Pedogenic carbonate is an important archive for paleoclimate, paleoecology, and paleoelevation studies. However, it can form under seasonal environmental conditions that differ significantly from the ...mean growing season environment or mean annual conditions, potentially complicating its use for proxy reconstructions. The observed seasonal temperature is typically, but not always, biased high relative to mean annual air temperature (MAT). To evaluate the annual timing of pedogenic carbonate formation, ten different soils were sampled across the western United States. Sites were selected to span a variety of precipitation regimes and soil orders. Precipitation regimes ranged from arid sites (mean annual precipitation (MAP) <20cm) that receive the majority of precipitation during the winter to wetter sites (MAP >50cm) dominated by summer precipitation. Pedogenic carbonate formation temperatures derived from clumped isotope measurements ranged between 6 and 22°C, with most samples falling at or below MAT. Clumped isotope temperatures were compared to monthly precipitation normals and modeled monthly values of evapotranspiration and soil water content. Results show that carbonate formation temperatures agree with the annual timing of soil water depletion, suggesting soil moisture content is a primary control on the timing of pedogenic carbonate formation. Although the seasonal bias is a function of environmental factors that are difficult to reconstruct in paleo-studies, the use of other paleosol proxies can help to assess if changes in clumped isotope temperatures are a function of changes in air temperature or hydrology. These results have important implications for the production of accurate paleoclimate and paleoelevation estimates.
•Seasonal timing of pedogenic carbonate formation is explored across four different precipitation regimes.•At multiple sites, clumped isotope data suggest carbonate formation during cool points of the year.•Seasonal fluctuations in soil water content can explain the carbonate formation temperatures observed at different sites.•Factors affecting soil water storage, such as texture, should be considered when using soil carbonate in paleoapplications.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Carbohydrate–Aromatic Interactions in Proteins Hudson, Kieran L; Bartlett, Gail J; Diehl, Roger C ...
Journal of the American Chemical Society,
12/2015, Volume:
137, Issue:
48
Journal Article
Peer reviewed
Open access
Protein–carbohydrate interactions play pivotal roles in health and disease. However, defining and manipulating these interactions has been hindered by an incomplete understanding of the underlying ...fundamental forces. To elucidate common and discriminating features in carbohydrate recognition, we have analyzed quantitatively X-ray crystal structures of proteins with noncovalently bound carbohydrates. Within the carbohydrate-binding pockets, aliphatic hydrophobic residues are disfavored, whereas aromatic side chains are enriched. The greatest preference is for tryptophan with an increased prevalence of 9-fold. Variations in the spatial orientation of amino acids around different monosaccharides indicate specific carbohydrate C–H bonds interact preferentially with aromatic residues. These preferences are consistent with the electronic properties of both the carbohydrate C–H bonds and the aromatic residues. Those carbohydrates that present patches of electropositive saccharide C–H bonds engage more often in CH−π interactions involving electron-rich aromatic partners. These electronic effects are also manifested when carbohydrate–aromatic interactions are monitored in solution: NMR analysis indicates that indole favorably binds to electron-poor C–H bonds of model carbohydrates, and a clear linear free energy relationships with substituted indoles supports the importance of complementary electronic effects in driving protein–carbohydrate interactions. Together, our data indicate that electrostatic and electronic complementarity between carbohydrates and aromatic residues play key roles in driving protein–carbohydrate complexation. Moreover, these weak noncovalent interactions influence which saccharide residues bind to proteins, and how they are positioned within carbohydrate-binding sites.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Drylands occupy nearly 40% of the land surface and comprise a globally significant carbon reservoir. Dryland‐atmosphere carbon exchange may regulate interannual variability in atmospheric CO2. ...Quantifying soil respiration rates in these environments is often complicated by the presence of calcium carbonates, which are a common feature of dryland soils. We show with high‐precision O2 measurements in a laboratory potted soil experiment that respiration rates after watering were similar in control and carbonate treatment soils. However, CO2 concentrations were up to 72% lower in the carbonate treatment soil because CO2 was initially consumed during calcite dissolution. Subsequently, CO2 concentrations were over 166% greater in the carbonate treatment soil as respiration slowed and calcite precipitated, releasing CO2. Elevated δ13C values of soil CO2 (>6‰ higher in the treatment than control) confirm that observed differences were due to calcite dissolution. These findings demonstrate that calcite dissolution and precipitation can occur rapidly enough to affect soil gas compositions and that changes in soil CO2 are not always directly related to changes in soil respiration rates. Studies of local soil respiration rates and carbon exchange are likely to be influenced by dissolution and precipitation of calcium carbonates in soils. We estimate that one fifth of global soil respiration occurs in soils that contain some amount of soil carbonate, underscoring the need to account for its obscuring effects when trying to quantify soil respiration and net ecosystem exchange on a regional or global scale.
Plain Language Summary
Carbon dioxide can be removed from the atmosphere by plants and stored in soils. Respiration in soils can later return it to the atmosphere. Measuring how much carbon dioxide is returned to the atmosphere is important for soils in water‐limited locations, because they may control how much carbon is stored in terrestrial environments from year‐to‐year. This measurement is complicated because the dissolution and formation of calcite, a mineral commonly found in arid soils, can also change the amount of soil carbon dioxide. We conducted a laboratory experiment with two soils, one containing calcite and one without calcite to study how calcite affects soil carbon dioxide. Our results show that calcite effects can be identified by simultaneously measuring the amount of carbon dioxide and oxygen present in soils. After a simulated rainfall, calcite dissolves and temporarily limits the amount of carbon dioxide returned to the atmosphere. However, this storage is only temporary, because carbon dioxide is gradually released back to the atmosphere as calcite reforms when the soil dries out. This work illustrates that calcite formation and dissolution can occur rapidly within soils. It also highlights the need to understand calcite behavior to accurately measure soil carbon fluxes.
Key Points
Soil calcite dissolution and precipitation can mask or inflate the actual respiration‐driven CO2 response to precipitation
High‐precision measurements of soil CO2 and O2 enable indirect monitoring of soil calcite dissolution and precipitation
Calcite dissolution and formation is rapid enough to affect soil CO2 δ13C values, suggesting semiclosed system behavior of soil gasses
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Allylation and conjunctive cross‐coupling represent two useful, yet largely distinct, reactivity paradigms in catalysis. The union of these two processes would offer exciting possibilities in organic ...synthesis but remains largely unknown. Herein, we report the use of allyl electrophiles in nickel‐catalyzed conjunctive cross‐coupling with a non‐conjugated alkene and dimethylzinc. The transformation is enabled by weakly coordinating, monodentate aza‐heterocycle directing groups that are useful building blocks in synthesis, including saccharin, pyridones, pyrazoles, and triazoles. The reaction occurs under mild conditions and is compatible with a wide range of allyl electrophiles. High chemoselectivity through substrate directivity is demonstrated by the facile reactivity of the β‐γ alkene of the starting material, whereas the ϵ‐ζ alkene of the product is preserved. The generality of this approach is further illustrated through the development of an analogous method with alkyne substrates. Mechanistic studies reveal the importance of the dissociation of the weakly coordinating directing group to allow the allyl moiety to bind and facilitate C(sp3)−C(sp3) reductive elimination.
Nickel catalysis enables regioselective 1,2‐allylmethylation of unactivated alkenes in a variety of N‐allyl heterocycles. The resulting alkene product remains untouched and can be easily diversified.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Total Synthesis of Kibdelomycin He, Chi; Wang, Yu; Bi, Cheng ...
Angewandte Chemie,
August 8, 2022, Volume:
61, Issue:
32
Journal Article
Peer reviewed
Open access
A modular total synthesis of kibdelomycin is disclosed that should enable structure–activity relationship (SAR) studies of this interesting class of antibiotics. The route uses simple building blocks ...and addresses lingering questions about its structural assignment and relationship to amycolamicin, a recently described natural product reported to have a similar structure. Initial antibacterial assays reveal that both C‐22 epimers (the N‐glycosidic linkage) of the natural product have similar activity while structurally truncated analogs lose activity.
A convergent synthesis of kibdelomycin, which leveraged readily available starting materials, enabled the efficient preparation of the potent antimicrobial and its C‐22 epimer in 16 steps (LLS). The modularity of the strategy provided rapid access to 8 analogs, allowing investigation of structural activity relation.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Paleosols comprise an important archive of continental paleoclimate information throughout geologic history, but resolving temperature seasonality poses a challenge to the application of ...paleosol-derived temperature proxies. Not only does seasonality exert a strong control on the biosphere, but it can also obfuscate accurate interpretation of proxy records of paleotemperature. In order to examine the effect temperature seasonality has on paleosol temperature proxies, soil temperature data were compiled from over 200 stations that comprise the NRCS Soil Climate Analysis Network. Observed and modeled seasonal soil temperature variations were then compared to quantify system behavior. Greater than predicted soil temperature seasonality is observed at nearly half of the sites, driven primarily by direct heating of the soil surface by solar radiation. The ground-heating effect becomes more pronounced at sites receiving less than 600 mm of annual precipitation, with an average effect > 4 °C when mean annual precipitation falls below 300 mm. Large ground heating effects cannot be presumed for all carbonate-bearing paleosols, but the effect can be approximated when combined with paleo-precipitation estimates. Approximately two-fifths of sites record less temperature variation than predicted. This reduction in soil temperature seasonality is a result of warmer cold-season soil temperatures, driven by processes such as snow cover insulation and explains why pedotransfer functions break down below mean annual air temperatures (MAAT) of 6 to 8 °C. Clumped isotope measurements of pedogenic carbonate from a stacked series of late Eocene paleosols from Northeastern Spain are also examined to demonstrate how the documented seasonal trends in modern soils can inform paleo-applications, by considering carbonate formation depth and predicted ground heating effect. These paleosol results are best explained by a MAAT of ∼27 °C with annual temperature seasonality of 25 °C.
The electrophilic C−H palladation and Heck-type alkenylation of the tetrahydro1,2-apyrimidine scaffold leads to exclusive formation of the C(7) adducts, and this palladium-catalyzed process is ...applicable to a broad range of alkenyl components. Mechanistic studies suggest that palladation is selective for C(7), and there was no evidence for C(9) metalation; the latter corresponds to the pathway observed previously with N-methylpyridone.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
For large-eddy simulation of turbulent premixed reacting flows, major challenges stem from the inability to resolve the flame in a computationally affordable manner. These challenges are most evident ...in combustors characterized by large domains and thin flames. In these applications, the thickened flame model may be used to extend the flame artificially to a numerically resolvable size through a thickening factor. Thicker flames exhibit suppressed wrinkling in the presence of turbulence, so an efficiency factor increases the flame speed without influencing flame thickness. In contrast to the detailed considerations of unresolved turbulent flame wrinkling, recent work shows that thickened flames do not respond correctly to resolved-scale stretch. In this work, errors in stretch-induced extinction are considered. The already established effect of thickening on extinction is illustrated, and the effect of efficiency factor is characterized in detail. Significant errors in extinction stretch rate are observed analytically and numerically in twin premixed counterflow flame simulations. In general, the original thickened flame formulation does not permit control over extinction, in contrast to its control over freely-propagating-flame thickness and speed. For reactant mixtures with a Lewis number greater than 1, a novel modification of the thickened flame formulation is presented, and through Lewis number adjustments, extinction errors are significantly reduced, while key flame thickening and speed properties of the original formulation are preserved. A test case featuring a turbulent premixed bluff-body-stabilized flame demonstrates that the extinction errors of the original formulation can lead to premature blowoff dynamics and significant statistical errors, if the grid is too coarse. The modified thickened flame model applied to the same grids addresses this issue and provides reasonable flame predictions on all grids, indicating the potential for extending this combustion model to resolutions of greater engineering relevance.
Full text
Available for:
BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
PDF
