Photosystem succumbs to new spectroscopic probe

The light-powered water-splitting reaction at the core of photosynthesis has succumbed to probing with a new spectroscopic technique - 2D HYSCORE - employed by scientists at Rensselaer Polytechnic Institute. Understanding the photosynthetic reaction mechanism in detail might one day allows scientists to copy what green plants do in using sunlight to split water and convert carbon dioxide to sugars, or better still to tap the energy from sunlight directly generate electricity.

K.V. Lakshmi and colleagues are undertaking research into the photosynthetic protein, Photosystem II. New results with the 2D electron paramagnetic resonance spectroscopic technique, HYperfine Sub-level CORrElation, published in the Journal of Physical Chemistry [DOI: 10.1021/acs.jpcb.5b01602] shows that they can capture the reactions that split water and hydrogen peroxide in metal-containing proteins or metallo-enzymes in nature.

"The solar-powered water-splitting photosynthetic protein complex, Photosystem II, catalyzes one of the most energetically demanding reactions in nature by using light energy to split water to dioxygen," explains Lakshmi. "However, the details of the water-splitting reaction have remained elusive due to the inability of conventional methods to probe the active site of metal-containing proteins, like Photosystem II."

Photosystem II is present in green plants and cyanobacteria (often incorrectly known as blue-green algae) and uses photon energy to lyse water molecules, release electrons and protons to fuel the photosynthetic production of chemical. The key reaction is solar-powered oxidation of water in a cluster of oxygen, manganese, and calcium ions, the "oxygen-evolving complex." This complex uses four photons of light to split two molecules of water in five distinct steps known as "S-states." These intermediate states are very difficult to study because they are so short-lived.

In 2012, Lakshmi and her team used advanced spectroscopic techniques to describe the atomic-level mechanism of S-2, the third step in the process. In 2013, they received a grant from the US Department of Energy to expand their research over three years to the remaining S-states.

They used 2D HYSCORE to study manganese catalase, a similar, but simpler, metal-containing protein that bacteria use to split hydrogen peroxide. Catalases are important metallo-enzymes that act as an antioxidant by using a cluster of two manganese ions to split the harmful chemical, hydrogen peroxide, to water and dioxygen in cells. They successfully determined the geometry of the intermediate active site, and also revealed the oxidation state of the individual metal ions and the atomic structure of the bound water ligand within the active site.

"We use manganese catalase as a model to develop and apply this new method," explains Lakshmi. "And the insight that we have gained on the mechanism of manganese catalase and the high-resolution spectroscopic methods that we have developed in this study greatly enhance the ultimate goal and bring us closer to determining the mechanism of the solar-powered water-splitting reaction of Photosystem II."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".