atomic centers. Examples include the three-center
bonds in diborane, the delocalized pi bonding of ben-
zene, and BRIDGED CARBOCATIONs.
multicenter reaction A synonym for PERICYCLIC
REACTION. The number of “centers” is the number of
atoms not bonded initially, between which single bonds
are breaking or new bonds are formed in the TRANSI-
TION STATE. This number does not necessarily corre-
spond to the ring size of the transition state for the
pericyclic reaction. Thus, a Diels-Alder reaction is a
“four-center” reaction. This terminology has largely
been superseded by the more detailed one developed
for the various pericyclic reactions.
See also CYCLOADDITION; SIGMATROPIC REAR-
RANGEMENT.
multicopper oxidases A group of ENZYMEs that
oxidize organic SUBSTRATEs and reduce dioxygen to
water. These contain a combination of copper ions
with different spectral features, called TYPE1 centers,
TYPE2 centers, and TYPE3 centers, where the type 2
and type 3 sites are clustered together as a triNUCLEAR
unit. Well-known examples are LACCASE, ascorbate
oxidase, and CERULOPLASMIN.
multident SeeAMBIDENT.
multienzyme A protein possessing more than one cat-
alytic function contributed by distinct parts of a polypep-
tide chain (DOMAINs), or by distinct SUBUNITs, or both.
multiheme Refers to a protein containing two or
more HEMEgroups.
multiple bond Some atoms can share multiple pairs
of electrons, forming multiple covalent bonds. A single
covalent bond is two atoms sharing a pair of electrons.
mu (μ) symbol Notation for a ligand (prefix) that
bridges two or more metal centers. The symbol μis
used for dipole moments.
See alsoBRIDGING LIGAND.
mutagen An agent that causes a permanent heritable
change (i.e., a mutation) into the DNA(deoxyribonu-
cleic acid) of an organism.mutagenesis The introduction of permanent herita-
ble changes, i.e., MUTATIONs, into the DNAof an organ-
ism. In the case of site-directed mutagenesis, the
substitution or modification of a single amino acid at a
defined location in a protein is performed by changing
one or more base pairs in the DNA using recombinant
DNA technology.
See alsoBASE PAIRING.mutation A heritable change in the NUCLEOTIDE
SEQUENCEof genomic DNA(or RNA in RNA viruses),
or in the number of GENEs or chromosomes in a cell,
that can occur spontaneously or be brought about by
chemical mutagens or by radiation (induced mutation).
See alsoRIBONUCLEIC ACID.mutual prodrug The association in a unique
molecule of two, usually synergistic, DRUGs attached to
each other, one drug being the carrier for the other and
vice versa.myocrysin SeeGOLD DRUGS.myoglobin A monomeric dioxygen-binding heme-
protein of muscle tissue, structurally similar to a SUB-
UNITof HEMOGLOBIN.myoglobin 189Multiple bonds. A bond between two atoms involving more than
one pair of electrons (e.g., a double bond)