Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 360

Evidence of strong disorganization of the aforementioned complexes was
provided by changes in the 77 K fluorescence excitation spectra. In excitation
spectra recorded at F685, 696 and 739 nm, the normal three-banded fluorescence
excitation profile with maxima at 416, 440, 475 and 485 nm disappeared and was
replaced by one Soret excitation maximum at 447 nm and an excitation shoulder at
421 nm (Amindari et al. 1995 ). The Soret excitation maximum at 447 nm belongs to
MV Chl a hexacoordinated to a small ligand such as diethyl ether at 77 K (Belanger
and Rebeiz 1984 ). The Soret excitation maximum at 421 nm probably corresponds
to the η 1 transition of Chl a .
17.4.2.5 Effect of Exogenous MV Pchlide a on Photodynamic
Damage in Isolated Cucumber Chloroplasts
As discussed in Chap. 9 , MV Pchlide a is the precursor of MV Chlide a . It differs
from DV Pchlide a by the presence of an ethyl instead of a vinyl group at position
two of the macrocycle. In its native state it is bound to the plastid membranes
(Smith and Rebeiz 1979 ). Membrane-bound MV Pchlide a exhibits emission
maxima between 629 and 658 nm, depending on its state of aggregation and the
stage of greening of the tissue (Cohen and Rebeiz 1978 ).
After 2 h of incubation with isolated chloroplasts in the light, 12 % of the added
MV Pchlide a disappeared probably as a consequence of photodestruction (Rebeiz
et al. 1984a , b ). During incubation in the light, MV Pchlide a exerted negligible
effects on the pigment pools of the chloroplast. Nevertheless it did cause disruption
of various pigment-protein complexes as evidenced by 77 K fluorescence
spectroscopy.
In one of three replicates the 77 K fluorescence emission and excitation profiles
after 2 h of incubation in the absence of added MV Pchlide a , were indistinguish-
able from those of the 0 h control. In the other two replicates, the amplitudes of the
694 emission became larger than that of the 685 nm emission latter was split into
two emissions, and became red shifted by about 8 nm (Amindari et al. 1995 ).
After incubation with 5,997 nmol of MV Pchlide a per 100 mg plastid protein for
2 h in the light, the 77 K fluorescence emission and excitation profiles underwent
profound changes. Essentially the organized structure of the chloroplast was
strongly disrupted. The 740 nm fluorescence emission disappeared and was
replaced by a long wavelength emission at 747 nm, thus indicating disruption of
the LHCI-730 protein-pigment complex (Amindari et al. 1995 ). The fluorescence
emission maxima at 686 and 696 nm also disappeared and were replaced by a single
emission maximum at 690 nm. This also indicated a certain disorganization of
the LHCII, LHCI-680 and CP47 and/or CP29 complexes. Excitation at 472 nm i.e.
close to the Soret excitation maximum of MV Chl b , elicited a Chl b emission
maximum at 660 nm. Appearance of Chl b fluorescence is usually an indication of a
certain degree of disruption of the structural relationship of Chl a and b in the