Publications

Catalyst platforms that promote multielectron charge delocalization offer an attractive approach to achieving the CO2 reduction reaction (CO2RR) with selectivity over the competing hydrogen evolution reaction (HER). Here, we show the importance of metal-ligand exchange coupling as an example of charge delocalization that can determine efficiency for photocatalytic CO2RR. A comparative evaluation of iron and cobalt complexes supported by the redox-active ligand tpyPY2Me establishes that the two-electron reduction of [Co(tpyPY2Me)]2+ ([Co]2+) occurs at potentials 770 mV more negative than the [Fe(tpyPY2Me)]2+ ([Fe]2+) analog by maximizing exchange coupling in the latter compound. The positive shift in reduction potential promoted by metal-ligand exchange coupling drives [Fe]2+ to be among the most active and selective molecular catalysts for photochemical CO2RR reported to date, maintaining up to 99% CO product selectivity with total turnover numbers (TON) and initial turnover frequencies (TOF) exceeding 30,000 and 900 min–1, respectively. In contrast, [Co]2+ shows much lower CO2RR activity, reaching only ca. 600 TON at 83% CO product selectivity under similar conditions accompanied by rapid catalyst decomposition. Spin density plots of the two-electron reduced [Co]0 complex implicate a paramagnetic open-shell doublet ground state compared to the diamagnetic open-shell singlet ground state of reduced [Fe]0, rationalizing the observed negative shift in two-electron reduction potentials from the [M]2+ species and lowered CO2RR efficiency for the cobalt complex relative to its iron congener. This work emphasizes the contributions of multielectron metal-ligand exchange coupling in promoting effective CO2RR and provides a starting point for the broader incorporation of this strategy in catalyst design.

Domesticated microalgae hold great promise for the sustainable provision of various bioresources for human domestic and industrial consumption. Efforts to exploit their potential are far from being fully realized due to limitations in the know-how of microalgal engineering. The associated technologies are not as well developed as those for heterotrophic microbes, cyanobacteria and plants. However, recent studies on microalgal metabolic engineering, genome editing and synthetic biology have helped immensely to enhance transformation efficiencies and are bringing new insights into this field. Therefore, this article summarizes recent developments in microalgal biotechnology and examines the prospect of generating specialty and commodity products through the processes of metabolic engineering and synthetic biology. After a brief examination of empirical engineering methods and vector design, the article focuses on quantitative transformation cassette design, elaborates on target-editing methods and emerging digital design of algal cellular metabolism to arrive at high yields of valuable products. These advances have enabled a transition of manners in microalgal engineering from single-gene and enzyme-based metabolic engineering to systems-level precision engineering, from cells created with genetically modified (GM) tags to those without GM tags, and ultimately from proof of concept to tangible industrial application. Finally, future trends are proposed in microalgal engineering, aiming to establish individualized transformation systems in newly identified species for strain-specific specialty and commodity products, while developing sophisticated universal toolkits in model algal species.

Lacking the atmosphere for temperature neutralization, objects in outer space without thermal control undergo large temperature swings. Effective temperature management technologies (TMTs) are essential to avoid undesirable effects caused by extreme thermal conditions. However, existing high-performance TMTs impose additional burden on the limited mass and power budgets of spacecrafts. Very recently, temperature-adaptive solar coatings (TASCs) and temperature-adaptive radiative coatings (TARCs) emerged as novel light-weight, energy-free temperature-regulation approaches for terrestrial objects with excellent thermal performance. Here, we simulate and present the great potential of TASCs and TARCs as future passive TMTs for space objects. A case study of a geosynchronous satellite with body-mounted solar panels covered by TARC exhibits an interior temperature swing as small as 20.3°C–25.6°C in an orbital period even with solar eclipses. These findings provide insight into the superior performance of TASCs and TARCs in space and will promote their application in extraterrestrial missions.

The ability to generate RNA aptamers for synthetic biology using in vitro selection depends on the informational complexity (IC) needed to specify functional structures that bind target ligands with desired affinities in physiological concentrations of magnesium. We investigate how selection for high-affinity aptamers is constrained by chemical properties of the ligand and the need to bind in low magnesium. We select two sets of RNA aptamers that bind planar ligands with dissociation constants (K(d)s) ranging from 65 nM to 100 microM in physiological buffer conditions. Aptamers selected to bind the non-proteinogenic amino acid, p-amino phenylalanine (pAF), are larger and more informationally complex (i.e., rarer in a pool of random sequences) than aptamers selected to bind a larger fluorescent dye, tetramethylrhodamine (TMR). Interestingly, tighter binding aptamers show less dependence on magnesium than weaker-binding aptamers. Thus, selection for high-affinity binding may automatically lead to structures that are functional in physiological conditions (1-2.5 mM Mg(2+)). We hypothesize that selection for high-affinity binding in physiological conditions is primarily constrained by ligand characteristics such as molecular weight (MW) and the number of rotatable bonds. We suggest that it may be possible to estimate aptamer-ligand affinities and predict whether a particular aptamer-based design goal is achievable before performing the selection.

The hydrogenation of isobutene (2-methylpropene) has been monitored near ambient pressure over a Pt(111) single crystal at 295 K by SFG vibrational spectroscopy. During hydrogenation tert-butyl groups and π-bonded isobutene species were the dominant surface species detected. Only small amounts of dehydrogenated species such as isobutylidyne could be observed unless the reaction conditions were very hydrogen poor. The hydrogenation rate for isobutene was at least one order of magnitude lower than for but-1-ene and cis-but-2-ene. Vacuum studies indicated that the rate of tert-butyl hydrogenation was much slower than that of isobutyl, which may force isobutane production to proceed from π-bonded isobutene through the slow kinetic step of isobutyl formation. Vacuum studies of isobutene dehydrogenation show evidence for a stable intermediate during the dehydrogenation of di-σ-bonded isobutene to isobutylidyne.

Retrieving images from large and varied collections using image content as a key is a challenging and importantproblem. In this paper we present a new image representation which provides a transformation from the raw pixel data to a small set of image regions which are coherent in color and texture space. This so-called "blobworld" representation is based on segmentation using the Expectation-Maximization algorithm on combined color and texture features. The texture features we use for the segmentation arise from a new approach to texture description and scale selection. We describe a system that uses the blobworld representation to retrieve images. An important and unique aspect of the system is that, in the context of similarity-based querying, the user is allowed to view the internal representation of the submitted image and the query results. Similar s3,sterns do not offer the user this view into the workings of the system; consequently, the outcome of many queries on these systems can be quite inexplicable, despite the availability of knobs for adjusting the similarity metric.