Biology Reference
In-Depth Information
Finally, the proposal that Pchlide
b
does not occur in nature goes counter to the
recent finding of Xu et al. (
2002
). These authors reported the accumulation of a
putative Pchlide
b
in a triple
Synechocystis
mutant (PS I-less/
chl
L
/
lhcb
+
/cao
+
)
grown under light activated heterogeneous growth (LAHG) conditions that also led
to the accumulation of pheophorbide
b
(
i.e
. demetalated Chlide
b
). The putative
Pchlide
b
exhibited an absorbance spectrum very similar to that reported by Schoch
et al. (
1995
) for their synthetic putative Pchlide
b
. In this case too, it is very
probable that the accumulated putative Pchlide
b
reported by Xu et al. (
2002
),
was demetalated along with the reported demetalation of Chlide
b
.
Although the mass spectroscopic data reported by Schoch et al. (
1995
) and Xu
et al. (
2002
) is compatible with the molecular weight of MV Pchlide
b,
it is very
possible that this is due to the vagary of mass spectroscopy caused by the formation
of a protopheophytin
a
adduct having the same molecular weight as MV Pchlide
b
.
12.3 Divinyl Protochlorophyllide
b
(DV Pchlide
b
)
So far it has not been possible to detect DV Pchlide
b
in plants (Fig.
12.2
).
12.4 Monovinyl Chlorophyllide
b
(MV Chlide
b
)
Monovinyl chlorophyllide
b
was first detected in greening (Duggan and Rebeiz
1981
,
1982
) and green higher plant tissues (Aronoff
1981
). The pool of MV Chlide
b
exhibited the spectrofluorometric properties of MV Chl
b
in diethyl ether at
298 and 77 K, but had the chromatographic mobility and solubility of a monocar-
boxylic phorbin. The presence of a free carboxylic group and a formyl group was
demonstrated by methylation with diazomethane and conversion to a Chlide
b
oxime upon reaction with hydroxylamine (Duggan and Rebeiz
1982
). The
concentration of Chlide
b
in green tissues was in the same range as that of MV
Pchlide
a
and MV Chlide
a
. It was estimated that less than 15 % of the Chlide
b
pool
could have arisen from chlorophyllase activity in vitro as confirmed by the extent of
hydrolysis of
14
C-labeled MV Chl
b
added to green tissues just before pigment
extraction (Duggan and Rebeiz
1982
) (Fig.
12.3
).
12.5 Divinyl Chlorophyllide
b
(DV Chlide
b
)
DV Chlide
b
has so far been detected only in the Nec 2 maize mutant (that used to
be known as the ON 8147 mutant) (C. A. Rebeiz, unpublished). This mutation is a
lethal mutation, the leaves are pale yellow, and accumulate only DV Chl
a
and
b
(Bazzaz
1981
). Nec2 maize leaves accumulate DV Chlide
b
to the extent of about
1.00 nmoles per gram of fresh leaves (C. A. Rebeiz, unpublished) (Fig.
12.4
).
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