system. However, the value of that concentration will
determine which of the reaction steps is rate-limiting. If
the particular concentrations of interest, which may
vary, are chosen as the standard state, then the rate-
limiting step is the one with the highest Gibbs energy.
See alsoPOTENTIAL-ENERGY PROFILE; POTENTIAL-
ENERGY(REACTION) SURFACE; REACTION COORDINATE.
Gibbs energy of activation(standard free energy of
activation), ∆t-Go (SI unit: kJ mol–1) The standard
Gibbs energy difference between the TRANSITION STATE
of a reaction (either an ELEMENTARY REACTIONor a
STEPWISE REACTION) and the ground state of the reac-
tants. It is calculated from the experimental rate con-
stant kvia the conventional form of the absolute rate
equation:
∆t-G= RT[ln(kB/h)–ln(k/T)]where kBis the Boltzmann constant and hthe Planck
constant (kB/h= 2.08358 × 1010 K–1s–1). The values of
the rate constants, and hence Gibbs energies of activa-
tion, depend upon the choice of concentration units (or
of the thermodynamic standard state).
See also ENTHALPY OF ACTIVATION; ENTROPY OF
ACTIVATION.
Gibbs free energy Energy liberated or absorbed in a
reversible process at constant pressure and constant
temperature.
Gilbert, Walter (1932– ) AmericanPhysicist, bio-
chemist Walter Gilbert was born on March 21, 1932,
in Boston, Massachusetts, to Richard V. Gilbert, an
economist at Harvard University, and Emma Cohen, a
child psychologist. Educated at home for the first few
years, Gilbert moved with the family to Washington,
D.C., in 1939, and he was educated there in public
schools, later attending the Sidwell Friends high school.
He attended college at Harvard and majored in
chemistry and physics, then went to the University of
Cambridge for two years, receiving a doctorate degree
in physics in 1957 for his work on dispersion relations
for elementary particle scattering. He returned to Har-
vard and, after a postdoctoral year and a year as Julian
Schwinger’s assistant, became an assistant professor of
biophysics in 1964 and professor of biochemistry in- In 1974 he became American Cancer Society
professor of molecular biology at Harvard. During the
late 1950s and early 1960s, he focused on theoretical
physics and worked with graduate students on prob-
lems in theory. After a few years, he shifted from the
mathematical formulations of theoretical physics to an
experimental field.
In the 1970s he developed a widely used technique
of using gel electrophoresis to read nucleotide
sequences of DNA segments. The same method was
developed independently by Frederick Sanger, and they
both won the Nobel Prize in chemistry in 1980 “for
their contributions concerning the determination of
base sequences in nucleic acids.”
In 1979 he joined a group of other scientists and
businessmen to form Biogen, a commercial genetic-
engineering research corporation, but resigned from
Biogen in 1984. He went back to Harvard and became
a chief proponent of the Human Genome Project, a
government-funded effort to compile a complete map
of the gene sequences in human DNA, where he contin-
ues to work today. He is married to poet Celia Gilbert,
and they have two children.
glass A homogeneous material with a random, liq-
uidlike molecular structure.glass electrode An electrode for measuring pH when
it is dipped into an aqueous solution containing H+
ions.glass transition temperature The temperature
where a polymer changes from hard and brittle to soft
and pliable.glycogen A glucose polymer (also known as animal
starch) stored in animal tissue.glycoprotein Glycoproteins are complexes in which
carbohydrates are attached covalently to asparagine
(N-glycans) or serine/threonine (O-glycans) residues of
peptides. Also known as a conjugated protein. A pro-118 Gibbs energy of activation