Biomedical Engineering Reference
In-Depth Information
Among the factors determining low energy activation of elementary chemical steps are
concerted and multi-electron mechanisms, mechanical stress on substrate and catalytic
groups and optimum polarity of the active site cavity.
Favorable Quantum-Mechanical Factors. The rate constants of an elementary step of
a chemical process (k) depend significantly on the value of resonance integral V which is
proportional to the overlap integral S. The latter characterizes the degree of positive
overlap of the electron wave functions. If the overlap is, as a rule, very significant, then
frequencies of electronic motion exceed the frequencies of nuclear motion with
characteristic times to In this case, adiabatic approximation is valid and k
does not depend on V. If the overlap is slight, i. e. the centers are separated by a large
distance or electronic transitions are symmetrically forbidden, then k is proportional to
Another quantum-mechanical selection rule, the principle of the total spin conservation
follows from the low of momentum conservation.
4.
5
.
Effective Synchronization of Nuclei in a Chemical Concerted Reaction. In a
concerted process the transition from initial state to transition states occurs upon the
motion of nuclei (taking about in a certain direction, which is the only possible
path that can lead to reaction products. Obviously, the statistic thermal nature of chemical
processes limits the number of nuclei, which can be involved in a signal elementary step.
In such cases, the value of synchronization factor
can be markedly less then 1.
Formation of Catalytic Ensembles. Regulatory Capacity. Formation of ordered
catalytic ensembles can greatly facilitate the accessibility of substrates in consecutive
chemical and enzyme reactions. Capacity of catalysts to be or not to be active in proper
space and proper time is of great importance especially in biological cells. A catalyst's
capacity for switching activity in the appropriate space and time is very important,
especially in biological cells.
6.
2.2. Electron Transfer
Electron transfer is one of the most ubiquitous and fundamental phenomena in chemistry,
physics and biology (Jortnter and Bixon, 1999a, b; Marcus, 1968, 1999; Sutin, 1999;
Marcus and Sutin, 1985). Non-radiative and radiative ET are found to be a key elementary
step in many important processes involving isolated molecules and super molecules, ions
and excess electrons in solution, condensed phase, surfaces and interfaces, electrochemical
systems and biology. A combination of X-ray crystallographic and physicochemical
experiments on isolated proteins and enzymes and kinetics investigations produces detailed
picture of initial events in those systems under investigation. Some of the most critical
steps in the functioning of photosynthetic reaction centers, mitochondrial enzymes,
nitrogenase, copper, heme and non-heme iron and molybdenum- containing enzymes and
proteins are the long-range electron transfer reactions (Marcus, 1999; Sutin 1999; Marcus
and Sutin 1986; Likhtenshtein, 1988a; Moser and Dutton, 1992: Farver and Pecht, 1999;
Jourtner and Bixon, 1999a; Bixon, 1992; Bixon and Jortner, 1999; Gray and Winkler,
1996; Gray and Ellis, 1994; MacLendon et al., 1999; Machonkin et al., 2000; and
references therein)
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