Publications

The synthesis of new isoreticular non-interpenetrated woven covalent organic frameworks (COFs) was achieved by linking aldehyde-functionalized copper(I) bisphenanthroline complexes with benzidine linkers in the presence of a bulky anion, diphenylphosphinate (PO₂Ph₂^-) to give metalated COF-506-Cu and, upon removal of copper(I), the demetalated COF-506. The structures of these COFs were determined by a combination of powder X-ray diffraction and electron microscopy techniques. Guest-accessibility to the pores of the two frameworks was examined by vapor and dye inclusion studies and compared to the already reported doubly-interpenetrated COF-505-Cu. Remarkably, COF-506 was found to take up guest molecules that exceed the size of the COF-506-Cu pores, thus giving credence to the notion of a novel mode of motional dynamics in solids we term 'adaptive inclusion'.

Plastics are ubiquitous in modern life. They are primarily made from synthetic carbon-based polymers— organic macromolecules made up of many repeating subunits called monomers—and are designed to be durable and resistant to degradation. Global plastics production has reached a rate of more than 400 million metric tons (MTs) per year, with more than 8 billion MTs produced in the past 50 years. Average production has increased by 36% in the past decade and is projected to grow to 700 million MTs in 2030—about 80 kg of plastics produced for every human on earth. Globally, 20% of discarded plastics are recycled (<10% in the United States), primarily using mechanical processes, and about 25% are incinerated for energy recovery. More than half are deposited into landfills or released into the environment. Thus, discarded plastics pose a long-term environmental challenge: For example, at the current rate of plastics production and disposal, the mass of plastics in the ocean is predicted to exceed the mass of fish by 2050. Current approaches to reducing unwanted plastics are insufficient to address the growing accumulation of discarded plastics. Although incineration eliminates unwanted plastics and recovers some of the energy used to make them, it uses up the potential resource and creates unwanted byproducts. Mechanical recycling—which involves shredding, heating, and remolding of the plastics—is more efficient than making them from petroleum products, using less than half as much energy to generate new plastics. However, mechanical recycling generally degrades, or downcycles, the polymers. Chemical recycling involves deconstruction of polymers by chemical processes to monomers for conversion back into polymers or to molecular intermediates that can be used as fuel or feedstock by the chemical industry. However, present methods (e.g., pyrolysis—high-temperature decomposition in the absence of oxygen), are energy-intensive and require further processing to make products. Chemical upcycling of polymers—the process of selectively converting discarded plastics into chemicals, fuels, or materials with higher value—holds the promise of changing the paradigm for discarded plastic from waste to valued resource. A significant opportunity exists for fundamental research to provide the foundational knowledge required to move toward a circular lifecycle for plastics, in which the chemical constituents of plastics are reformed into polymers or repurposed to give them another life. To identify the fundamental challenges and research opportunities that could accelerate the transformation of discarded plastics to higher-value fuels, chemicals, and materials, the US Department of Energy, Office of Science, Office of Basic Energy Sciences sponsored a Roundtable on Chemical Upcycling of Polymers, which was held near Washington, DC, on April 30–May 1, 2019. This roundtable identified four Priority Research Opportunities to address the complex chemical transformations and physical processes underlying the upcycling of discarded plastics.

Abstract Objective The objective of this study was to examine the hypothesis that abdominal and gluteal adipocyte turnover, lipid dynamics, and fibrogenesis are dysregulated among insulin‐resistant (IR) compared with insulin‐sensitive (IS) adolescents with obesity. Methods Seven IS and seven IR adolescents with obesity participated in a 3‐h oral glucose tolerance test and a multi‐section magnetic resonance imaging scan of the abdominal region to examine body fat distribution patterns and liver fat content. An 8‐week 70% deuterated water ( 2 H 2 O) labeling protocol examined adipocyte turnover, lipid dynamics, and fibrogenesis in vivo from biopsied abdominal and gluteal fat. Results Abdominal and gluteal subcutaneous adipose tissue (SAT) turnover rates of lipid components were similar among IS and IR adolescents with obesity. However, the insoluble collagen (type I, subunit α2) isoform measured from abdominal, but not gluteal, SAT was elevated in IR compared with IS individuals. In addition, abdominal insoluble collagen Iα2 was associated with ratios of visceral‐to‐total (visceral adipose tissue + SAT) abdominal fat and whole‐body and adipose tissue insulin signaling, and it trended toward a positive association with liver fat content. Conclusions Altered extracellular matrix dynamics, but not expandability, potentially decreases abdominal SAT lipid storage capacity, contributing to the pathophysiological pathways linking adipose tissue and whole‐body IR with altered ectopic storage of lipids within the liver among IR adolescents with obesity.

Abstract According to classic stomatal optimization theory, plant stomata are regulated to maximize carbon assimilation for a given water loss. A key component of stomatal optimization models is marginal water‐use efficiency (mWUE), the ratio of the change of transpiration to the change in carbon assimilation. Although the mWUE is often assumed to be constant, variability of mWUE under changing hydrologic conditions has been reported. However, there has yet to be a consensus on the patterns of mWUE variabilities and their relations with atmospheric aridity. We investigate the dynamics of mWUE in response to vapor pressure deficit (VPD) and aridity index using carbon and water fluxes from 115 eddy covariance towers available from the global database FLUXNET. We demonstrate a non‐linear mWUE‐VPD relationship at a sub‐daily scale in general; mWUE varies substantially at both low and high VPD levels. However, mWUE remains relatively constant within the mid‐range of VPD. Despite the highly non‐linear relationship between mWUE and VPD, the relationship can be informed by the strong linear relationship between ecosystem‐level inherent water‐use efficiency (IWUE) and mWUE using the slope, m *. We further identify site‐specific m * and its variability with changing site‐level aridity across six vegetation types. We suggest accurately representing the relationship between IWUE and VPD using Michaelis–Menten or quadratic functions to ensure precise estimation of mWUE variability for individual sites.

High- (and medium-) entropy alloys have emerged as potentially suitable structural materials for nuclear applications, particularly as they appear to show promising irradiation resistance. Recent studies have provided evidence of the presence of local chemical order (LCO) as a salient feature of these complex concentrated solid-solution alloys. However, the influence of such LCO on their irradiation response has remained uncertain thus far. In this work, we combine ion irradiation experiments with large-scale atomistic simulations to reveal that the presence of chemical short-range order, developed as an early stage of LCO, slows down the formation and evolution of point defects in the equiatomic medium-entropy alloy CrCoNi during irradiation. In particular, the irradiation-induced vacancies and interstitials exhibit a smaller difference in their mobility, arising from a stronger effect of LCO in localizing interstitial diffusion. This effect promotes their recombination as the LCO serves to tune the migration energy barriers of these point defects, thereby delaying the initiation of damage. These findings imply that local chemical ordering may provide a variable in the design space to enhance the resistance of multi-principal element alloys to irradiation damage.

Abstract Xylan is a major component of the plant cell wall and the most abundant noncellulosic component in the secondary cell walls that constitute the largest part of plant biomass. Dicot glucuronoxylan consists of a linear backbone of β(1,4)-linked xylose residues substituted with α(1,2)-linked glucuronic acid (GlcA). Although several genes have been implicated in xylan synthesis through mutant analyses, the biochemical mechanisms responsible for synthesizing xylan are largely unknown. Here, we show evidence for biochemical activity of GUX1 (for GlcA substitution of xylan 1), a member of Glycosyltransferase Family 8 in Arabidopsis (Arabidopsis thaliana) that is responsible for adding the glucuronosyl substitutions onto the xylan backbone. GUX1 has characteristics typical of Golgi-localized glycosyltransferases and a K m for UDP-GlcA of 165 μm. GUX1 strongly favors xylohexaose as an acceptor over shorter xylooligosaccharides, and with xylohexaose as an acceptor, GlcA is almost exclusively added to the fifth xylose residue from the nonreducing end. We also show that several related proteins, GUX2 to GUX5 and Plant Glycogenin-like Starch Initiation Protein6, are Golgi localized and that only two of these proteins, GUX2 and GUX4, have activity as xylan α-glucuronosyltransferases.