The cytochrome bc1 and related complexes are essential energyconserving components of mitochondrial and bacterial electron transport chains. They orchestrate a complex sequence of electron and proton transfer reactions resulting in the oxidation of quinol, the reduction of a mobile electron carrier, and the translocation of protons across the membrane to store energy in an electrochemical proton gradient. The enzyme can also catalyze substantial rates of superoxide production, with deleterious physiological consequences. Progress on understanding these processes has been hindered by the lack of observable enzymatic intermediates. We report the first direct detection of a semiquinone radical generated by the Q o site using continuous wave and pulsed EPR spectroscopy. The radical is a ubisemiquinone anion and is sensitive to both specific inhibitors and mutations within the Qo site as well as O2, suggesting that it is the elusive intermediate responsible for superoxide production. Paramagnetic interactions show that the new semiquinone species is buried in the protein, probably in or near the Qo site but not strongly interacting with the 2Fe2S cluster. The semiquinone is substoichiometric, even with conditions optimized for its accumulation, consistent with recently proposed models where the semiquinone is destabilized to limit superoxide production. The discovery of this intermediate provides a critical tool to directly probe the elusive chemistry that takes place within the Qo site.electron transfer ͉ free radical ͉ photosynthesis ͉ reactive oxygen species ͉ respiration T he cytochrome (cyt) bc 1 , b 6 f, and related complexes, collectively termed cyt bc complexes, are essential components of the respiratory and photosynthetic electron transport chains in mitochondria, many bacteria, and chloroplasts (1-3). These complexes oxidize quinol and reduce one-electron redox carriers while generating an electrochemical gradient of protons, termed the proton motive force (pmf ), which drives the synthesis of ATP and other bioenergetic processes. The natural substrate is ubiquinol (UQH 2 ) in the case of the mitochondrial and bacterial cyt bc 1 complexes, and the mobile carrier is cyt c in mitochondria or photosynthetic bacteria. The general mechanistic framework for the cyt bc complexes is the Q-cycle, first proposed by Mitchell (4-6) and modified by many others (e.g., refs. 7-14).In ''standard'' versions of the Q-cycle (2, 9, 15) a unique bifurcated oxidation of QH 2 occurs in the Q o site, located on the positively charged side (p-side) of the membrane. An initial single electron transfer to the ''Rieske'' 2Fe2S cluster produces a free radical semiquinone (SQ) intermediate (the anionic form or the neutral form, depending on the exact sequence of electron and proton transfers). The Rieske 2Fe2S cluster is the first in a series of carriers, termed the ''high potential chain,'' which in mitochondria and certain bacteria includes cyt c 1 followed by a soluble (or mobile) cyt c. Under normal conditions, the SQ intermediate ...

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We have investigated the relationship between different types and amounts of bulk defects and the surface morphology of TiO 2 (110) single crystals prepared by annealing in ultrahigh vacuum and in oxygen. Rutile TiO 2 (110) specimens were cut from the same crystal and were heated in a furnace to different temperatures which resulted in different states of reduction (colors of the crystals). After characterization of the bulk defects with electron paramagnetic resonance (EPR), the specimens were studied with scanning tunneling microscopy (STM), low-energy He + ion scattering (LEIS), and work function measurements. EPR reveals that darker rutile crystals exhibit higher concentrations of extended Ti 3+ related bulk defects such as crystallographic shear planes (CSP), with a decrease in substitutional and interstitial defects as compared to lighter crystals. Surface structures with (1 × 2) features are preferably formed upon UHV annealing on these darker crystals. LEIS measurements show that all of the crystals' (110) surfaces are reoxidized upon annealing in 18 O 2 (573 K, 1 × 10 -6 mbar, 10 min) and that the 18 O surface content is proportional to the bulk reduction state. UV-visible adsorption spectra and resistivity measurements also scale with the reduction states of crystals. Only the (1 × 1) structure is observed on the surface of slightly reduced crystals. Annealing in oxygen induces additional metastable structures, i.e., TiO 2 clusters on blue crystals and rosette networks on dark blue crystals.

Interest in high-spin organic materials is driven by opportunities to enable far-reaching fundamental science and develop technologies that integrate light element spin, magnetic, and quantum functionalities. Although extensively studied, the intrinsic instability of these materials complicates synthesis and precludes an understanding of how fundamental properties associated with the nature of the chemical bond and electron pairing in organic materials systems manifest in practical applications. Here, we demonstrate a conjugated polymer semiconductor, based on alternating cyclopentadithiophene and thiadiazoloquinoxaline units, that is a ground-state triplet in its neutral form. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-to-low spin energy gap of 9.30 × 10−3 kcal mol−1. The strongly correlated electronic structure, very narrow bandgap, intramolecular ferromagnetic coupling, high electrical conductivity, solution processability, and robust stability open access to a broad variety of technologically relevant applications once thought of as beyond the current scope of organic semiconductors.