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+ containing PSII 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 state using continuum electrostatics. The calculations show that the pKa is higher by 4 pH units in the Mn4O5Sr2+cluster.
Highlights
Calcium and strontium belong to group 2 alkaline earth metals in the periodic table
We have calculated the pKa of the water that bridges Mn4 and the Ca2+/Sr2+ in the S2 using continuum electrostatics
Experimental13 and theoretical studies14–16 have proposed that the S2-S3 transition passes through an intermediate step in which the S2 EPR signal changes from the multiline g=2 signal to the g=4.1
Summary
Calcium and strontium belong to group 2 alkaline earth metals in the periodic table. they are chemically similar and have a stable oxidation state of +2. 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+. We compare the energies of two structures of the S2 g=4.1 state, A in which HIS190 and W3 are neutral (Figure 1A) and B with protonated HIS190+ and W3 is a OH- bridge between Mn4 and Ca2+ (Figure 1B) in both Mn4O5Ca2+ and Mn4O5Sr2+ clusters.
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