Georgia Tech graduate student Brandon Polander prepares for an FT-IR experiment. The green laser light is used to photo excite the spinach photosytem II sample. Photo: Gary Meek |
Splitting hydrogen and oxygen from water using conventional electrolysis
techniques requires considerable amounts of electrical energy. But green plants
produce oxygen from water efficiently using a catalytic technique powered by
sunlight—a process that is part of photosynthesis and so effective that it is
the Earth’s major source of oxygen.
If mimicked by artificial systems, this photocatalytic process could provide
abundant new supplies of oxygen and, possibly hydrogen, as a byproduct of producing
electricity. However, despite its importance to the survival of the planet,
scientists don’t fully understand the complex process plants use to harness the
sun’s energy.
A paper published in the Proceedings of the National Academy of Sciences
moves scientists closer to that understanding by showing the importance of a
hydrogen bonding water network in that portion of the photosynthetic machinery
known as photosystem II. Using Fourier transform infrared spectroscopy (FT-IR)
on photosystem II extracted from ordinary spinach, researchers at the Georgia
Institute of Technology tested the idea that a network of hydrogen-bonded water
molecules plays a catalytic role in the process that produces oxygen.
“By substituting ammonia, an analog of the water molecule that has a similar
structure, we were able to show that the network of hydrogen-bonded water
molecules is important to the catalytic process,” said Bridgette Barry, a
professor in Georgia Tech’s School
of Chemistry and
Biochemistry and the Petit Institute for Bioengineering and Biosciences. “Substituting ammonia for water inhibited the activity of the photosystem and
disrupted the network. The network could be reestablished by addition of a
simple sugar, trehalose.”
The research was supported by the National Science Foundation (NSF).
In the chloroplasts of green plants, algae, and cyanobacteria, oxygen is
produced by the accumulation of photo-induced oxidizing equivalents in a
structure known as the oxygen-evolving complex (OEC). The OEC contains
manganese and calcium ions. Illumination causes oxidation of manganese ions in
the OEC. Short laser flashes can be used to step through the reaction cycle,
which involves four sequential light-induced oxidation reactions. Oxygen is
produced on the fourth step, and then is released from the OEC.
This so-called S state cycle resets with the binding of the substrate,
water. Scientists have proposed that a hydrogen bond network, which includes
multiple water molecules bound to manganese ions, calcium ions, and protein
amide carbonyl (C=O) groups, forms an electrostatic network surrounding the
OEC. In this scenario, the extensive hydrogen-bond network would then serve as
a component of the catalyst, which splits off oxygen.
To study the process, Barry and graduate student Brandon Polander used
precision FT-IR spectroscopy to describe how the network reacts to a short
laser flash. The second harmonic of a pulsed Nd-Yag laser was used as the light
source. This illumination causes the OEC to undergo one step in its catalytic
cycle, the so-called S1 to S2 transition. An infrared
spectrum was recorded before and after a laser flash to the photosystem sample,
which was isolated from supermarket spinach.
The exquisite sensitivity of FT-IR spectroscopy allowed them to measure
changes in the bond strength of the protein C=O groups. The energies of these
C=O groups were used as markers of hydrogen bond strength. The brief laser
flash oxidized a manganese ion and caused a change in the strength of the C=O
bond, which reported an increase in hydrogen bonding to water molecules. When
ammonia was added as an inhibitor, a decrease in C=O hydrogen bonding was
observed instead. Addition of trehalose, which is known to change the ordering
of water molecules at the surface of proteins, blocked this effect of ammonia.
The study describes the coordinated changes that must occur in the protein
to facilitate the reaction and shows that the strength of the hydrogen-bonded
network is important.
“This research helps to clarify how ammonia inhibits the photosystem, which
is something that researchers have been wondering about for many years,” Barry
explained. “Our work suggests that ammonia can inhibit the reaction by
disrupting this network of hydrogen bonds.”
The research also suggests that in design of artificial devices that carry
out this reaction, sustaining a similar hydrogen-bonding network may be
important. The stabilizing effect of trehalose discovered by Polander and Barry
may also be important.
Beyond the importance of understanding the photosynthetic process, the work
could lead to new techniques for producing hydrogen and oxygen using sunlight.
One possibility would be to add a biomimetic photocatalytic process to a photovoltaic
system producing electricity from the sun.
“In terms of providing new sources of energy, we still have lessons to learn
from plants about how they carry out these critical processes,” Barry said. “It
would be a great advance for the planet to have new, sustainable, and
inexpensive processes to carry out this reaction.”
Ultimately, she hopes the full water oxidizing cycle can be explored and
potentially harnessed or imitated for oxygen and energy production.
“We are only looking at a single part of the overall reaction now, but we
would like to study the entire cycle, in which oxygen is produced, to see how
the interactions in the water network change and how the interactions with the
protein change,” Barry said. “The work is another step in understanding how
plants carry out this amazing series of photosynthetic reactions.”
