction between
magnetically equivalent nuclei does not appear, and
thus has no effect on the multiplicity of the respective
NMR signals. Magnetically equivalent nuclei are nec-
essarily also chemically equivalent, but the reverse is
not necessarily true.171