Barry Prince operating some of the lasers.
The Engine Room of Life Revealed
Elmars Krausz describes photosynthesis as a finely tuned chemical reaction that is “like avoiding burning your straw
house down while cooking a big roast dinner for every meal”. By investigating how this balance is struck he hopes to
find applications in renewable sources of high-energy, storable and transportable fuels.
ife on Earth is supported by a continuous flow of
energy emanating from the Sun in the form of visible
light. This is captured and converted into chemical
energy during photosynthesis.
Chemical energy is needed to fuel the many activities in
living cells. Although life can exist without photosynthesis,
it can only make use of chemical energy available from
minerals. While this is limited, life forms functioning in this
way flourish around warm undersea volcanic vents.
Photosynthesis starts with light, water and carbon dioxide
and converts these to carbohydrates. This process is a chemical reduction and is dependent on a ready source of electrons.
Primitive photosynthetic bacteria in Archaean times used
dihydrogen sulphide (rotten egg gas) as an electron source
but supplies were again limited, restricting opportunities.
The dramatic evolutionary change that created many of
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the circumstances needed for us to evolve and thrive on
this planet occurred when one very clever organism found
a way of extracting electrons from an overwhelmingly abundant substance on Earth, dihydrogen oxide (water).
Capitalising on this powerful adaptation, photosynthetic
organisms utilising water as a source of electrons quickly
dominated the biosphere. The byproduct of extracting electrons from water is oxygen. The presence of oxygen in the
atmosphere enabled the formation of a protective ozone
layer that allowed life to come out of the oceans. An oxygenic
atmosphere also dramatically changed the surface of the
Earth by eroding rocks and releasing essential nutrients.
We need to breathe oxygen continuously to liberate the
stored energy of the foods we eat.
P680: The Fierce Dragon
The amazingly intricate and efficient nanomachinery of
photosynthesis is embedded deep within the membranes
of specially designed cells. These membranes are crisscrossed with specialised spiral proteins that hold chlorophylls, which absorb and transfer light energy.
These proteins also host an array of special catalytic
centres that can both accept and donate electrons. These
centres control, conduct and store the electron flow in
photosynthesis just like switches, capacitors and resistors
in an incredibly minute electrical circuit.
Most of the chlorophylls act as antennae, concentrating
light energy by acting as miniature solar collectors. The
excitation energy that chlorophyll gains from absorbing a
photon of light can be very quickly transferred to neighbouring chlorophylls, but only when the neighbour is energetically similar or slightly “downhill”. This is called
“excitation transfer”.
There are typically hundreds of antenna molecules in a
photosynthetic assembly, and the light energy absorbed by
many molecules during excitation transfer can be rapidly and
efficiently funnelled down to lower energy chlorophylls.
Reaction centres are set up at the bottom of the funnel.
They gain energy from the harvesting properties of all
antennae. Chemical transformations only occur in reaction
centres, where light is converted into electricity far more efficiently than is possible with silicon photocells, although it
happens in a similar way.
Reaction centres are special assemblies of chlorophyll or
similar molecules, and are labelled according to the longest
wavelength of light at which they are thought to absorb.
For example, the P870 of a purple bacterium reaction centre
absorbs light at a wavelength of 870 nm. This is in the very
deep red of the spectrum.
The primary and most powerful process in photosynthesis
occurs at P680, which was thought to start absorbing at 680
nm. Laser pointers often work around 680 nm, so this is a
deep red wavelength.
An important fundamental principle is that light of shorter
wavelengths carries more energy per quantum and thus is
able to perform more powerful chemistry. The tricky balance
is to make the best use of the available energy in sunlight
without chemical reactions becoming too violent and
destroying the proteins that host the entire process. Oxygenic
photosynthesis is a bit like avoiding burning your straw
house down while cooking a big roast dinner for every meal.
When the chlorophylls of P680 are excited, P680 spits
out an electron and light energy is transformed into an electrical potential strong enough to destroy water. This action
is indeed the most powerful process in biology.
The destruction of water to form molecular oxygen is
not achieved in a single step. It requires four steps, each
step involving a photon of light and extracting a single electron in the four-electron oxidation of two molecules of water
to molecular oxygen.
Generations of scientists have tried to discover what
P680 is, what makes it so powerful, how oxygen is ultimately
evolved and how all this is done with consummate efficiency. Much is known and there is a wide range of speculations, yet a fundamental understanding of this most
important system remains elusive.
The Breakthrough
Far more is known about the photosynthetic reaction centres
of non-oxygen-evolving sulphur bacteria. These have been
isolated, crystallised and studied in great detail.
But it has been far more difficult to study P680. Not only
is the oxygenic reaction centre more complex, it is tightly
bound to light-harvesting proteins CP47 and CP43, from
which it is not easily separated.
We have discovered that this separation radically degrades
its capacities and blurs and changes its spectral properties.
We feel it is then not truly representative of native P680.
However, most studies have been made on such degraded
samples and this is probably the source of many
difficulties.
At the Australian National University, Paul Smith has
been able to use market spinach to prepare purified assemblies that consist only of the absolutely minimal assembly
(core complex) that still evolves oxygen. No one else has
been able to prepare such pure and active samples. We have
been studying them in great detail and comparing them with
data from more degraded samples.
Smith’s samples are still amazingly active when cooled to
2° above absolute zero (–271°C) and illuminated with the
most minute amount of light. When these illuminations are
performed with a high-resolution laser, we can track the
speed at which excitation transfer is moving around the
core complex and how effective different colours of light are
at inducing photochemistry.
This led to two surprises and a great deal of soulsearching. First, we found that light excitation transfer from
the light-harvesting proteins CP43 and CP47 occurs at a
snail’s pace (although speed here is very much a relative
thing). Instead of taking less than a picosecond (one millionth
of one millionth of a second) as expected, it takes more
than 100 times longer for the light energy to reach P680.
This rate makes it slower than the charge separation
process observed in more primitive bacterial reaction
centres. Thus the dynamics and energetics of the oxygenic
reaction centre need to be completely rethought. Maybe
nature has found it necessary to reduce the rate at which the
dragon is fed, perhaps to protect it from indigestion.
An even more surprising observation was that light of
wavelengths far longer then expected led to efficient charge
separation. P680 gains its name because it was thought that
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Joe Hughes filling our world-beating spectrometer with liquid
helium at –269°C.
A model of the oxygenic core complex. Ribbons are transmembrane proteins in which active elements are embedded. Excitation transfer from CP43 and CP47 to the reaction centre D1/D2
containing P680 is surprisingly slow.
its lowest energy charge-separating state occurred at a wavelength of 680 nm. We found that wavelengths as long as 700
nm, deeper in the red, led to charge separation.
This may not seem that much but Barry Prince, Joe
Hughes and I were forced to completely rethink P680. It
forced us away from the bacterial reaction centre as a model,
a paradigm deeply entrenched in the literature. The bacterial model seems natural enough as P680 most likely evolved
from earlier sulfur bacteria. But maybe it didn’t!
What It Can Mean
P680 lives right at the edge. There is only just enough energy
in visible light to split water. The more powerful P680 is as
an oxidant, the more likely it is to blow up its factory infrastructure by damaging the fragile photosynthetic proteins in
which it operates – like trying to fry eggs in a wicker basket
without damaging the basket. We need to know how nature
manages this and learn from it.
P680 uses sunlight to provide us with so much of what we
need to exist. Over many millions of years it has created
the non-renewable reservoir of billions of tonnes of oil, coal
and natural gas that we are now squandering. We must find
renewable sources of high-energy, storable and transportable
fuels. Unfortunately a wood-fired plane won’t fly too well
and would make a lot of smoke.
Clean, efficient fuels can be created by mimicking some
of the unique capacities of natural systems or by bioengineering natural systems. There are photosynthetic organisms that can generate both hydrogen and oxygen while
absorbing carbon dioxide in the process! They produce highgrade renewable energy while absorbing the major greenhouse gas! We need to coax these hydrogen-producing
systems into being more productive in ways that suit us. By
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discovering the mechanism by which it is done we will be
able to mimic nature nature’s ability to use sunlight to split
water into hydrogen and oxygen with ease and efficiency.
Researchers are also experimenting with plant photosynthetic proteins as solar cells to produce electrical power
for a spinach-powered laptop (sciencenews.org/
articles/20040605/fob2.asp). The efficiencies already achieved
have been quite good and comparable with silicon solar
cells.
From Little Things Big Things Grow
We are really excited about our research into the fundamental processes in photosynthesis. At the Australian
National University there are small teams working on charge
separation, oxygen generation, carbon dioxide fixation and
mimicking nature.
There are sure to be valuable outcomes from our research.
Yet, to be driven by short-term and tightly defined outcomes
in research is unwise and ultimately unproductive, especially as so little emphasis is being placed on this work by
both government and industry in Australia and the US. And
it has been proven time and time again that the most important discoveries so often come from unexpected directions
and fundamental research. When researchers were first
studying semiconductors in the 1950s they were ridiculed
and told that their devices were hopelessly inadequate and
uncompetitive against vacuum tubes (valves) in use at that
time.
Science is driven by the passion to understand; by a
continuing persistence in a deep sea of impossible possibilities versus possible impossibilities; and by the willingness
and clarity to swim with a discovery wherever it takes you.
And it works.
Research is one of the best investments a country can
make. It definitely works. From little things big things grow.
Elmars Krausz is Professor and Group Leader of Laser and Optical Spectroscopy at the
Australian National University’s Research School of Chemistry.