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Copper

Stable isotopes and natural abundances

Copper has two naturally occurring stable isotopes: ⁶³Cu and ⁶⁵Cu, which exist in natural abundances of 69.17 and 30.83%, respectively. The isotopic composition of Cu is reported in delta notation (in ‰) relative to a NIST SRM 976 standard:

$\delta ^{65}Cu=\left[{\frac {(^{65}Cu/^{63}Cu)_{sample}}{(^{65}Cu/^{63}Cu)_{NIST976}}}\right]$

Chemistry

Copper has two redox states: Cu¹⁺ and Cu²⁺. Cu has an electronic configuration of [Ar] 3d10 4s1. Upon oxidation to Cu¹⁺, one electron is removed from the 4s shell, giving an electronic configuration of [Ar] 3d10 4s0. Upon further oxidation to Cu²⁺, an electron is removed from the 3d shell, giving an electronic configuration of [Ar] 3d9 4s0. Due to its full d-orbital, Cu¹⁺ has diamagnetic resonance. In contrast, Cu²⁺ has one unpaired electron in its d-orbital, giving it paramagnetic resonance.

Equilibrium isotope fractionation

Redox speciation of Cu¹⁺ and Cu²⁺ fractionates Cu isotopes. ⁶³Cu²⁺ is preferentially reduced over ⁶⁵Cu²⁺, leaving the residual Cu²⁺ enriched in ⁶⁵Cu. The equilibrium fractionation factor for speciation between Cu²⁺ and Cu¹⁺ (α_Cu(II)-Cu(I)) is 1.00403 [Zhu et al., 2002].

Biological relevance

Copper can be found in the active sites of most enzymes that catalyze redox reactions (i.e., oxidoreductases), as it facilitates single electron transfers while reversibly oscillating between the Cu¹⁺ and Cu²⁺ redox states. Enzymes typically contain between one (mononuclear) to four (tetranuclear) copper centers, which enable enzymes to catalyze different reactions. These copper centers are generally coordinated to N-, O- and S-containing groups, including histidine, aspartic acid, glutamic acid, cysteine and methionine. Copper's powerful redox capability makes it critically important for biology, but comes at a cost: Cu¹⁺ is a highly toxic metal to cells because it readily abstracts single electrons from organic compounds and cellular material, leading to production of free radicals. Thus, cells have evolved specific strategies for carefully controlling the activity of Cu¹⁺ while exploiting its redox behavior.

Examples of copper-based enzymes

Copper proteins function as electron or oxygen carriers, oxidases, mono- and dioxygenases, superoxide dismutases (SOD) and nitrogen oxide (NO_x) reductases. In mollusks, copper-containing hemocyanins bind and transport O₂ through the blood, much like hemoglobin in humans. One of the three major families of SOD contains Cu and Zn as its metal cofactors. In E. coli, the copper protein multicopper oxidase CueO oxidizes toxic Cu¹⁺ to Cu²⁺ to regulate copper homeostasis. Particulate methane monooxygenase (pMMO) is a copper-containing protein in methanotrophs that oxidizes methane to methanol for both energy and carbon acquisition. Methane oxidation by pMMO is kinetically faster than by soluble methane monooxygenase (sMMO), the iron-containing counterpart to pMMO.

Biological fractionation

The natural ⁶⁵Cu/⁶³Cu varies according to copper's redox form and the ligand to which copper binds. Oxidized Cu²⁺ preferentially coordinates with hard donor ligands (e.g., N- or O-containing ligands), while reduced Cu(I) preferentially coordinates with soft donor ligands (e.g., S-containing ligands) [Fujii et al., 2013]. As ⁶⁵Cu is preferentially oxidized over ⁶³Cu, these isotopes tend to coordinate with hard and soft donor ligands, respectively [Fujii et al., 2013]. Cu isotopes can fractionate upon Cu-bacteria interactions from processes that include Cu adsorption to cells, intracellular uptake, metabolic regulation and redox speciation [Zhu et al., 2002; Navarrete et al., 2011]. Fractionation of Cu isotopes upon adsorption to cellular walls appears to depend on the surface functional groups that Cu complexes with, and can span positive and negative values [Navarrete et al., 2011]. Furthermore, bacteria preferentially incorporate the lighter Cu isotope intracellularly and into proteins. For example, E. coli, B. subtilis and a natural consortia of microbes sequestered Cu with apparent fractionations (ε⁶⁵Cu) ranging from ~-1.0 to -4.4‰ [Navarrete et al., 2011]. Additionally, fractionation of Cu upon incorporation into the apoprotein of azurin was -9.8‰ in P. aeruginosa, and -15.3‰ in E. coli, while ε⁶⁵Cu values of Cu incorporation into Cu-metallothionein and Cu-Zn-SOD in yeast were -17.1 and -11.8‰, respectively [Zhu et al., 2002].

Measurement

⁶⁵Cu/⁶³Cu ratios are measured via multi-conductor inductively coupled plasma mass spectrometry (MC-ICP-MS), which ionizes samples using inductively coupled plasma.

Natural variations in Cu isotopes and concentrations

Natural variations in Cu isotopic compositions of different materials. Data from: Albarede et al., 2011; Weinstein et al., 2011; Jouvin et al., 2012; Larson et al., 2003; Albarede et al., 2016; Vance et al., 2008; Boyle et al., 2012

δ⁶⁵Cu variations in organisms

To first order, δ⁶⁵Cu values in organisms are driven by dietary Cu isotopic compositions. In plants, δ⁶⁵Cu values vary between the different components (seeds, stem and leaves) from -1 to +0.4‰ [Weinstein et al., 2011; Jouvin et al., 2012]. In animals, δ⁶⁵Cu values vary among the different organs. δ⁶⁵Cu values of livers from sheep and mice fed a diet of δ⁶⁵Cu = 0‰ were -1.5‰, while δ⁶⁵Cu values of their kidneys were +1.5‰ [Balter and Zazzo, 2011]. Serum in human blood is typically ⁶⁵Cu-depleted relative to erythrocytes, with these blood components having a range of Cu isotopic compositions from -0.7 to +0.9‰ [Albarede et al., 2011].

Vertical Cu concentration profile in the Pacific ocean [Bruland, 1980]

Vertical δ⁶⁵Cu profile in the Atlantic ocean [Boyle et al., 2012]

Environmental δ⁶⁵Cu variations

Due to equilibrium and biological processes that fractionate Cu isotopes in the marine environment, the bulk isotopic composition of copper (δ⁶⁵Cu = +0.6 to +1.5‰) is different from the δ⁶⁵Cu values of the riverine input (δ⁶⁵Cu = +0.02 to +1.45‰, with discharge-weighted average δ⁶⁵Cu = +0.68‰) to the oceans [Vance et al., 2008; Boyle et al., 2012]. Equilibrium processes that fractionate Cu isotopes include high temperature ion exchange and redox speciation between mineral phases, and low temperature ion exchange between aqueous species or redox speciation between inorganic species [Zhu et al., 2002]. In riverine and marine environments, ⁶⁵Cu/⁶³Cu ratios are driven by preferential adsorption of ⁶³Cu to particulate matter and preferential binding of ⁶⁵Cu to organic complexes [Vance et al., 2008]. Additionally, Cu is strongly cycled in the surface and deep ocean. Cu concentrations are ~5 nM in the deep Pacific [Bruland, 1980] and ~1.5 nM in the deep Atlantic [Boyle et al., 2012]. The deep/surface ratio of Cu in the ocean is typically <10. Depth concentration profiles for Cu are roughly linear due to biological recycling and scavenging processes [Bruland and Lohan, 2003] in addition to adsorption to particles. Similarly, δ⁶⁵Cu values in the Atlantic ocean do not markedly vary with depth [Boyle et al., 2012].

Applications to medicine

Copper isotopes in human plasma may serve as a marker for various types of cancer. In general, the serum of patients with colon, breast and liver cancer appear to be ⁶⁵Cu-depleted relative to the serum of healthy patients, while liver tumors are ⁶⁵Cu-enriched [Albarede et al., 2016]. In one study, the blood of patients with hepatocellular carcinomas was found to be depleted in ⁶⁵Cu by 0.4‰ relative to the blood of non-cancer patients [Balter et al., 2015].

Zinc

Stable isotopes and natural abundances

Zinc has five stable isotopes: