Biology Reference
In-Depth Information
F
F
F
|
|
|
H - C = C - H
H - C = C - H
|
F
trans
-1,2-difluoroethylene
cis
-1,2-difluoroethylene
FH
F F
|
|
|
|
H-C-C-H
H-C-C-H
|
|
|
|
HF
HH
trans
-1,2-difluoroethane
cis
-1,2-difluoroethane
Fig. 3.5 Distinguishing between
configurations
and
conformations
.(
Upper
) A configuration
refers to the arrangement of atoms in a molecule that cannot be changed without breaking or
forming at least one covalent bond. One of the two C-C bonds must be broken and reformed to
convert the trans-1,2-difluoreethylene to the cist isomer. (
Lower
) A conformation is the arrange-
ment of atoms in a molecule that can be changed by bond rotations without breaking or forming
any covalent bonds. No covalent bond needs to be broken to convert the trans-1,2-difuoroethane
conformation to the cis conformer.
Conformers
are defined as the molecular structures that can be
interconverted without breaking any covalent bonds
4. Type I shapes are sensitive to microenvironmental conditions (e.g., temperature,
pH, ionic strength, electric field gradient, mechanical stress gradient, etc.), while
Type II shapes are relatively insensitive to such factors.
5. It was postulated that Type I shapes are utilized to transmit information through
space, while Type II shapes are used to transmit information through time
(Ji 1988).
Therefore, it may be reasonable to conclude that one possible reason for there
being two (and only two) kinds of molecular interactions and shape changes in
molecular and cell biology is to mediate
information transfer
through
space
and
time
in living systems.
3.3 The Principle of Microscopic Reversibility
In formulating possible mechanisms for an enzyme-catalyzed reaction, it is impor-
tant to obey two principles - the generalized Franck-Condon principle (GFCP)
introduced in Sect.
2.2.3
and the principle of microscopic reversibility (PMR)
described below. PMR is well known in the field of chemical kinetics (Gould
1959; Hine 1962; Laidler 1965) and statistical mechanics (Tolman 1979), and is
succinctly stated by Hine (1962, pp. 69-70) in the form that is useful in enzymology:
the mechanism of reversible reaction is the same, in microscopic detail
for the
...
...
reaction in one direction as in the other under a given set of conditions.
(3.9)
...
