Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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+ 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+.

8
The categories of this paper are: hep-ex
13
The categories of this paper are: quant-ph
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
5
The categories of this paper are: gr-qc
20
The categories of this paper are: acc-phys
24
The categories of this paper are: dg-ga
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
10
The categories of this paper are: q-bio
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
11
The categories of this paper are: hep-lat