Postcards from the photosynthetic edge: Femtosecond snapshots of photosynthetic water oxidation

"An effective method of solar-based water-splitting is essential for artificial photosynthesis to succeed but developing such a method has proven elusive," says Vittal Yachandra, a chemist with Berkeley Lab's Physical Biosciences Division and one of the leaders of this study. "Using femtosecond x-ray pulses for the simultaneous collection of both x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) data at room temperature, we have gone around the four-step catalytic cycle of photosynthetic water oxidation in photosystem II. This represents a major advance towards the real time characterization of the formation of the oxygen molecule in photosystem II, and has yielded information that should prove useful for designing artificial solar-energy based devices to split water."

Photo-oxidation of water by photosystem II is responsible for most of the oxygen in Earth's atmosphere. At the core of photosystem II is a manganese-calcium (Mn₄Ca) metalloenzyme complex that when energized by solar photons catalyzes a four photon-step cycle of oxidation states (S₀-to-S₃) that ultimately yields molecular oxygen. Scientists need to observe intact x-ray crystallography of the Mn₄Ca ion in action but the molecule is highly sensitive to radiation. The LCLS is the world's only source of x-rays capable of providing femtosecond pulses at the high intensities that allow intact photosystem II crystals to be imaged before they are destroyed by exposure to the x-ray beams.

"In an earlier study at the LCLS, we reported combined XRD and XES data from photosystem II samples in the dark S₁ state and the one visible-flash illuminated S₂ (1-flash) state," says Junko Yano, a chemist also with Berkeley Lab's Physical Biosciences Division and also a leader of this research. "In this new study we report data from the S₃ (2-flash) and S₀ (3-flash) states, which are the intermediate states directly before and after the evolution of the oxygen molecule. In addition, we report data for the first time from a light-induced transient state between the S₃ and S₀ states, which opens the window for elucidating the mechanism of oxygen-oxygen bond formation that occurs between these two states."

XRD data of all the flash states studied revealed an anomalous diffraction signal from Mn that is not complicated by signals from the overall protein matrix of carbon, nitrogen, oxygen and other metals, or even by the Ca atom, which is a part of the five atom Mn₄Ca metalloenzyme complex.

"The detection of this anomalous Mn scattering signal not only validates the quality of our data, but also the procedures used for analyzing the data," says computational scientist Nicholas Sauter. Sauter and Paul Adams, both with Berkeley Lab's Physical Biosciences Division and both contributors to this study, are leading an effort to develop new and better methods for analyzing data from the LCLS.

Yachandra and Yano believe that the detection of an anomalous Mn scattering signal opens up the possibility for detecting changes pertaining only to the Mn cluster as it advances through the S-state cycles and the oxygen-oxygen bond formation, which is where the catalytic action is taking place.

"Knowing how this happens is important for understanding the design principles used in natural photosynthesis," Yachandra says.