Toward understanding the S2-S3 transition in the Kok Cycle of Photosystem II: Lessons from Sr-Substituted Structure

TL;DR

This study uses DFT calculations to compare the energetics of the S2 to S3 transition in PSII with Ca2+ and Sr2+ substitutions, revealing differences in deprotonation energetics and pKa that impact water oxidation efficiency.

Contribution

It provides a detailed computational comparison of the S2-S3 transition energetics in Sr-substituted and native PSII, highlighting the effects of Sr substitution on water deprotonation and pKa.

Findings

Deprotonation of water W3 is more favorable in native Ca2+ than Sr2+ clusters.

The pKa of bridging water is higher in Sr2+ substituted PSII by 4 pH units.

Sr substitution affects the energetics of the water oxidation cycle in PSII.

Abstract

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+ as an oxygen evolving catalyst. Here, we use Density Functional Theory (DFT) to compare the energetics of the S2 to S3 transition in the Mn4O5Ca2+ and Mn4O5Sr2+ clusters. The calculations show that deprotonation of the water bound to Ca2+ (W3), required for the S2 to S3 transition, is energetically more favorable in Mn4O5Ca2+ than Mn4O5Sr2+. In addition, we have calculated the pKa of the water that bridges Mn4 and the Ca2+/Sr2+ in the S2 using continuum electrostatics. The calculations show that the pKa is higher by 4 pH units in the Mn4O5Sr2+.