Amino Acid Complexation Fractionates Nickel Isotopes: Implications for Tracing Nickel Cycling in the Environment

Nickel (Ni) is an essential cofactor in many proteins. Ni stable isotopes have been shown to undergo isotope fractionation in microorganisms and plants. However, the mechanisms driving this fractionation are poorly understood. Here, we present experimental data on Ni isotope fractionation during binding by common Ni-binding amino acids: glutamate (carboxylate side chain), histidine (imidazole side chain), and cysteine (sulfhydryl side chain). We used an equilibrium Donnan dialysis approach to separate free versus bound Ni and measured the isotopic composition of both pools via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Our results reveal that the glutamate and cysteine favor heavy ⁶⁰Ni (Δ^60/58Ni_glutamate = +0.80 ± 0.08; Δ^60/58Ni_cysteine = +1.27 ± 0.08‰), while histidine causes little isotope shift (-0.12 ± 0.16‰). We then conducted experiments using a short peptide that is a structural analogue for acetyl-CoA synthetase and Ni-iron hydrogenase metal-binding sites. The peptide preferentially bound the heavy ⁶⁰Ni with a Δ^60/58Ni_peptide value of +0.74 ± 0.04‰. The Ni isotope effect associated with peptide binding corresponded directly to the fractionation expected based on the coordinating ligands. This work represents an important first step in understanding the mechanistic controls on Ni isotope fractionation and the drivers of Ni isotope fractionation in biological and environmental systems.