Biomedical Engineering Reference
In-Depth Information
exchangeable centers and the chemical nature of the bridge connecting the centers may be
used for evaluating such dependences for the resonance integral in the ET equations (Eq.
2.27).
A vast literature is connected with the quantitative investigation of exchange processes
(see, for example, Zamaraev et al., 1981; Ermolaev, et al., 1997; Likhtenshtein, 1995; and
references therein). As it seen in Fig. 2.7, experimental data on the dependence of
and
on the distance between the centers
lies on two curves, which are approximated
by the following equation (Likhteshtein, 1996)
For systems in which the centers are separated by a “non-conductive” medium (molecules
or groups with saturated chemicals bond)
equal
For systems in which the
radical centers are linked by “conducting” conjugated bonds,
is
We can consider the ratios
as parameters of attenuation of the exchange interaction of TTET and SE through the given
medium. Taking into account Eqs. 2.27 and 2.28 with values n = 4 for TTET and n = 2 for
SE and ET, and Eq. 2.29, we have an expression for the dependence of the attenuation
parameters for SE and ET on the distance between remote donor and acceptor centers
with for a “non-conducting” medium and for a
“conducting” bridge. The value of is found to be close to that obtained by
analysis of on the distance in model and biological systems Fig. 2.7.
An examination of the empirical data on the exchange integral values for the spin-
spin interactions in systems with known structure, e.g. biradicals, transition metal
complexes with paramagnetic ligands and monocrystals of nitroxide radicals, allows the
value of the attenuation parameter
for the exchange interaction through a given group X
to be estimated. By our definition, the
is
where R is a nitroxide or organic radical, P is a paramagnetic complex or radical and X, Y,
and Z are chemical groups in the bridge between R and P.
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