Speciation analysis of a non-covalent nickel-histidine complex system using hydrophilic interaction liquid chromatography coupled to ESI Q-TOFMS/MS and ICP-MS Majidah Alsaeedi, Huda Alghamdi, Phyllis E. Hayes,Anna M. Hogan and Jeremy D. Glennon Innovative Chromatography Group, Irish Separation Science Cluster (ISSC) Ireland, School of Chemistry, and the Analytical & Biological Chemistry Research Facility (ABCRF), University College Cork, Ireland. Nickel (Ni) is a trace heavy metal of importance in a wide range of biological processes and human conditions. With Ni(II) as the dominant oxidation state, elucidating the mechanisms and complex species in Ni(II) transportation in living systems, toxicity and allergies, and bioavailability pathways is key to understanding its biological effects and location. Histidine (His) is an essential amino acid that contributes to proteins' structure and activity and in the coordination of Cu(II) and Ni(II). The aqueous Nickel(II): Histidine (Ni(II): His) complex system consists primarily of two stepwise complex species, Ni(II)(His) 1 and Ni(II)(His) 2 , in the pH range of 4 to 12. Four separation columns, including Poro-shell EC-C 18 , Halo RP-amide, Poro-shell bare silica-HILIC, and Z-cHILIC fully porous columns, were evaluated to separate Ni(II): His species at various Ni(II) to His ratios. It is shown that different stationary phase functionalities display contrasting separation mechanisms. The optimized separation for each stationary phase was discussed regarding retention mechanisms and chromatographic stability of Ni(II): His species. The Z-cHILIC exhibited stability and ability to distinguish between the Ni(II): His species with a fast separation within 120 seconds. The HILIC method utilizing the Z-cHILIC column was initially optimized for separating Ni(II): His species using UV detection with a mobile phase consisting of 70 % ACN and sodium acetate buffer at pH 6. The method was optimized in terms of the organic modifier content, buffer pH, and concentration. The identity of Ni(II)His 1 and Ni(II)His 2 species was confirmed by LC- electrospray ionization- quadrupole time of flight mass spectrometry (LC-ESI-Q-TOFMS/MS) at negative mode and the offline analysis LC-inductively coupled plasma-mass spectrometry (ICP-MS). References 1. Franklin L, Walker S, Hill G 2020. A DFT study of isolated histidine interactions with metal ions (Ni2+, Cu2+, Zn2+) in a six- coordinated octahedral complex. Journal of Molecular Modeling 26. 2. Farcasanu IC, Mizunuma M, Nishiyama F, Miyakawa T 2005. Role of L-Histidine in Conferring Tolerance to Ni2+ in Sacchromyces cerevisiae Cells. Bioscience, Biotechnology, and Biochemistry 69(12):2343-2348. 3. Köster J, Shi R, von Wirén N, Weber G 2011. Evaluation of different column types for the hydrophilic interaction chromatographic separation of iron-citrate and copper-histidine species from plants. J Chromatogr A 1218(30):4934-4943. 4. Chivers PT, Benanti EL, Heil-Chapdelaine V, Iwig JS, Rowe JL 2012. Identification of Ni-(l-His)2 as a substrate for NikABCDE-dependent nickel uptake in Escherichia coli. Metallomics 4(10):1043-1050.
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