Calcium and the Hydrogen-Bonded Water Network in the Photosynthetic Oxygen-Evolving Complex.
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 S1 to S2 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.
Department of Chemistry and Biochemistry and the Petit Institute for Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
Brandon C
BC
Polander
Department of Chemistry and Biochemistry and the Petit Institute for Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
Bridgette A
BA
Barry
amide carbonyl frequency; photosystem II; vibrational spectroscopy; water activation; water oxidation