homoleptic Transition-metal or main group com-
pounds having only one type of LIGANDare said to be
homoleptic, e.g., TaMe 5.
See alsoHETEROLEPTIC.
homologous series A series of organic compounds
in which each member differs from the next by a spe-
cific number and kind of atoms (structures differ by
some regular increment) and shares the same basic for-
mula, for example, the alkane series: methane (CH 4 ),
ethane (C 2 H 6 ), propane (C 3 H 8 ), butane (C 4 H 10 ), etc.
homologue Used to describe a compound belonging
to a series of compounds differing from each other by a
repeating unit, such as a methylene group, a peptide
residue, etc.
homolysis The cleavage of a bond (homolytic cleav-
age or homolytic fission) so that each of the molecular
fragments between which the bond is broken retains
one of the bonding electrons.
homonuclear Consisting of only one element.
Hopkins, Frederick Gowland(1861–1947) British
Biochemist Frederick Gowland Hopkins was born on
June 20, 1861, at Eastbourne, England, to a bookseller
in Bishopsgate Street, London, who died when Freder-
ick was an infant.
In 1871 he attended the City of London School,
and at the early age of 17, he published a paper in The
Entomologist on the bombardier beetle. He went to
University College, London, where he became the assis-
tant to Sir Thomas Stevenson, an expert on poisoning.
In 1888 he became a medical student at Guy’s Hospi-
tal, London.
In 1894 he graduated with a degree in medicine
and taught physiology and toxicology at Guy’s Hospi-
tal for four years, and in 1898 he moved to Cambridge.
He was appointed fellow and tutor at Emmanuel Col-
lege, Cambridge.
Hopkins established biochemistry as a field in
Great Britain. He discovered how to isolate the amino
acid tryptophan and identified its structure, discovered
enzymes, and isolated glutathione. For his research on
discovering growth-stimulating vitamins, which he
called “accessory substances,” he was awarded the
Nobel Prize in 1929 in medicine or physiology. He
actually isolated vitamins C, A, and D.
Hopkins was knighted in 1925 and received the
Order of Merit in 1935. Hopkins died in 1947, at the
age of 86. The Sir Frederick Gowland Hopkins Memo-
rial Lecture of the Biochemical Society, named in his
honor, is presented by a lecturer to assess the impact of
recent advances in his or her particular field on devel-
opments in biochemistry. The award is made every two
or three years, and the lecturer is presented with a
medal and £1,000.hormone A substance produced by endocrine glands,
released in very low concentration into the blood-
stream, and which exerts regulatory effects on specific
organs or tissues distant from the site of secretion.host A MOLECULAR ENTITYthat forms COMPLEXes
with organic or inorganic guests, or a chemical species
that can accommodate guests within cavities of its crys-
tal structure. Examples include CRYPTANDs and
CROWNs (where there are ion-dipole attractions
between heteroatoms and positive ions), HYDROGEN-
BONDEDmolecules that form clathrates (e.g., hydro-
quinone and water), and host molecules of INCLUSION
COMPOUNDs (e.g., urea or thiourea). VAN DERWAALS
FORCES and HYDROPHOBIC INTERACTIONs bind the
guest to the host molecule in clathrates and inclusion
compounds.Hückel (4n + 2) rule Monocyclic planar (or almost
planar) systems of trigonally (or sometimes digonally)
hybridized atoms that contain (4n + 2) π-electrons
(where nis a nonnegative integer) will exhibit aromatic
character. The rule is generally limited to n= 0–5.
This rule is derived from the Hückel MO (molecu-
lar orbital) calculation on planar monocyclic conju-
gated hydrocarbons (CH)mwhere mis an integer equal
to or greater than 3, according to which (4n+ 2) π-
electrons are contained in a closed-shell system. Exam-
ples of systems that obey the Hückel rule include:Hückel (4n + 2) rule 131