Gavotte - Douglas

https://digitalcommons.library.umaine.edu/mmb-ps/3129/thumbnail.jp

What Are the Oxidation States of Manganese Required To Catalyze Photosynthetic Water Oxidation?

Photosynthetic O₂ production from water is catalyzed by a cluster of four manganese ions and a tyrosine residue that comprise the redox-active components of the water-oxidizing complex (WOC) of photosystem II (PSII) in all known oxygenic phototrophs. Knowledge of the oxidation states is indispensable for understanding the fundamental principles of catalysis by PSII and the catalytic mechanism of the WOC. Previous spectroscopic studies and redox titrations predicted the net oxidation state of the S₀ state to be (Mn(III))₃Mn(IV). We have refined a previously developed photoassembly procedure that directly determines the number of oxidizing equivalents needed to assemble the Mn₄Ca core of WOC during photoassembly, starting from free Mn(II) and the Mn-depleted apo-WOC complex. This experiment entails counting the number of light flashes required to produce the first O₂ molecules during photoassembly. Unlike spectroscopic methods, this process does not require reference to synthetic model complexes. We find the number of photoassembly intermediates required to reach the lowest oxidation state of the WOC, S₀, to be three, indicating a net oxidation state three equivalents above four Mn(II), formally (Mn(III))₃Mn(II), whereas the O₂ releasing state, S₄, corresponds formally to (Mn(IV))₃Mn(III). The results from this study have major implications for proposed mechanisms of photosynthetic water oxidation.This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through grant DE-FG02-10ER16195. D.R.J.K. was supported by an American Chemical Society Alternative Energy postdoctoral fellowship and the Dreyfus postdoctoral fellowship in environmental chemistry

Sustained water oxidation by [Mn4O4]7+ core complexes inspired by oxygenic photosynthesis

The bioinspired Mn-oxo cubane complex, [Mn4O4L6]+ 1b+ (L = (p-MeO-Ph)2PO2), is a model of the photosynthetic O2-evolving complex. It is able to electro-oxidize water at 1.00 V (vs Ag/AgCl) under illumination by UV−visible light when suspended in a proton-conducting membrane (Nafion) coated onto a conducting electrode. Electrochemical measurements, and UV−visible, NMR, and EPR spectroscopies are interpreted to indicate that 1b+ is the dominant electro-active species in the Nafion, both before and after catalytic cycling, and thus correlates closely with activity. The observation of a possible intermediate and free phosphinate ligand within the Nafion suggests a catalytic mechanism involving photolytic disruption of a phosphinate ligand, followed by O2 formation, and subsequent reassembly of the cubane structure. Several factors that influence catalytic turnover such as the applied potential, illumination wavelength, and energy have been examined in respect of attaining optimum catalytic activity. Catalytic turnover frequencies of 20−270 molecules O2 h−1 catalyst−1 at an overpotential of 0.38 V plus light (275−750 nm) and turnovers numbers \u3e1000 molecules O2 catalyst−1 are observed. The 1b+-Nafion system is among the most active and durable molecular water oxidation catalysts known

The Effect of Works Councils on Employment Change

Despite recent changes in the relationship between unionism and various indicators of firm performance, there is one seeming constant in the Anglophone countries: unions at the workplace are associated with reduced employment growth of around -2.5% a year. Using German data, we examine the impact of the works council – that country’s form of workplace representation – on employment change, 1993-2001. The German institution appears to have much the same negative effect on employment growth. That said, survival bias seems to play a small role, and works councils do not seem to further slow the tortuous pace of employment adjustment in Germany

Formalising planning and information search in naturalistic decision-making

International audienceDecisions made by mammals and birds are often temporally extended. They require planning and sampling of decision-relevant information. Our understanding of such decision-making remains in its infancy compared with simpler, forced-choice paradigms. However, recent advances in algorithms supporting planning and information search provide a lens through which we can explain neural and behavioral data in these tasks. We review these advances to obtain a clearer understanding for why planning and curiosity originated in certain species but not others; how activity in the medial temporal lobe, prefrontal and cingulate cortices may support these behaviors; and how planning and information search may complement each other as means to improve future action selection