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Discovery of high-spin manganese centers sheds light on photosynthesis

In a small manganese oxide cluster, teams from HZB and HU Berlin have discovered a particularly exciting compound: two high-spin manganese centers in two very different oxidation states. This complex is the simplest model of a catalyst that occurs as a slightly larger cluster in natural photosynthesis, where it enables the formation of molecular oxygen. The discovery is considered an important step towards a complete understanding of photosynthesis.

The study is in the Journal of the American Chemical Society.

In chemistry, oxidation states of metals are used to classify chemically relevant valence electrons into those that already form bonds with other atoms and those that could potentially form bonds. This is a simple way to classify the different possibilities for individual elements to participate in reactions.

Transition metals such as manganese can easily switch between oxidation states, which is particularly important for catalytic reactions. Interestingly, some oxidation states are extremely rare, although they are thought to play a central role—one example is high-spin manganese (V), which could have a crucial role in the formation of molecular oxygen through natural photosynthesis.

Molecular oxygen formation via photosynthesis

Despite extensive research, only two examples of these high-spin manganese(V) centers were known, both containing only one manganese atom. In natural photosynthesis, however, four manganese atoms and one calcium atom are involved in the formation of O₂.

Photosynthesis is a light-activated catalytic reaction that produces carbohydrates and oxygen from water and carbon dioxide, and evolved over two billion years ago in cyanobacteria and algae. It is only through photosynthesis that life on Earth as we know it has become possible.

High-spin manganese (V) center discovered

Now, groups at Humboldt-Universität zu Berlin and HZB have discovered the long-sought high-spin manganese (V) center in a small manganese oxide cluster. The cluster contains only five atoms in total and looks very simple: two oxygen atoms form bridges between two manganese atoms, one of which is bound to a third oxygen atom as a terminal ligand.

"This is the simplest form of a bonding motif that also occurs in natural photosynthesis, which makes this discovery very exciting," says HZB researcher Konstantin Hirsch.

A very special sample

The search for the elusive high-spin manganese(V) centers was carried out by Olesya Ablyasova for her doctoral thesis at HZB. She quickly realized that her sample was special: not only did it contain manganese(V) in the rare high-spin state, but this high oxidation state was also coupled to a second manganese center in a low oxidation state of +2.

This can be seen as a strong formal charge separation in a small volume of just a handful of atoms. This extreme difference in oxidation states, +2 and +5, of the two manganese atoms in a cluster is very unusual, and has surprised the team at least as much as the discovery of the unusual species they had originally looked for.

The result could not be predicted using standard computational methods; instead, the team of theoretical chemists led by Michael Römelt at HU Berlin had to use sophisticated methods to achieve agreement with the experimental data.

Unlocking the secrets of photosynthesis at BESSY II

"This discovery is very encouraging and we will now continue our search for high-spin manganese(V) centers in even larger clusters that are closer to the inorganic cluster in natural photosynthesis. We hope that one day we will be able to unlock the secret of how nature produces all the oxygen molecules that surround us and that we breathe every day," says Ablyasova.

The unique experimental capabilities at BESSY II have played a key role in the investigation: HZB operates an ion trap experiment for X-ray spectroscopy, designed to study the oxidation and spin states of highly reactive species.

"By freezing these species in the gas phase, we can access potential reactive intermediates in chemical reactions that would be very short-lived under standard conditions," explains Hirsch. "Perhaps we can even show that high-spin manganese(V) is more common than expected, now that we know how to look for it."