Biomedical Engineering Reference
In-Depth Information
serve as an electron donor (D*) and the nitroxide fragment as an acceptor (A). The same
group allows the estimation of the local apparent dielectric constant of the medium
near the donor by the measurement of the relaxation shift of the D fluorescence spectra and
of the medium near the acceptor by the analysis of the nitroxide ESR spectra. It was
shown (Rubtsova et al., 1994) that for the chromophore fragment imbedded in
hydrophobic portion of BSA and for the nitroxide fragment protruded in 50%
water-ethylene glycol solution. On this basis the values of Gibbs energy
and reorganization energy for ET in the DA pairs were estimated with the use
Eqs. 2.8 and 2.9 respectively. Using Eq. 2.6 and the aforementioned value of
was found.
Though rational estimations of the reorganization energy in protein and other
complicated biological objects were done, the precise calculation of
remains a
challenging problem.
Electronic coupling (resonance integral)
The non-adiabatic long-range electron transfer (LRET) has been proven to be one of the
key stages of many processes in enzymes, proteins and model systems. Therefore,
theoretical calculation and experimental determination of the resonance integral (V) and its
dependence on the distance between donor and acceptor centers appears to be a
fundamental problem.
Information garnered from studies with simple homogeneous media and artificial
systems in which these centers are tethered by a bridge of appropriate chemical nature in
comparison to natural objects provides insight into what occurs mechanistically in both
systems (Gust and Moor, 1992, Sessler, 1992, Wasielewski, 1992, 2002; Wesielwski et al.,
2000; Ponce et al., 2000; Tezcan et al., 2001; Likhtenshtein, 1993,1996; and references
therein).
The theoretical and experimental results in non-biological objects can be briefly
summarized as follows:
In systems in which the donor and acceptor centers are in direct contact with each
other or connected by a “conducting” bridge (conjugated bonds), electron transfer rates are
very fast The transition occurs markedly slower when the donor-
acceptor mutual orientation is not favorable for positive orbital overlap and, therefore, the
electron coupling V is small.
Separation of D and A centers by “non-conducting” media resulted in the strong
dependence of the ET rate on distance between D and A and the marked effect of the
chemical nature of saturated molecules and bonds between the pair. This dependence can
be quantitatively characterized be the decay factor, (Eq. 2.27). The following values of
were found: 3-4 (vacuum), 1.6 - 1.75 (water), 1.2 (organic solvents) and 1.08 -
1.2 (synthetic D-bridge-A molecules). The effects of distance and the number of
intermediate saturated groups (n) on photoinduced electron transfer between a donor and
acceptor are discussed in (Verhoeven, 1999).
Fig. 2.8 shows that the logarithm of maximum rates (Eq. 2.16) spanning 12 order of
magnitude for intraprotein ET reaction as a function of the edge-to edge distance generates
an approximate linear relationship with
1
.
2.
(Moser and Dutton, 1992). A similar
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