Abstract:
Low temperature (77K) linear dichroism spectroscopy was used to characterize pigment
orientation changes accompanying the light state transition in the cyanobacterium,
Synechococcus sp. pee 6301, and cold-hardening in winter rye (Secale cereale L. cv.
Puma). Samples were oriented for spectroscopy using the gel squeezing method
(Abdourakhmanov et aI., 1979) and brought to 77K in liquid nitrogen.
The linear dichroism (LD) spectra of Synechococcus 6301 phycobilisome/thylakoid
membrane fragments cross-linked in light state 1 and light state 2 with glutaraldehyde showed
differences in both chlorophyll a and phycobilin orientation. A decrease in the relative
amplitude of the 681nm chlorophyll a positive LD peak was observed in membrane fragments
in state 2. Reorientation of the phycobilisome (PBS) during the transition to state 2 resulted in
an increase in core allophycocyanin absorption parallel to the membrane, and a decrease in rod
phycocyanin parallel absorption. This result supports the "spillover" and "PBS detachment"
models of the light state transition in PBS-containing organisms, but not the "mobile PBS"
model. A model was proposed for PBS reorientation upon transition to state 2, consisting of a
tilt in the antenna complex with respect to the membrane plane.
Linear dichroism spectra of PBS/thylakoid fragments from the red alga, Porphyridium
cruentum, grown in green light (containing relatively more PSI) and red light (containing
relatively more PSll) were compared to identify chlorophyll a absorption bands associated
with each photosystem. Spectra from red light - grown samples had a larger positive LD signal
on the short wavelength side of the 686nm chlorophyll a peak than those from green light -
grown fragments. These results support the identification of the difference in linear dichroism
seen at 681nm in Synechococcus spectra as a reorientation of PSll chromophores.
Linear dichroism spectra were taken of thylakoid membranes isolated from winter rye
grown at 20°C (non-hardened) and 5°C (cold-hardened). Differences were seen in the
orientation of chlorophyll b relative to chlorophyll a. An increase in parallel absorption was
identified at the long-wavelength chlorophyll a absorption peak, along with a decrease in
parallel absorption from chlorophyll b chromophores. The same changes in relative pigment
orientation were seen in the LD of isolated hardened and non-hardened light-harvesting antenna
complexes (LHCII). It was concluded that orientational differences in LHCII pigments were
responsible for thylakoid LD differences. Changes in pigment orientation, along with
differences observed in long-wavelength absorption and in the overall magnitude of LD in
hardened and non-hardened complexes, could be explained by the higher LHCII
monomer:oligomer ratio in hardened rye (Huner et ai., 1987) if differences in this ratio affect
differential light scattering properties, or fluctuation of chromophore orientation in the isolated
LHCII sample.
