CHEMICAL MODELS OF ENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 179

Commonly accepted mechanisms of the MMO reactions are based on the concept of
the activation of dioxygen or the use of “shunts” such as Nevertheless, the new
mechanism of methane oxidation via an intermediate complex containing
pentacoordinated carbon has been forwarded (Shilov, 1997, Karasevich et al., 1998,
1999). This suggestion is based on experiments on the multiple H-D exchange and
methane oxidation catalyzed by platinum (II) complexes, for instance).
Formation of methyl platinum (IV) chloride complex in methane oxidation was
confirmed by its NMR spectrum.

6.4. Light energy conversion

There are two main objections to the mimicking of the photosynthetic process of light
energy conversion in reaction centers via the mechanism of charge photoseparation on
cation and anion radicals: 1) establishing factors affecting fast stages of primary electron
transfer in donor-acceptor pairs using femto-, pico-, and nanosecond time domain
techniques and 2) building donor-acceptor structures capable of retaining the
photoseparated structures long enough for secondary chemical reactions of the charges
to occur. Artificial reaction centers can also form the basis for optoelectronic devices.
They may be incorporated into the lipid bilayer membranes of artificial vesicles, where
they function as components of light-driven proton pumps that generate trans -membrane
proton motive force for synthesis of ATP via an ATP synthase enzyme.
For this purpose a congruent and systematic set of well-designed models has been
synthesized and the photochemical and photophysical propeties of these models were
characterized (Sessler, 1992; McLendon and Hakel992; Gust et al., 1999, 2001; Miller
et al., 2000; Wasielewski, 1992; 2002; Wasielewski et al., 1998, 2000; HammerstrÖm et
al., 2001; Heinen et al., 2002; and references therein). The effect of the chemical
structure of donor and acceptor centers, the energy of the donor center in its excited
state, the distance between the centers and their mutual orientation, the nature and length
of the spacer tethered donor and acceptor and solvent and temperature were investigated.
A series of Zn porphyrin-quinone dyads and two porphyrins-quinone tryads have
been synthesized (Sessler, 1992; and references therein). In the first group of complexes
the photoinduced charge separation (PCS) occurred on a time scale of < 1 ps, while in
the triad the excitation formed a transition species for about 60 ps. In these systems the
thermal recombination was found to be very fast. The distance dependence of PCS
through norbornyl bridges of varying length has been examined (Verhoeven, 1999). The
values of the decay factor were determed as for PCS and
for the thermal recombination. Similar multicomponent systems have been synthesized
and investigated by elaborated time-resolved transient absorption and ESR techniques in
1990's (Wasielewski et al., 1998; Gust et al., 1999; and references therein)
Recently, new insight into detailed mechanisms of photochemical processes in
donor-acceptor pairs (DA) has emerged. To study the role of bridge energy levels on
electron transfer rates, a series of rod-like donor-bridge-acceptor (D-B-A) molecules in
which a 4-aminonaphthalene- ,8-imide (ANI) electron donor is linked to a 1,8:4,5-

naphthalenediimide acceptor (NI) via the 1,4 positions on a phenyl bridge was

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