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1. Revisiting Elevated δ ¹³ C Values of Sediment on Modern Carbonate Platforms

   https://doi.org/10.1029/2023GL107703  

   Trower, Elizabeth_J; Hibner, Brianna_M; Lincoln, Tyler_A; Dodd, Jacqueline_E; Hagen, Cedric_J; Cantine, Marjorie_D; Gomes, Maya_L (July 2024, Geophysical Research Letters)

   Abstract The measured carbon isotopic compositions of carbonate sediments (δ¹³C_carb) on modern platforms are commonly¹³C‐enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ¹³C_carbvalues of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ¹³C_DICvalues on a modern carbonate platform. These data demonstrate that δ¹³C_carbvalues on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ¹³C value of atmospheric CO₂and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the¹³C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs. 

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2. NMR “Crystallography” for Uniformly ( ¹³ C, ¹⁵ N)‐Labeled Oriented Membrane Proteins

   https://doi.org/10.1002/anie.201915110  

   Awosanya, Emmanuel O.; Lapin, Joel; Nevzorov, Alexander A. (January 2020, Angewandte Chemie International Edition)

   Abstract In oriented‐sample (OS) solid‐state NMR of membrane proteins, the angular‐dependent dipolar couplings and chemical shifts provide a direct input for structure calculations. However, so far only¹H–¹⁵N dipolar couplings and¹⁵N chemical shifts have been routinely assessed in oriented¹⁵N‐labeled samples. The main obstacle for extending this technique to membrane proteins of arbitrary topology has remained in the lack of additional experimental restraints. We have developed a new experimental triple‐resonance NMR technique, which was applied to uniformly doubly (¹⁵N,¹³C)‐labeled Pf1 coat protein in magnetically aligned DMPC/DHPC bicelles. The previously inaccessible¹H_α–¹³C_αdipolar couplings have been measured, which make it possible to determine the torsion angles between the peptide planes without assuming α‐helical structure a priori. The fitting of three angular restraints per peptide plane and filtering by Rosetta scoring functions has yielded a consensus α‐helical transmembrane structure for Pf1 protein. 

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3. NOAH-( ¹⁵ N/ ¹³ C)-CEST NMR supersequence for dynamics studies of biomolecules

   https://doi.org/10.1039/D2CC02015G  

   Cabrera Allpas, Rodrigo; Hansen, Alexandar L.; Brüschweiler, Rafael (August 2022, Chemical Communications)

   An NMR supersequence is introduced for the rapid acquisition of 15 N-CEST and methyl- 13 C-CEST experiments in the same pulse sequence for applications to proteins. The high sensitivity and accuracy allows the simultaneous quantitative characterization of backbone and side-chain dynamics on the millisecond timescale ideal for routine screening for alternative protein states. 

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4. Calibration of the carbon isotope composition (δ ¹³ C) of benthic foraminifera: Benthic δ ¹³ C Calibration

   https://doi.org/10.1002/2016PA003072  

   Schmittner, Andreas; Bostock, Helen C.; Cartapanis, Olivier; Curry, William B.; Filipsson, Helena L.; Galbraith, Eric D.; Gottschalk, Julia; Herguera, Juan Carlos; Hoogakker, Babette; Jaccard, Samuel L.; et al (June 2017, Paleoceanography)
5. Meta‐analysis of primary producer amino acid δ ¹⁵ N values and their influence on trophic position estimation

   https://doi.org/10.1111/2041-210X.13678  

   Ramirez, Matthew_D; Besser, Alexi_C; Newsome, Seth_D; McMahon, Kelton_W (August 2021, Methods in Ecology and Evolution)

   Abstract Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TP_CSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TP_CSIAestimation is an understanding of beta (β) values—the differences between trophic and source AA δ¹⁵N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue‐specificβvalue variability than typically appreciated.This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources ofβvalue variability and its contribution to TP_CSIAuncertainty. We first synthesized all published primary producer AA δ¹⁵N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ¹⁵N andβvalue variability. Lastly, we evaluated the sensitivity of TP_CSIAestimates to uncertainty in meanβ_Glx‐Phevalues and Glx‐Phe trophic discrimination factors (TDF_Glx‐Phe).We show that variation inβ_Glx‐Phevalues is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non‐vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed toβ_Glx‐Phevalue variability, but we found no clear distinction among C₃, C₄and CAM plantβ_Glx‐Phevalues. Notably, we found that vascular plantβ_Glx‐Lysvalues (+2.5 ± 1.6‰) are considerably less variable thanβ_Glx‐Phevalues, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi‐trophic level sensitivity analyses demonstrate that TP_CSIAestimates are highly sensitive to changes in bothβ_Glx‐Pheand TDF_Glx‐Phevalues but that the relative influence ofβvalues dissipates at higher trophic levels.Our results highlight that primary producerβvalues are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting forβvalue variability to improve TP_CSIAestimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ¹⁵N values to better understand the mechanistic drivers ofβvalue variation. 

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