Electrocatalytic proton reduction to form dihydrogen (H 2 ) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main-group-element-based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (À 4.6 mA cm À 2 at À 1.7 V vs. Fc/Fc + ) and high selectivity (H 2 Faradaic efficiency of 94 % at À 1.7 V vs. Fc/Fc + ) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOF max ) for H 2 production was obtained as 1099 s À 1 . Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron-chemical-electron-chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

Bacterial modular type I polyketide synthases (PKSs) are complex multidomain assembly line proteins that produce a range of pharmaceutically relevant molecules with a high degree of stereochemical control. Due to their colinear properties, they have been considerable targets for rational biosynthetic pathway engineering. Among the domains harbored within these complex assembly lines, ketoreductase (KR) domains have been extensively studied with the goal of altering their stereoselectivity by site‐directed mutagenesis, as they confer much of the stereochemical complexity present in pharmaceutically active reduced polyketide scaffolds. Here we review all efforts to date to perform site‐directed mutagenesis on PKS KRs, most of which have been done in the context of excised KR domains on model diffusible substrates such as β‐keto N‐acetyl cysteamine thioesters. We also discuss the challenges around translating the findings of these studies to alter stereocontrol in the context of a complex multidomain enzymatic assembly line.

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We present ground-based atmospheric measurements of ratios between anthropogenic carbon monoxide (CO an ) and carbon dioxide (CO 2an ) (β −1 ratios) in Cookeville, Tennessee (36.1628°N, 85.5016°W), a medium-sized city located within the Eastern Highland Rim region of the United States, obtained using a continuous wavelength-scanned cavity ring-down spectrometer (CRDS). In contrast to the summer season, the winter and spring β −1 values are reasonably high, where a tight correlation between the above-background mole fractions of CO and CO 2 is exhibited (as given by the high R 2 value). The winter season is characterized by relatively high CO an than biospheric CO 2 signals (CO 2Bio ) due to fossil fuel heating sources and reduced biospheric uptake of CO 2 . The lowest estimated seasonal background CO 2 and CO mole fractions during the study period are 407.1 ± 11.3 ppm and 144.9 ± 2.1 ppb, respectively. During the summer, the biogenic CO source from isoprene oxidation collocates with direct anthropogenic CO sources, leading to uncertainties in the calculated summertime β −1 values. For 2017, β −1 values (ppb:ppm) of 9.7 ± 0.4, 5.3 ± 0.4, and 2.0 ± 0.2 were obtained for the winter, spring and summer seasons, respectively. In 2018, a similar seasonal variability in the β −1 ratios was obtained with values of 8.7 ± 0.5, 7.4 ± 0.7, and 2.6 ± 0.5 for winter, spring, and summer seasons, respectively. While the estimated percentage contribution of the oxidation reaction between the OH radical and isoprene (CH 2 C(CH 3 )−CHCH 2 + OH) to the total summertime CO budget is less than 50%, the relative amounts may be significant enough to imply that any future study based on the CO as a tracer of combustion emission should account for its photochemical production through biogenic volatile organic compounds in the summertime.