Journal
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
Volume 4, Issue 5, Pages 786-791Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jz400071k
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Funding
- NSF [MCB 08-42246]
- Direct For Biological Sciences
- Div Of Molecular and Cellular Bioscience [0842246] Funding Source: National Science Foundation
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In photosynthesis, photosystem II evolves oxygen from water at a Mn4CaO5 cluster (OEC). Calcium is required for biological oxygen evolution. In the OEC, a water network, extending from the calcium to four peptide carbonyl groups, has recently been predicted by a high-resolution crystal structure. Here, we use carbonyl vibrational frequencies as reporters of electrostatic changes to test the presence of this water network. A single flash, oxidizing Mn(III) to Mn(IV) (the S-1 to S-2 transition), upshifted the frequencies of peptide C = O bands. The spectral change was attributable to a decrease in C = O hydrogen bonding. Strontium, which supports a lower level of steady state activity, also led to an oxidation induced shift in C = O frequencies, but treatment with barium and magnesium, which do not support activity, did not This work provides evidence that calcium maintains an electrostatically responsive water network in the OEC and shows that OEC peptide carbonyl groups can be used as solvatochromic markers.
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