Publications

In order to design more efficient photoelectrochemical cells and photovoltaic devices, it is necessary to identify the limiting processes and to determine the kinetics of the controlling reactions. In this paper we report on a technique for the analysis of relaxation of photoprocesses at illuminated semiconductor electrodes in the frequency domain by transformation of the perturbating function and the transient response. Results are presented for simulated photocurrent transients showing that diagnostic parameters can be extracted from both real‐axis and imaginary‐axis transformation.

Isopentenyl diphosphate (IPP) is the common, five-carbon building block in the biosynthesis of all carotenoids. IPP in Escherichia coli is synthesized through the nonmevalonate pathway, which has not been completely elucidated. The first reaction of IPP biosynthesis in E. coli is the formation of 1-deoxy-D-xylulose-5-phosphate (DXP), catalyzed by DXP synthase and encoded by dxs. The second reaction in the pathway is the reduction of DXP to 2-C-methyl-D-erythritol-4-phos- phate, catalyzed by DXP reductoisomerase and encoded by dxr. To determine if one or more of the reactions in the nonmevalonate pathway controlled flux to IPP, dxs and dxr were placed on several expression vectors under the control of three different promoters and transformed into three E. coli strains (DH5alpha, XL1-Blue, and JM101) that had been engineered to produce lycopene. Lycopene production was improved significantly in strains transformed with the dxs expression vectors. When the dxs gene was expressed from the arabinose-inducible araBAD promoter (P(BAD)) on a medium-copy plasmid, lycopene production was twofold higher than when dxs was expressed from the IPTG-inducible trc and lac promoters (P(trc) and P(lac), respectively) on medium-copy and high-copy plasmids. Given the low final densities of cells expressing dxs from IPTG-inducible promoters, the low lycopene production was probably due to the metabolic burden of plasmid maintenance and an excessive drain of central metabolic intermediates. At arabinose concentrations between 0 and 1.33 mM, cells expressing both dxs and dxr from P(BAD) on a medium-copy plasmid produced 1.4-2.0 times more lycopene than cells expressing dxs only. However, at higher arabinose concentrations lycopene production in cells expressing both dxs and dxr was lower than in cells expressing dxs only. A comparison of the three E. coli strains transformed with the arabinose-inducible dxs on a medium-copy plasmid revealed that lycopene production was highest in XL1-Blue.

Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The rapidly expanding universe of information, media, and learning experiences available through digital technology is creating unique opportunities and vulnerabilities for children and adolescents. These issues are particularly salient during the developmental window at the transition from childhood into adolescence. This period of early adolescence is a time of formative social and emotional learning experiences that can shape identity development in both healthy and unhealthy ways. Increasingly, many of these foundational learning experiences are occurring in on-line digital environments. These expanding vulnerabilities and opportunities are being further amplified for young adolescents growing up in low resourced settings around the world. Cultural and contextual factors influence access, use, and appropriation of digital technology. Further, neurobehavioral changes associated with the onset of puberty often coincide with entry into social media and more autonomous use of technology. In low-and-middle-income countries (LMICs), disparities in access, use, and appropriation of digital media can amplify prevailing economic gaps, and compound gender inequalities during early adolescence. In LMICs, adolescents are often the early adopters of mobile technology and social media platforms. While the impact of social media on the well-being, particularly mental health, of young adolescents has been a focus of research in high-income countries (HICs), much less is known about the impacts of social media use on young adolescents in LMICs. In this paper, we review what is known about the interaction between digital media and early adolescent development. We highlight crucial gaps in the evidence in LMICs; and describe some hypotheses and areas for future research to address these compelling issues.

The qualitative nature of human immunodeficiency virus replication dynamics is examined by using the known action of regulatory proteins. The combination of activation of transcription by the Tat protein and the influence of the Rev protein on processing of RNA leads to a regulatory loop that can have multiple expression rates. In the lower state, the regulatory loop leads to low, or no, production of virus, which corresponds to its quiescent state. Conversely, when the regulatory loop is in the upper state, active production of virus is maintained over time. These features of the kinetic model have implications for both human immunodeficiency virus-related illness and therapeutics and predict testable experimental results in vitro.

Dr. Ritchie writes: "The perspective “Structural Nanocomposites” (Y. Dzenis, 25 January, p. 419) describes a quest for improved structural materials and indicates that composites with nanoscale reinforcements would have “exceptional mechanical properties.” Is this true? Why would reinforcements that are small in size or volume offer any particular benefit over larger-scale reinforcements? As the Perspective correctly asserts, if the composite material is to be used for a small-volume structure, clearly the reinforcements must also be small. In addition, small-volume reinforcements are stronger, as has been known since the early days of research on whiskers (1). In this regard, reinforcement by carbon nanotubes, for example, which are thought of as one of the strongest materials in existence (2), would seem ideal. The problem with this notion is that new materials are not limited by strength, but by resistance to fracture (also known as fracture toughness). It is not by accident that most critical structures, such as bridges, ships, and nuclear pressure vessels, are manufactured from materials that are low in strength but high in toughness. Indeed, the majority of toughening mechanisms mentioned by Dzenis—i.e., crack deflection, plastic deformation, and crack bridging—are promoted by increasing, not decreasing, reinforcement dimensions [e.g., (3)]. Is it any surprise that “results obtained so far are disappointing”?... Dr. Dzenis's reply is included.