Ligand Pool Research Articles

Reaction-Agnostic Featurization of Bidentate Ligands for Bayesian Ridge Regression of Enantioselectivity.

Chiral ligands are important components in asymmetric homogeneous catalysis, but their synthesis and screening can be both time-consuming and resource-intensive. Data-driven approaches, in contrast to screening procedures based on intuition, have the potential to reduce the time and resources needed for reaction optimization by more rapidly identifying an ideal catalyst. These approaches, however, are often nontransferable and cannot be applied across different reactions. To overcome this drawback, we introduce a general featurization strategy for bidentate ligands that is coupled with an automated feature selection pipeline and Bayesian ridge regression to perform multivariate linear regression modeling. This approach, which is applicable to any reaction, incorporates electronic, steric, and topological features (rigidity/flexibility, branching, geometry, and constitution) and is well-suited for early stage ligand optimization. Using only small data sets, our workflow capably predicts the enantioselectivity of four metal-catalyzed asymmetric reactions. Uncertainty estimates provided by Bayesian ridge regression permit the use of Bayesian optimization to efficiently explore pools of prospective ligands. Finally, we constructed the BDL-Cu-2023 data set, composed of 312 bidentate ligands extracted from the Cambridge Structural Database, and screened it with this procedure to identify ligand candidates for a challenging asymmetric oxy-alkynylation reaction.

Photoinduced copper-catalyzed asymmetric cyanoalkylalkynylation of alkenes, terminal alkynes, and oximes.

The asymmetric dicarbofunctionalization of alkenes via a radical relay process can provide routes to diverse hydrocarbon derivatives. Three-component carboalkynylation, limited to particular alkyl halides and using readily available cycloketone oxime esters as redox-active precursors, is restricted by the available pool of suitable chiral ligands for broadening the redox potential window of copper complexes and simultaneously creating the enantiocontrol environment. Herein, we report a new hybrid tridentate ligand bearing a guanidine-amide-pyridine unit for photoinduced copper-catalyzed cyanoalkylalkynylation of alkenes. Leveraging the copper catalyst's redox capability is achieved via merging the electron-rich ligand with a readily organized configuration and enhanced absorption in the visible light range, which also facilitates the enantioselectivity. The generality of the catalyst system is exemplified by the efficacy across a number of alkenes, terminal alkynes and cycloketone oxime esters, working smoothly to give alkyne-bearing nitriles with good yields and excellent enantioselectivity. A mechanistic study reveals that the chiral copper catalyst meets the requirements of possessing sufficient reduction ability, good light absorption properties, and appropriate steric hindrance.

Natural Inhibitors of Mammalian α-Amylases as Promising Drugs for the Treatment of Metabolic Diseases.

α-Amylase is a generally acknowledged molecular target of a distinct class of antidiabetic drugs named α-glucosidase inhibitors. This class of medications is scarce and rather underutilized, and treatment with current commercial drugs is accompanied by unpleasant adverse effects. However, mammalian α-amylase inhibitors are abundant in nature and form an extensive pool of high-affinity ligands that are available for drug discovery. Individual compounds and natural extracts and preparations are promising therapeutic agents for conditions associated with impaired starch metabolism, e.g., diabetes mellitus, obesity, and other metabolic disorders. This review focuses on the structural diversity and action mechanisms of active natural products with inhibitory activity toward mammalian α-amylases, and emphasizes proteinaceous inhibitors as more effective compounds with significant potential for clinical use.

Distributions of humic substances in an estuarine region (Otsuchi Bay, Japan) determined using electrochemical and optical methods

Humic substances are major components of the dissolved organic carbon pool in aquatic environments that can bind to trace metals like copper and iron as organic ligands. The unique chromophoric and fluorescence properties of humic substances enabled measurements using conventional optical methods but recently a new electrochemical method using cathodic stripping voltammetry has emerged. In this study, we employed both methods to measure humic substances and elucidate their distribution patterns in Otsuchi Bay in Japan. The electroactive humic substances (eHS) detected using the electrochemical method were higher in the rivers (range 0.24 to 4.3 mg L−1) than in the coastal bay (range 0.08 to 0.69 mg L−1). Both methods revealed significantly higher humic concentrations in the surrounding rivers than in Otsuchi Bay, indicating that riverine sources are an important source of humic substances to Otsuchi Bay, consistent with previous observations in riverine systems globally. eHS correlated well with optical indices of dissolved organic matter (DOM) including humic and marine humic-like fluorescent DOM and chromophoric DOM. Furthermore, comparison with data from the literature suggests that about 20% of the weaker Cu-binding organic ligand pool in Otsuchi Bay may consist of eHS. As Cu-binding organic ligands, eHS may be important in buffering temporal or seasonal increases in Cu fluxes in estuarine regions, with possible implications for nutrient bioavailability and toxicity for primary producers.

Siderophore production and utilization by marine bacteria in the North Pacific Ocean

AbstractSiderophores are strong iron‐binding molecules produced and utilized by microbes to acquire the limiting nutrient iron (Fe) from their surroundings. Despite their importance as a component of the iron‐binding ligand pool in seawater, data on the distribution of siderophores and the microbes that use them are limited. Here, we measured the concentrations and types of dissolved siderophores during two cruises in April 2016 and June 2017 that transited from the iron‐replete, low‐macronutrient North Pacific Subtropical Gyre through the North Pacific Transition Zone (NPTZ) to the iron‐deplete, high‐macronutrient North Pacific Subarctic Frontal Zone (SAFZ). Surface siderophore concentrations in 2017 were higher in the NPTZ (4.0–13.9 pM) than the SAFZ (1.2–5.1 pM), which may be partly attributed to stimulated siderophore production by environmental factors such as dust‐derived iron concentrations (up to 0.51 nM). Multiple types of siderophores were identified on both cruises, including ferrioxamines, amphibactins, and iron‐free forms of photoreactive siderophores, which suggest active production and use of diverse siderophores across latitude and depth. Siderophore biosynthesis and uptake genes and transcripts were widespread across latitude, and higher abundances of these genes and transcripts at higher latitudes may reflect active siderophore‐mediated iron uptake by the local bacterial community across the North Pacific. The variability in the taxonomic composition of bacterial communities that transcribe putative ferrioxamine, amphibactin, and salmochelin transporter genes at different latitudes further suggests that the microbial groups involved in active siderophore production and usage change depending on local conditions.

MELD-Bracket Ranks Binding Affinities of Diverse Sets of Ligands.

Affinity ranking of structurally diverse small-molecule ligands is a challenging problem with important applications in structure-based drug discovery. Absolute binding free energy methods can model diverse ligands, but the high computational cost of the current methods limits application to data sets with few ligands. We recently developed MELD-Bracket, a Molecular Dynamics method for efficient affinity ranking of ligands [ JCTC 2022, 18 (1), 374-379]. It utilizes a Bayesian framework to guide sampling to relevant regions of phase space, and it couples this with a bracket-like competition on a pool of ligands. Here we find that 6-competitor MELD-Bracket can rank dozens of diverse ligands that have low structural similarity and different net charges. We benchmark it on four protein systems─PTB1B, Tyk2, BACE, and JAK3─having varied modes of interactions. We also validated 8-competitor and 12-competitor protocols. The MELD-Bracket protocols presented here may have the appropriate balance of accuracy and computational efficiency to be suitable for ranking diverse ligands from typical drug discovery campaigns.

Advancing our knowledge of antigen processing with computational modelling, structural biology, and immunology.

Antigen processing is an immunological mechanism by which intracellular peptides are transported to the cell surface while bound to Major Histocompatibility Complex molecules, where they can be surveyed by circulating CD8+ or CD4+ T-cells, potentially triggering an immunological response. The antigen processing pathway is a complex multistage filter that refines a huge pool of potential peptide ligands derived from protein degradation into a smaller ensemble for surface presentation. Each stage presents unique challenges due to the number of ligands, the polymorphic nature of MHC and other protein constituents of the pathway and the nature of the interactions between them. Predicting the ensemble of displayed peptide antigens, as well as their immunogenicity, is critical for improving T cell vaccines against pathogens and cancer. Our predictive abilities have always been hindered by an incomplete empirical understanding of the antigen processing pathway. In this review, we highlight the role of computational and structural approaches in improving our understanding of antigen processing, including structural biology, computer simulation, and machine learning techniques, with a particular focus on the MHC-I pathway.

Spontaneous resolution of two chiral metal–organic frameworks through local geometric and lattice frustration effects

Two chiral metal–organic frameworks that are differentiated by their Cd-centred helical twists are prepared by spontaneous chiral resolution from rigid, aliphatic, and achiral precursors.

Ru3 (CO)12 -Catalyzed Modular Assembly of Hemilabile Ligands by C-H Activation of Phosphines with Isocyanates.

Hemilabile ligands have been applied extensively in transition metal catalysis, but preparations of these molecules typically require multistep synthesis. Here, modular assembly of diverse phosphine-amide ligands, including related axially chiral compounds, is first reported through ruthenium-catalyzed C-H activation of phosphines with isocyanate directed by phosphorus(III) atoms. High reactivity and regioselectivity can be obtained by using a Ru3 (CO)12 catalyst with a mono-N-protected amino acid ligand. This transformation significantly expands the pool of phosphine-amide ligands, some of which have shown excellent efficiency for asymmetric catalysis. More broadly, the discovery constitutes a proof of principle for facile construction of hemilabile ligands directly from the parent monodentate phosphines by C-H activation with ideal atom, step and redox economy. Several dinuclear ruthenium complexes were characterized by single-crystal X-ray diffraction analysis revealing the key mechanistic features of this transformation.

Towards the identification of humic ligands associated with iron transport through a salinity gradient

Humic ligands from boreal rivers have been identified as important sources of iron-binding ligands to the coastal marine environment but remain poorly characterized. A novel method using Fourier transformed infrared spectroscopy (FTIR) was used to identify and quantify iron-binding ligands present in a boreal river in Newfoundland, Canada. 20 to 35% of the total iron load was carried through an artificial salinity gradient, and remained in solution at 35 salinity. Using FTIR combined with linear regression and 2D correlation analysis, we identified two pools of organic ligands, with different behaviour with regards to iron across the salinity gradient. The weaker ligand pool consisted of alkenes, ethers, and esters, and was found to release iron to flocculation at low salinities, and not contribute to iron transport into the marine environment. The stronger ligand group contained carboxylic acids and aliphatic functional groups. This group appears to contain two subgroups, one which was able to retain iron in the dissolved phase at 35 salinity, and another that flocculated out with iron across the salinity gradient. The strong ligands that retain iron in solution through the salinity gradient provide a much-needed source of the micronutrient to the coastal and marine environment, while the other subgroup sequesters iron and carbon in estuarine sediments. The balance between these two subgroups appears to be controlled by the hydrographic and weather conditions at the time of sampling, suggesting a dynamic ligand-iron relationship throughout the year, impacting the biogeochemical cycles of both iron and carbon in contrasting ways.

A juxtamembrane basolateral targeting motif regulates signaling through a TGF-β pathway receptor in Drosophila.

In polarized epithelial cells, receptor-ligand interactions can be restricted by different spatial distributions of the 2 interacting components, giving rise to an underappreciated layer of regulatory complexity. We explored whether such regulation occurs in the Drosophila wing disc, an epithelial tissue featuring the TGF-β family member Decapentaplegic (Dpp) as a morphogen controlling growth and patterning. Dpp protein has been observed in an extracellular gradient within the columnar cell layer of the disc, but also uniformly in the disc lumen, leading to the question of how graded signaling is achieved in the face of 2 distinctly localized ligand pools. We find the Dpp Type II receptor Punt, but not the Type I receptor Tkv, is enriched at the basolateral membrane and depleted at the junctions and apical surface. Wit, a second Type II receptor, shows a markedly different behavior, with the protein detected on all membrane regions but enriched at the apical side. Mutational studies identified a short juxtamembrane sequence required for basolateral restriction of Punt in both wing discs and mammalian Madin-Darby canine kidney (MDCK) cells. This basolateral targeting (BLT) determinant can dominantly confer basolateral localization on an otherwise apical receptor. Rescue of punt mutants with transgenes altered in the targeting motif showed that flies expressing apicalized Punt due to the lack of a functional BLT displayed developmental defects, female sterility, and significant lethality. We also show that apicalized Punt does not produce an ectopic signal, indicating that the apical pool of Dpp is not a significant signaling source even when presented with Punt. Instead, we find that basolateral presentation of Punt is required for optimal signaling. Finally, we present evidence that the BLT acts through polarized sorting machinery that differs between types of epithelia. This suggests a code whereby each epithelial cell type may differentially traffic common receptors to enable distinctive responses to spatially localized pools of extracellular ligands.

Slow Kinetics of Iron Binding to Marine Ligands in Seawater Measured by Isotope Exchange Liquid Chromatography-Inductively Coupled Plasma Mass Spectrometry.

Current understanding of dissolved iron (Fe) speciation in the ocean is based on two fundamentally different approaches: electrochemical methods that measure bulk properties of a heterogeneous ligand pool and liquid chromatography mass spectrometry methods that characterize ligands at a molecular level. Here, we describe a method for simultaneously determining Fe-ligand dissociation rate constants (kd) of suites of naturally occurring ligands in seawater by monitoring the exchange of ligand-bound 56Fe with 57Fe using liquid chromatography-inductively coupled mass spectrometry. Values of kd were determined for solutions of ferrichrome and ferrioxamine E. In seawater, the dissociation rate constant of ferrichrome (kd = 10 × 10-8 s-1) was greater than that of ferrioxamine E (kd = 3.6 × 10-8 s-1). The rates for both compounds were over twice as fast in seawater compared with pure water, suggesting that seawater salts accelerate dissociation. Isotope exchange experiments on organic extracts of natural seawater indicated that ligand-binding sites associated with chromatographically unresolved dissolved organic matter exchanged Fe more quickly (kd = 1.8 × 10-5 s-1) than amphibactin siderophores (kd = 2.15 × 10-6 s-1) and an unidentified siderophore with m/z 709 (kd = 9.6 × 10-6 s-1). These findings demonstrate that our approach can bridge molecular-level ligand identification with kinetic and thermodynamic metal-binding properties.

Feedbacks Between Ocean Productivity and Organic Iron Complexation in Reaction to Changes in Ocean Iron Supply

Low concentrations of iron, an important micronutrient for photosynthetic organisms, limit growth in large parts of the ocean. The solubility and availability of iron is to a large degree determined by organic iron-binding molecules, so-called ligands. While ligands come from a variety of sources, many of them are produced in autotrophic or heterotrophic production in the ocean, leading to the possibility of feedbacks between marine primary production and iron availability. The diversity of ligands, reaching from siderophores, small molecules involved in bacterial iron uptake, to breakdown products and long-lived macromolecules like humic substances, means that feedbacks could be both negative and positive or there may even be no feedback at all. Here we investigate first, how the cycling of this ligand pool can be described simplistically in a model such that it reproduces the observed global distribution of dissolved iron and phosphorus as closely as possible. We show that inclusion of a ligand similar to refractory dissolved organic carbon leads to an improved agreement to observations in our model. Inclusion of a second, shorter-lived siderophore-like ligand does not strongly affect this agreement. In a second step we then study how feedbacks affect how iron distribution and oceanic productivity react to changes in external supply of iron. We show that, to be consistent with present-day iron distribution, the dominant feedback is positive, increasing the sensitivity of global biological productivity and hence carbon cycling to changes in iron supply. The strength of the feedback increases with increasing ligand life-time. The negative feedback associated with siderophore-like ligands has the potential to mitigate the positive feedback, especially at the surface and for global export production, but more research on the production and decay of siderophores is needed for a better quantification. Ocean biogeochemical models that assume a constant ligand concentration and hence neglect possible feedbacks may therefore underestimate the reaction of the global carbon cycle to the strong increase in dust deposition under future or glacial climate conditions.

Synthesis and Preclinical Evaluation of 177Lu-Labeled Radiohybrid PSMA Ligands for Endoradiotherapy of Prostate Cancer.

The prostate-specific membrane antigen (PSMA)-targeted radiohybrid (rh) ligand [177Lu]Lu-rhPSMA-7.3 has recently been assessed in a pretherapeutic dosimetry study on prostate cancer patients. In comparison to [177Lu]Lu-PSMA I&T, application of [177Lu]Lu-rhPSMA-7.3 resulted in a significantly improved tumor dose but also higher kidney accumulation. Although rhPSMA-7.3 has been initially selected as the lead compound for diagnostic application based on the characterization of its gallium complex, a systematic comparison of the most promising 177Lu-labeled rhPSMA ligands is still missing. Thus, this study aimed to identify the rhPSMA ligand with the most favorable pharmacokinetics for 177Lu-radioligand therapy. Methods: The 4 isomers of [177Lu]Lu-rhPSMA-7 (namely [177Lu]Lu-rhPSMA-7.1, -7.2, -7.3, and -7.4), along with the novel radiohybrid ligands [177Lu]Lu-rhPSMA-10.1 and -10.2, were compared with the state-of-the-art compounds [177Lu]Lu-PSMA I&T and [177Lu]Lu-PSMA-617. The comparative evaluation comprised affinity studies (half-maximal inhibitory concentration) and internalization experiments on LNCaP cells, as well as lipophilicity measurements. In addition, we determined the apparent molecular weight (AMW) of each tracer as a parameter for human serum albumin (HSA) binding. Biodistribution studies and small-animal SPECT imaging were performed on LNCaP-tumor bearing mice at 24 h after injection. Results: 177Lu labeling of the radiohybrids was performed according to the established procedures for the currently established PSMA-targeted ligands. All ligands showed potent binding to PSMA-expressing LNCaP cells, with affinities in the low nanomolar range and high internalization rates. Surprisingly, the most pronounced differences regarded the HSA-related AMW. Although [177Lu]Lu-rhPSMA-7 isomers demonstrated the highest AMW and thus strongest HSA interactions, [177Lu]Lu-rhPSMA-10.1 showed an AMW lower than for [177Lu]Lu-rhPSMA-7.3 but higher than for the 177Lu-labeled references PSMA I&T and PSMA-617. In biodistribution studies, [177Lu]Lu-rhPSMA-10.1 exhibited the lowest kidney uptake and fastest excretion from the blood pool of all rhPSMA ligands while preserving a high tumor accumulation. Conclusion: Clinical investigation of [177Lu]Lu-rhPSMA-10.1 is highly warranted to determine whether the favorable pharmacokinetics observed in mice will also result in high tumor uptake and decreased absorbed dose to kidneys and other nontarget tissues in patients.

3-Aminophthalic acid, a new cereblon ligand for targeted protein degradation by O'PROTAC.

In this study, we identified 3-aminophthalic acid as a new ligand of cereblon (CRBN) E3 ubiquitin ligase and developed a phthalic acid-based O'PROTAC for degradation of the ERG transcription factor. This phthalic acid-based O'PROTAC presented an efficacy in degrading ERG comparable to those displayed by pomalidomide-based ERG O'PROTACs. Moreover, phthalic acid-being more chemically stable and economical than classical immunomodulatory drugs (IMiDs)-represents, as a ligand, a new alternative for the development of PROTACs, especially O'PROTACs.

Subsurface iron accumulation and rapid aluminum removal in the Mediterranean following African dust deposition

Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built on laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near-surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminum (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. The Fe/Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al/Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (∼ 100–1000 m), while dFe input from dust was only transient in the surface mixed layer. The rapid transfer of dust to depth, the Fe-binding ligand pool in excess to dFe in subsurface (while nearly saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (< 0.5 nmol kg−1) cause us to question the residence time for this dust-derived subsurface reservoir and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.

Accelerating Molecular Dynamics Enrichments of High-Affinity Ligands for Proteins.

Molecular docking algorithms are used to seek the most active compounds from a pool of ligands. In principle, molecular dynamics (MD) simulations with accurate physical potentials and sampling could yield better enrichments, but they are computationally expensive. Here, we describe a method called MELD-Bracket that utilizes biased replica exchange ladders in MD in order to compete different ligands against each other within a fast bracket style "binding tournament". MELD-Bracket finds best-binders rapidly when ligands are well separated in their binding affinities.

Which are the Keystones in the Dynamic AHR–CYP1A1 Signaling Network?

Kyoreva et al. (2021) showed recently that the enzymatic activity of CYP1A1 is a critical regulator of skin inflammation. In a commentary by van den Bogaard and Perdew (2021), these authors are focusing on the potential relationship between CYP1A1 and the role of a natural AHR ligand under the paragraph “Identification of the natural AHR ligand pool in the skin: Needles in a haystack?”

The complex provenance of Cu-binding ligands in the South-East Atlantic

Organic ligands play a key role in the marine biogeochemical cycle of copper (Cu), a bio-essential element, regulating its solubility and bioavailability. However, the sources, abundance, and distribution of these ligands are still poorly understood. In this study, we examined vertical Cu speciation profiles from the South-East Atlantic (GEOTRACES section GA08). Profiles were collected from a range of ocean conditions, including the Benguela upwelling region, the oligotrophic South Atlantic Gyre, and the Congo River outflow. In general, the lack of a significant correlation between most of the parameters assessed here with Cu speciation data obscures the provenance of Cu-binding ligands, suggesting that Cu speciation in the South-East Atlantic is influenced by a complex interplay between biotic and abiotic processes. Nevertheless, the total dissolved Cu (CuT) illustrated an allochthonous origin in the working area, while Cu-binding ligands showed both an allochthonous and a biogenic, autochthonous origin. Pigment concentrations showed that the phylogeography of different microorganisms influenced the spatial features of the Cu-binding ligand pool in the South-East Atlantic. Allochthonous Cu-binding ligand sources in the upper water column are likely associated with dissolved organic matter which originated from the Congo River and the Benguela upwelling system. Deep water ligand sources could include refractory dissolved organic carbon (DOC), resuspended benthic inputs, and lateral advected inputs from the shelf margin. The degradation of L1-type ligands and/or siderophores in low oxygen conditions may also be a source of L2-type ligands in the deep. Free Cu ion levels (1.7 to 156 fM), the biologically available form of CuT, were below the putative biolimiting threshold of many marine organisms. Two classes of ligands were found in this study with total ligand concentrations ([LT]) ranging from 2.5 to 283.0 nM and conditional stability constants (logKCuL, Cu2+cond) ranging from 10.7 to 14.6. The Cu speciation values were spatially variable across the three subregions, suggesting that biogeochemical processes and sources strongly influence Cu speciation.

Evidence for the production of copper-complexing ligands by marine phytoplankton in the subarctic northeast Pacific

The availability and toxicity of copper (Cu) to marine phytoplankton involve complexation of Cu by dissolved organic ligands of uncertain origin and structure. As part of a GEOTRACES process study we used immobilized copper(II)-ion affinity chromatography (IMAC) to isolate and obtain information about the distribution and potential sources of Cu-complexing ligands along Line P in the subarctic northeast Pacific. Filtered seawater samples were collected from up to five stations during cruises in June 2016, June 2017, August 2017, and September 2018. Copper ligand concentrations were consistently higher at coastal station P4 than at other stations, particularly in surface waters. Ligand concentrations in the upper 40 m at station P26 increased between 2016 and 2018 following the 2014–2016 warming anomaly in the northeast Pacific, whereas an increase in ligand concentration from June to August 2017 coincided with seasonal increases in dissolved organic carbon (DOC) and net community production. At all stations, the depth of highest ligand concentration and maximum chlorophyll concentration coincided. A positive linear correlation between ligand and chlorophyll concentrations suggests that marine phytoplankton are a significant source of Cu ligands along Line P. The lower than expected ligand concentrations at station P26 in June 2016 are consistent with changes in phytoplankton ecology associated with the 2014–2016 warming anomaly. Comparing results from coastal and oceanic waters with those obtained previously in the Canadian Arctic suggests that terrigenous organic matter and marine humic substances contribute to the pool of Cu ligands captured by IMAC.