Toward understanding the S2-S3 transition in the Kok cycle of Photosystem II: Lessons from Sr-substituted structure

Understanding the water oxidation mechanism in Photosystem II (PSII) stimulates the design of biomimetic artificial systems that can convert solar energy into hydrogen fuel efficiently. The Sr2+-substituted PSII is active but slower than with the native Ca2+ containing PSII as an oxygen evolving catalyst. Here, we use Density Functional Theory (DFT) to compare the energetics of the S-2 to S-3 transition in the Mn4O5Ca2+ and Mn4O5Sr2+ clusters. The calculations show that deprotonation of the water bound to Ca2+ (W3), required for the S-2 to S-3 transition, is energetically more favorable in Mn4O5Ca2+ than Mn4O5Sr2+. In addition, we have calculated the pK(a) of the water that bridges Mn4 and the Ca2+/Sr2+ in the S-2 state using continuum electrostatics. The calculations show that the pK(a) is higher by 4 pH units in the Mn4O5Sr2+ cluster.

Automated summary

Understanding the water oxidation mechanism in Photosystem II can stimulate the design of biomimetic artificial systems for efficient conversion of solar energy into hydrogen fuel. The activity of Sr2+-substituted PSII is higher but slower compared to native Ca2+-containing PSII as an oxygen evolving catalyst. DFT calculations show that deprotonation of water bound to Ca2+ is more energetically favorable for the S-2 to S-3 transition than for Sr2+. Additionally, calculations reveal a higher pK(a) in the Mn4O5Sr2+ cluster compared to Mn4O5Ca2+ by 4 pH units.