Biology Reference
In-Depth Information
being on either of the iron ions. That is, in the Franck-Condon state, the two iron
ions are chemically equivalent, within the limits set by the
Heisenberg Uncertainty
Principle
(Reynolds and Lumry 1966). The Franck-Condon complex (i.e., the
reaction system at the Franck-Condon state) can now relax back to the reactant
state or proceed forward to the product state, depending on the sign of the Gibbs
free energy change,
D
G, accompanying the redox reaction. If
D
G given by Eq.
2.24
is negative, the reaction proceeds forward (from left to right), and if it is positive,
the reaction proceeds backward (from right to left).
=
G
initial
¼ D
G
0
RT log *Fe
þ
2
Fe
þ
3
D
G
¼
G
final
(2.24)
where G
final
and G
initial
are the Gibbs free energy levels of the final and initial states
of the reaction system,
D
G
0
is the standard Gibbs free energy (i.e.,
D
G at unit
concentrations of the reactants and products), R is the universal gas constant, T is
the absolute temperature of the reaction medium, [*Fe
+2
] is the concentration of the
radioactively labeled ferrous ion (to be distinguished from the unlabeled ferrous
ion, Fe
+2
), and [Fe
+3
] is the concentration of the ferric ion.
2.2.3 The Generalized Franck-Condon Principle (GFCP)
or the Principle of Slow and Fast Processes (PSFP)
It was postulated (Ji 1974a) that the
Franck
-
Condon principle
need not be
restricted to electron transfer processes in molecular spectroscopy or inorganic
electron transfer reaction in aqueous media but could be extended to physicochem-
ical processes that involve coupling between two processes whose rates differ
significantly. The generalized version of the Franck-Condon principle was also
referred to as the
Principle of Slow and Fast Processes
(PSFP) (Ji 1991, pp. 52-56),
which states that:
Whenever an observable process, P, results from the coupling of two partial processes, one
slow (S) and the other fast (F), with F proceeding faster than S by a factor of 10
2
or more,
then S must precede F. (2.25)
Statement 2.25 as applied to enzymic catalysis can be schematically represented
as follows:
h
i
Þ
z
,
Þ
z
A
þ
A
þ
B
B
ð
B
Þ
r
$
ð
B
ð
A
þ
$
ð
A
þ
Þ
p
(2.26)
where A and B are the donor (or source) and the acceptor (or sink) of a particle
denoted by
(which can be any material entities, either microscopic or macro-
scopic), and the parentheses indicate the immediate environment (also called
microenvironment) surrounding the reactant system, i.e., (A
+B)
r
, or the product
system, i.e., (A + B
)
p
, where the subscripts r and p stand for reactant and product,