Why Did Nature Choose Manganese over Cobalt to Make Oxygen Photosynthetically on the Earth?

All contemporary oxygenic phototrophs─from primitive cyanobacteria to complex multicellular plants─split water using a single invariant cluster comprising Mn4CaO5 (the water oxidation catalyst) as the catalyst within photosystem II, the universal oxygenic reaction center of natural photosynthesis. This cluster is unstable outside of PSII and can be reconstituted, both in vivo and in vitro, using elemental aqueous ions and light, via photoassembly. Here, we demonstrate the first functional substitution of manganese in any oxygenic reaction center by in vitro photoassembly. Following complete removal of inorganic cofactors from cyanobacterial photosystem II microcrystal (PSIIX), photoassembly with free cobalt (Co2+), calcium (Ca2+), and water (OH-) restores O2 evolution activity. Photoassembly occurs at least threefold faster using Co2+ versus Mn2+ due to a higher quantum yield for PSIIX-mediated charge separation (P*): Co2+ → P* → Co3+QA-. However, this kinetic preference for Co2+ over native Mn2+ during photoassembly is offset by significantly poorer catalytic activity (∼25% of the activity with Mn2+) and ∼3- to 30-fold faster photoinactivation rate. The resulting reconstituted Co-PSIIX oxidizes water by the standard four-flash photocycle, although they produce 4-fold less O2 per PSII, suggested to arise from faster charge recombination (Co3+QA ← Co4+QA-) in the catalytic cycle. The faster photoinactivation of reconstituted Co-PSIIX occurs under anaerobic conditions during the catalytic cycle, suggesting direct photodamage without the involvement of O2. Manganese offers two advantages for oxygenic phototrophs, which may explain its exclusive retention throughout Darwinian evolution: significantly slower charge recombination (Mn3+QA ← Mn4+QA-) permits more water oxidation at low and fluctuating solar irradiation (greater net energy conversion) and much greater tolerance to photodamage at high light intensities (Mn4+ is less oxidizing than Co4+). Future work to identify the chemical nature of the intermediates will be needed for further interpretation.

Gravitational pseudoaccommodation in patients with aphakic iris-claw intraocular lenses.

PURPOSE: To assess whether iris-claw intraocular lenses (IOLs) undergo gravitation-dependent changes in position and refraction. SETTING: Tertiary referral center, Bern, Switzerland. DESIGN: Observational case study. METHODS: Patients with a history of pars plana vitrectomy and IOL exchange with implantation of an aphakic iris-claw IOL (Artisan) were included in this study. Objective refraction was obtained with a handheld autorefractometer, and the IOL position was measured by ultrasound biomicroscopy with the patient prone, sitting, and supine. RESULTS: Twenty-one eyes of 19 patients with retropupilary IOLs (13) or prepupillary IOLs (8) were included. The mean spherical equivalent (SE) in the sitting position was -0.81 diopter (D) ± 0.95 (SD), and the mean distance from the endothelium to the anterior edge of the IOL was 3.35 ± 0.72 mm. The mean SE in the supine position was -0.61 ± 1.28 D, whereas the mean SE in the prone position was -1.34 ± 1.17 D (P = .0030). The IOL position changed from 3.50 mm in the supine position to 3.06 mm in the prone position (P < .0001). CONCLUSIONS: The aphakic iris-claw IOL was subject to significant movement related to gravity. The change in the refractive effect suggests that there is a degree of pseudoaccommodation caused by the forward shift of the aphakic IOL in the face-down position. FINANCIAL DISCLOSURE: None of the authors has a financial or proprietary interest in any material or method mentioned.