ABSTRACT
Astaxanthin is an important ketocarotenoid widely used in industries. However, its application is limited because of its low yield. Sodium citrate (Na-citrate), one of the major carbon sources for microorganisms, can promote cell growth and product accumulation. The basidiomycetous red yeast Xanthophyllomyces dendrorhous was thus used to study the effect of Na-citrate on cell growth and astaxanthin synthesis. The highest biomass and astaxanthin yield (6.0 g/L and 22.5 mg/L) were obtained in shake-flask when 3 g/L Na-citrate was added at 24 h and were 1.8 and 2.0 times higher than those of the control group, respectively. Furthermore, metabolomics and real-time reverse transcription PCR (qRT-PCR) analysis were conducted to study the metabolic pathways of X. dendrorhous in response to Na-citrate. The qRT-PCR assay revealed that Na-citrate facilitated glucose consumption, promoted the metabolic flux from glycolysis, and regulated the tricarboxylic acid (TCA) cycle, providing more energy and substrates for the synthesis of astaxanthin. The gene analysis revealed that adding Na-citrate significantly upregulated the expression of six key genes (ICL, HMGS, crtE, crtYB, crtI, and crtS) involved in pathways related to astaxanthin biosynthesis. These results suggest that exogenous Na-citrate treatment is a potentially valuable strategy to stimulate astaxanthin production in X. dendrorhous.
ABSTRACT
Electrochemical synthesis has been rapidly developed over the past few years, while a vast majority of the reactions proceed through a radical pathway. Understanding the properties of radical intermediates is crucial in the mechanistic study of electrochemical transformations and will be beneficial for developing new reactions. Nevertheless, it is rather difficult to determine the "live" radical intermediates due to their high reactivity. In this work, the formation and structure of sulfonamide N-centered radicals have been researched directly by using the time-resolved electron paramagnetic resonance (EPR) technique under electrochemical conditions. Supported by the EPR results, the reactivity of N-centered radicals as a mediator in the hydrogen atom transfer (HAT) approach has been discussed. Subsequently, these mechanistic study results have been successfully utilized in the discovery of an unactivated C(sp3)-H arylation reaction. The kinetic experiments have revealed the rate-determined step is the anodic oxidation of sulfonamides.
ABSTRACT
INTRODUCTION: Policymakers and researchers describe the COVID-19 epidemics by waves without a common vocabulary on what constitutes an epidemic wave, either in terms of a working definition or operationalization, causing inconsistencies and confusions. A working definition and operationalization can be helpful to characterize and communicate about epidemics. METHODS: We propose a working definition of epidemic waves in the ongoing COVID-19 pandemic and an operationalization based on the public data of the effective reproduction number R. RESULTS: Our operationalization characterizes the numbers and durations of waves (upward and downward) in 179 countries. DISCUSSIONS: The proposed working definition of epidemic waves provides a common and consistent vocabulary that can enable healthcare organizations and policymakers to make better description and assessment of the COVID crisis to make more informed resource planning, mobilization, and allocation temporally in the continued COVID-19 pandemic.
