Photosystem II and the Oxygen-Evolving Complex
Manganese-Calcium Cubane Clusters, Kok's Four-Flash S-State Clock, and Exciton Funneling
Virtually every molecule of dioxygen ($\text{O}_2$) in Earth's atmosphere—some $1.2 \times 10^{18}\text{ kilograms}$ of gas—was ripped out of liquid water by a single catalytic machine: Photosystem II. Evolved by cyanobacteria more than 2.4 billion years ago, its water-splitting active site has remained conserved down to the exact atomic ligands across every cyanobacterium, alga, and land plant.
Splitting two stable water molecules into one $\text{O}_2$ gas molecule, four protons, and four electrons at pH 7 requires a redox potential above $+0.82\text{ V}$. To drive that endothermic reaction with overpotential to spare, the photo-oxidized chlorophyll radical cation $\text{P}_{680}^{\bullet+}$ inside Photosystem II reaches $+1.25\text{ V}$. It is the strongest oxidizing agent known in biology.
Pierre Joliot illuminated dark-adapted Chlorella algae with a series of two-microsecond xenon flashes in 1969 and measured the microscopic oxygen bursts at a platinum electrode. No oxygen appeared on the first or second flash. Instead, oxygen pulsed on the third flash and peaked on every fourth flash thereafter ($3, 7, 11, 15$) in a damped period-four oscillation.
Bessel Kok explained Joliot's period-four oscillation in 1970 with the five-state S-state clock. A single photon ejects only one electron from $\text{P}_{680}$, whereas oxidizing two water molecules to $\text{O}_2$ removes four electrons. Each catalytic center therefore steps sequentially through five oxidation states ($\text{S}_0 \to \text{S}_1 \to \text{S}_2 \to \text{S}_3 \to \text{S}_4$) before releasing $\text{O}_2$ in one millisecond.
Jian-Ren Shen and Nobuo Kamiya solved the $1.9\text{-\AA}$ X-ray crystal structure of cyanobacterial Photosystem II in 2011, revealing the atomic architecture of the Oxygen-Evolving Complex. Four manganese ions, one calcium ion, and five oxo bridges form an asymmetric $\text{Mn}_4\text{CaO}_5$ distorted cubane chair. That inorganic cluster hangs suspended inside the lumenal loops of the D1 and CP43 subunits.
How does the single-electron photochemistry of $\text{P}_{680}^{\bullet+}$ couple to the four-electron $\text{Mn}_4\text{CaO}_5$ cluster $14\text{ \AA}$ away? Bridging the gap is a single redox-active amino acid side chain: Tyrosine-161 on the D1 subunit ($\text{Tyr}_{\text{Z}}$). When $\text{P}_{680}^{\bullet+}$ grabs an electron from $\text{Tyr}_{\text{Z}}$ in nanoseconds, $\text{Tyr}_{\text{Z}}$ hands its phenolic proton to adjacent His190 to prevent electrostatic buildup.
Even under blazing midday sun, a single chlorophyll molecule absorbs only about ten photons per second—far too slow to keep the water-splitting clock running before its high-valence states decay. Photosystem II solves the flux problem with an optical funnel. Each reaction center is surrounded by 250 antenna chlorophylls that channel excitons inward via Förster resonance in 50 picoseconds.
At the core of Photosystem II, the homologous D1 and D2 subunits form a symmetric heterodimer holding two mirror-image branches of chlorophyll, pheophytin, and plastoquinone cofactors. Yet local electrostatic fields around the D1 pocket steer 100 percent of photo-excited electrons exclusively down the D1 active branch. Charge separation transfers an electron to pheophytin in three picoseconds.
Wielding a $+1.25\text{-V}$ oxidizing potential inside an oxygen-saturated pocket exacts a relentless self-destructive price. Stray triplet chlorophyll states react with $\text{O}_2$ to generate singlet oxygen radicals that chemically scar the D1 core protein after every $10^6$ to $10^7$ photons. In full summer sunlight, the D1 subunit burns out once every 20 to 30 minutes.
Rather than discarding and rebuilding the entire 700-kilodalton, 20-subunit Photosystem II complex every half hour, chloroplasts and cyanobacteria run a surgical D1 repair line. The damaged complex migrates to unstacked thylakoid membranes where FtsH and Deg proteases extract and shred the burned-out D1 polypeptide alone. A freshly synthesized D1 copy then slots straight into the waiting scaffold.
When a passing cloud clears and solar photon flux suddenly outstrips downstream carbon fixation, thylakoid lumen pH drops below 5.5. That acid spike protonates the PsbS sensor protein and triggers the xanthophyll cycle. Light-harvesting antennas switch within seconds into Non-Photochemical Quenching mode, bleeding off up to 80 percent of excess absorbed sunlight harmlessly as heat.
Using femtosecond X-ray free-electron lasers (XFEL) between 2014 and 2023, Vittal Yachandra, Junko Yano, Jan Kern, and Jian-Ren Shen filmed the $\text{Mn}_4\text{CaO}_5$ cubane at room temperature. During the $\text{S}_2 \to \text{S}_3$ step, the cluster opens its lattice. A sixth oxygen atom ($\text{O}_6$) from a second water molecule inserts right alongside $\text{O}_5$ to forge the $\text{O–O}$ bond.