Biology Reference
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to apoproteins, the spectroscopic properties of Pchl(ide)
a
chromophores change
drastically (see below). It should be kept in mind that the bulk of Pchl(ide)
a
-Hs is
made up of Pchlide
a
-Hs.
In Pchl(ide)
a
-Hs, various Pchl(ide)
a
chromophores are bound to different
apoproteins by non-covalent forces. This is evidenced by the ready extraction of
the Pchl(ide)
a
chromophores by organic solvents such as acetone. Association of
the chromophores with apoproteins, probably involve (a) axial coordination of the
Pchl(ide)
a
central Mg-atom to nucleophyllic amino acid side chains (Kolossov
et al.
2003
; Rebeiz and Belanger
1984
), and (b) hydrogen bonding between the keto
group of the cyclopentanone ring of the Pchl(ide)
a
chromophore and appropriate
amino acid side chains (Kolossov et al.
2003
; Rebeiz and Belanger
1984
). Pigment-
pigment interaction may involve axial coordination of the keto group of the
cyclopentanone ring of one Pchl(ide)
a
chromophore to the central Mg-atom of
another Pchl(ide)
a
chromophore as suggested by Katz et al. (
1966
) for Chl-Chl
association in hydrophobic environments, as well as
interactions of Pchl(ide)
a
chromophores (Boucher and Katz
1967
). Axial coordination of the histidine
nitrogen of apoproteins to the central Mg-atom (Deisenhofer and Michel
1991
)of
Pchl(ide)
a
has not been established for various Pchl(ide) a-Hs.
Π
Π
8.2.1.2 Spectroscopic Properties of Various Pchlide
a
-Hs
The existence of at least two spectroscopically different Pchl(ide)
a
-Hs was first
reported by Hill and coworkers (Hill et al.
1953
). Using a Zeiss microspectroscope
they observed that in etiolated barley leaves, a band absorbing at 650 nm disappeared
(was phototransformed, i.e. was photoconverted to a Chl-like compound) as the light
was turned on and was replaced by the appearance of two new absorbance bands: one
near 670 nm which corresponded to newly formed Chl
a
-like compound, and one at
635 nm, which did not appear to be convertible to Chl. These results gave rise to the
notion that etiolated tissues contained two spectroscopically different Pchl(ide)
a
-H
complexes. A longer wavelength (LW), phototransformable (t) complex absorbing at
650 nm, and a shorter wavelength (SW), non-phototransformable (nt) complex,
absorbing at 635 nm.
To explain the difference between the LW and SW Pchl(ide)
a
-Hs, Butler and
Briggs proposed, on the basis of freezing and thawing treatments of plant tissues, that
aggregation of pigment molecules in etioplasts shifts the absorption maximum to
longer wavelengths, while disaggregation of pigment molecules shifts the absorption
maximum to shorter wavelengths (Butler and Briggs
1966
). Using freezing and
thawing as well as extraction, heat and acid treatments, Dujardin and Sironval
(
1970
) suggested the presence of three universal Pchl(ide)
a
-Hs in plants, namely:
an aggregated, phototransformable species absorbing at 647-648 nm that involves
pigment-protein and pigment-pigment interactions, a second phototransformable
species absorbing at 639-640 nm which involves only pigment-protein interactions,
and a non-phototransformable species absorbing at 627-628 nm, which is loosely
bound to proteins. They also proposed that pigment-pigment interaction is not
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