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Physical and Theoretical Chemistry
Laser and Optical Spectroscopy
Professor Elmars Krausz
http://rsc.anu.edu.au/research/krausz.php
Light, chemistry and photophysics are
the most natural, cooperative and synergistic of partners. The
energy in a single particle (photon) of visible light is just that
amount needed to perform chemical transformations. Nature takes
supreme advantage of this synergy and teamwork in the process of
photosynthesis, so essential to most life on the planet.
Spectroscopy
delves into the subtle yet absolutely distinctive interactions
of light, both visible and invisible, with the many forms of matter.
Normally, light can be absorbed, emitted or just scattered. Using
lasers, other far less familiar processes are possible. The wide
range of spectroscopic techniques now available may be utilised as
analytical or diagnostic tools, right down to the extreme limit -
measurements on single molecules.
At
a more fundamental level, spectroscopy provides potent methods with
which to probe the innermost secrets of all chemical species.
Spectroscopy also maps out the detailed electronic structure of all
the different forms of matter. This information is critical to the
understanding of both chemical bonding and chemical reactivity.
Laser
technology continues to evolve and in the process revolutionise
spectroscopy. Laser light is very different to ordinary light
(sunlight or lamplight, etc.). Lasers can be made of one single
colour, to a precision better than one part in a billion. Light
pulses can be compressed into incredibly short pulses or amplified to
a point where the very distinction between light and matter becomes
blurred. Put simply, lasers induce processes to occur that are just
not seen with "ordinary" light sources.
Our
group performs spectroscopic measurements on a wide range of
materials and systems: organic and inorganic, molecular, ionic,
amorphous, crystalline, and increasingly, biological. Our great
strength is the ability to design, develop and invent special
experiments and apparatus to target particular questions. A molecule
may behave very differently in solution to when it is 'trapped' in
the special environment of a protein or crystal. These critical
environmental influences may be identified and probed via the
application of laser selective spectroscopy.
The
study of Photosystem II continues to dominate our activities.
Professor Elmars Krausz spoke on some of our recent Photosystem II
results at the Conference on Physical Chemistry in Christchurch New
Zealand in February. Dr Sindra Peterson also presented work at the
Photosynthesis Gordon Conference in July in Rhode Island, US, as well
as giving a well received seminar in Stanford. Dr Barry Prince and Dr
Sindra Peterson presented work at the Australian Biophysics Society
meeting in Melbourne.
A
major success this year was granting of an ARC LEIF bid, to develop
and construct two new generation metallo enzyme
magneto-optical spectrometers. One system is to be located at the
University of Queensland with Dr Mark Riley and the second at the
ANU. Both systems are to be constructed at the RSC using experience
and technologies we have gained over the last decade. Further funding
success was the granting of a joint ARC Discovery grant with
Professor Rob Elliman of the Research School of Physical Sciences to
perform optical studies of silicon nanocrystals.
Electrochromism and EPR associated with Radical Formation in
Photosystem II
We
have discovered that PSII core complexes undergo surprisingly rapid
and efficient photochemistry at low temperatures (1.7K), with high
yields of the quinone radical anion. This anion gives rise to a large
and characteristic electrochromic (Stark) shift on the close-by
pheophytin, from details in optical spectra. In a 'physiological'
PSII illumination process, the electron donor would be the Manganese
cluster of the oxygen evolving centre, but this (so called S state)
process is inhibited at low temperatures. Alternative electron donors
usually considered are cytochrome b559, chlorphylls, ß-carotenes
and the two redox active tyrosines. None of the above candidates
appear to be the majority donor in samples where the cytochrome is
oxidised and we are attempting to identify the 'mystery radical' via
optical and EPR studies. (with S. Peterson, B. Prince, and
R. Pace, P. Smith [Dept. Chemistry, ANU])
Spectroscopy of Oriented Photosystem II
The
crystal structure of Photosystem II has established many aspects of
the overall displacement and orientation of the pigment molecules in
PSII. This in turn has allowed an increasingly detailed analysis of
high resolution optical spectra. It is possible to orient PSII
enriched membrane fragments of PSII by evaporation of a solubilised
sample onto a glass surface. The measurement of linear dichroism
spectra, taken of a tilted plate which supports the stacked
membranes, indicate that symmetry between the pigments in the D1 and
D2 proteins in PSII is absent. Features attributed to the largest
electronic coupling in P680 may arise from two pigments within D1 and
not from the 'special pair' of chlorophylls. (with S. Peterson, C.
Dobson, and R. Pace, P. Smith [Dept. Chemistry, ANU]
Single Crystal Spectroscopy of Photosystem II
Crystallisation
of PSII core complexes can initially lead to the formation of
very fine (green) needles, of only a few microns thickness. Although
such crystals are too small for structural studies, they are the
appropriate thickness to measure absorption features associated with
the pigments in the PSII core complex. Our experience with single
crystal spectroscopy has allowed us to perform low temperature
polarised spectral measurements of these tiny, fragile crystals,
although at present they appear to undergo photochemical damage
during the sample mounting process. (with S. Peterson, and R.
Pace, P. Smith [Dept. Chemistry, ANU], M. Parker [St. Vincent's Inst.
of Medical Research, Melbourne])
Synechoccoccus Vulcanus, a Hot Opportunity
This
thermophillic bacterium is one of two related species that have
succumbed to crystallisation of its PSII core complex, leading to
subsequent X-ray structural determination. We have been fortunate
enough to be invited to collaborate with a group performing
structural studies. We have already seen that there are significant
differences between plant and cyanobacterial core complexes.
Measurements on Vulcanus sample preparations having high activity,
purity, and stability, and for which the structure is known, is
certain to advance our understanding of P680 and PSII in general. We
have shown (above) that it is feasible to perform spectroscopy on
small single crystals of such materials. (with S. Peterson, and R.
Pace, P. Smith [Dept. Chemistry, ANU], J.-R. Shen [Riken Harima
Inst., Kyoto, Japan])
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