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Fig. 15.4 Schematics of the MBP-sublocation model in a PSU. All abbreviations and conventions
are as in Fig.
15.1
15.3 Resonance Excitation Energy Transfer from
Metabolic Tetrapyrroles to Various Chl-Protein
Complexes Indicate that Resonance Excitation
Energy Transfer Takes Place from Multiple
Heterogeneous Sites
Fluorescence resonance energy transfer involves the transfer of excited state energy
from an excited donor “D*” to an unexcited acceptor “A” (Calvert and Pitts
1967
;
Lakowicz
1999
; Turro
1965
). The transfer is the result of dipole-dipole interaction
between donor and acceptor and does not involve the exchange of a photon. The
rate of energy transfer depends upon (a) the extent of overlap of the emission
spectrum of the donor and the absorption spectrum of the acceptor, (b) the relative
orientation of the donor and acceptor transition dipoles, and (c) the distance
between donor and acceptor molecules. As soon as the excited donor “D*” and
unexcited acceptor “A” states are coupled by an appropriate interaction, they
become degenerate if there is an excited state of the acceptor “A”, which requires
exactly the same excitation energy available in “D*”. When such a condition exists,
excitation of one of the degenerate states leads to a finite probability that the same
excitation will appear in the other degenerate state (Turro
1965
). This probability
increases with time but is inversely proportional to the sixth power of the fixed
distance separating the centers of the donor and acceptor molecules. It has been
estimated that dipole-dipole energy transfer between donor and acceptor molecules
may occur up to a separation distance of 50-100
˚
(Calvert and Pitts
1967
).
Resonance excitation energy transfer from three tetrapyrrole donors to the Chl
a
of
Chl-protein complexes were monitored, namely: from protoporphyrin IX (Proto),
divinyl (DV) Mg-Proto and its methyl ester and monovinyl (MV) and DV Pchlide
a
.
DV Proto is a common precursor of heme and Chl. It is the immediate precursor of
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