Hindawi Publishing Corporation Bioinorganic Chemistry and Applications Volume 2016, Article ID 8296365, 8 pages http://dx.doi.org/10.1155/2016/8296365 Research Article Reversible Oxygenation of 2,4-Diaminobutanoic Acid-Co(II) Complexes Xiang Cheng, Yan Huang, Hui Li, Fan Yue, Hongmei Wen, and Jide Wang Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education and Xinjiang Uyghur Autonomous Region, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China Correspondence should be addressed to Jide Wang; [email protected] Received 20 May 2016; Accepted 26 July 2016 Academic Editor: Guillermo Mendoza-Diaz Copyright © 2016 Xiang Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper introduces the structural characterization and studies on reversible oxygenation behavior of a new oxygen carrier Co(II)- 2,4-diaminobutanoic acid (DABA) complex in aqueous solution. The composition of the oxygenated complex was determined by gas volumetric method, molar ratio method, and mass spectrometry, and the formula of the oxygenated complex was determined to be [Co(DABA)2O2]. In aqueous solution, the complex can continuously uptake and release dioxygen and exhibit excellent reversibility of oxygenation and deoxygenation ability. This complex can maintain 50% of its original oxygenation capacity after 30 cycles in 24 h and retain 5% of the original oxygenation capacity after more than 260 cycles after 72 h. When a ligand analogue was linked to histidine (His), the new complex exhibited as excellent reversible oxygenation property as His-Co(II) complex. Insight into the relationship between structural detail and oxygenation properties will provide valuable suggestion for a new family of oxygen carriers. 1. Introduction performanceinsolidstate[20,21],atlowtemperature,or in nonaqueous media [22, 23]. However, most known model Oxygen carriers are a series of natural/synthesized products compounds exhibit poorly reversible oxygenation behavior in that can reversibly bind dioxygen [1]. Studies on oxygen aqueous solution. Only histidine-Co(II) and iodo-histidine- carriers present important implications in revealing the Co(II) [24–26] complexes exhibit excellent reversibility of mechanism of dioxygen transfer and storage in vivo, for dioxygen uptake in aqueous solution to date. The lack of example, in hemoglobin and myohemoglobin [2–5]. Amino accumulation on the relationship between structural details acids can form various metal complexes with Fe(II), Cu(I), and oxygenation properties leads to a knowledge gap in Co(II), and other metal ions [1, 6]. These types of metal the development of novel, inexpensive oxygen carriers. We complexes are active centers of biological macromolecules, previously supposed that the special structure of histidine- such as peptidyl glycine -hydroxylating monooxygenase, Co(II) is the key to the reversibility of dioxygen uptake [14, dopamine -monooxygenase, myoglobin, and hemoglobin 24].Toprovethisconcept,wedesignedandselectedseveral [7–9]. Researches on metal complexes of amino acids can histidine ligand analogues, such as -(2-Pyridyl)--alanine be an aid to prove the biological activities and function [24] and 2, 3-diaminopropionic acid [27], and investigated mechanisms of these complexes in organisms. The design the oxygenation of the resulting Co(II) complexes. Both com- and synthesis of transition metal complexes with good plexes showed excellent reversible oxygenation as histidine- affinity to dioxygen and long reversible oxygenation life Co(II) and supported our assumption. We then selected UV- are key to develop natural oxygen carriers. The most com- Vis spectrophotometry to detect the oxygenation properties monly used central metal ions are Fe(II), Cu(I), Co(II), of potential oxygen carriers. The technique was selected and Mn(II) [10–16], and typical ligands include polyamine, because of its simplicity and usefulness in investigating Schiff base, amino acids, and porphyrins [14, 15, 17–19]. oxygenation in a solution, as UV-Vis absorption spectrum of Certain compounds show good reversible oxygen absorbing theoxygenatedcobaltcomplexissignificantlydistinctfrom 2 Bioinorganic Chemistry and Applications its deoxygenated species. Therefore, the oxygenation and spectra collected in full-scan positive ion mode were obtained reversibility of the complexes can be observed precisely by by scanning over the mass range of m/z 50 to m/z 900. using UV-Vis spectrophotometry through monitoring color Nitrogen (99.999% purity) was used as desolvation and nebu- variations. lization gas, and ultrapure argon (99.999% purity) was used as In this paper, the reversible oxygenation of 2,4- collision gas. A full-scan mass spectrum was obtained by flow −2 −3 diaminobutanoic acid-Co(II) was investigated by UV-Vis injection analysis of individual 1.0 × 10 mol⋅dm solutions spectra and oxygenmetry in aqueous solution at room in water. Acetonitrile/water (50 : 50, v/v) was used as the temperature. The formation of the oxygenated complex mobile phase. The concentration of DABA and the complex −2 −3 was characterized by UV-Vis spectrophotometry, infrared employed for MS determination was 2.0 × 10 mol⋅dm . (IR) spectroscopy, and mass spectrometry (MS), and the composition of the oxygenated complex was determined 2.5. Molar Ratio Method. Eight aliquots of DABA solution by molar ratio method and gas volumetric method. The −4 −3 (DABA =4.0× 10 mol⋅dm ) were separately added to reversible cycles of oxygenation and the oxygenation life 10.0 mL volumetric flasks at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, of the complex were tested in aqueous solution, and all −4 −3 and 4.0 mL. About 1.0 mL of 2.0 × 10 mol⋅dm Co(II) results demonstrated that the complex exhibited excellent solution was then added to each flask, and a pH 10 borate reversibility in its oxygenation. This work represents a buffer solution was added to volume. O2 was bubbled into the shift away from histidine and histidine analogues, which sample solutions for 10 min. The absorbance of the solutions have previously been confirmed to undergo reversible was determined using a UV2450 spectrophotometer with a oxygenation/deoxygenation events, and provides a link 1cmcuvetteat314nm. to biological systems that utilize iron and copper for this purposeinvivo. 2.6. Gas Volumetric Method. A 10.0 mL of Co(II) (1.0 × −1 −3 −1 −3 10 mol⋅dm ) and 10.0 mL of DABA (2.0 × 10 mol⋅dm ) 2. Experimental weremixedtogetherinaconicalflaskinaglovebox.The aqueous solution was immediately placed on a eudiometer, 2.1. Materials. L-2,4-Diaminobutanoic acid hydrochloride which had been filled with oxygen in advance. The decreasing (DABA⋅2HCl) and Co(OAc)2⋅4H2Owereobtainedfrom volume of oxygen with time was measured until volume Shanghai Hanhong Chemical Co. and Shanghai Chemical remained constant. The final volume of oxygen was then Regents Co., respectively. The reagents were used without fur- −3 recorded. ther purification. A CO2-free NaOH solution (0.1 mol⋅dm ) was prepared by standardization with potassium acid phtha- −3 late. An HCl solution (0.1 mol⋅dm )waspreparedfromcon- 2.7. Oxygenmetry. The concentration of dissolved2 O in centrated hydrochloric acid and standardized with anhydrous solution would be consumed during oxygenation, which displays the oxygenation evolution of the complex. Hence, sodium carbonate. High-purity (99.99%) O2 and N2 were used. the concentrations of dissolved O2 in solution were measured using a dissolved oxygen meter over a pH range of 2 to 10 ∘ ∘ −3 at 25 C ± 0.1 C. KCl (0.1 mol⋅dm ) was used to maintain 2.2. UV-Vis Spectrophotometry. Sample solutions were deter- the solution at a constant ionic strength. A 25.0 mL of −3 −3 mined by UV-Vis spectrophotometry. N2 and O2 were 1.2 × 10 mol⋅dm ligand solution was transferred to a alternately bubbled into the solution during testing. Optical ∘ ∘ titration pool and maintained at pH < 2 by adding 2.0 mL absorption spectra were recorded at 25 C ± 0.1 Cona −3 of 0.10 mol⋅dm HCl solution to prevent oxygenation prior Shimadzu UV2450 spectrophotometer using a 1 cm cuvette × −4 ⋅ −3 over the spectral range of 250–600nm. to testing. Afterward, 25.0 mL of 6.0 10 mol dm Co(II) solution was added to the pool, and the solution was sealed with 20.0 mL of cyclohexane to prevent air penetration. 2.3. IR Spectroscopy. The ligand and complex aqueous solu- 2 −3 Therefore, the concentration changes of the dissolved O in tions (C =1.0mol⋅dm ) were analyzed by Bruker EQUINOX solution are reasonably related to the oxygenation of the 55 liquid IR spectroscopy after air exposure for 1 h. The pH Co(DABA)2 complex. The diluted NaOH solution was used value of the complex solution was adjusted to 10 by adding to adjust the pH values of the solution from 2 to 10, and the −3 dilute NaOH solution (0.1 mol⋅dm ). O2 concentrations in solution were detected at each pH. After thepHreached10,diluteHClwasaddedtoadjustsolutionpH 2.4. Mass Spectrometry Method. MS was performed on a values from 10 to 2 and to observe the reversibility of releasing Waters Quattro Premier XE mass spectrometer equipped dioxygen from the complex. with an electrospray ionization source (Micromass, Manch- ester, UK). The mass analyzer was operated in positive 2.8. Construction of the Absorption (A)-pH Curve. 25.0 mL of −4 −3 ionization mode for all analytes, and the optimized param- 4.0 × 10 mol⋅dm solution was initially adjusted to pH 2 ∘ eters were as follows: source temperature, 120 C; desolvation × −4 ⋅ −3 ∘ with dilute HCl solution and 25.0 mL of 2.0 10 mol dm temperature, 450 C; capillary voltage, 3.5 kV; desolvation gas Co(II) solution was added and mixed. The absorption (A) −1 −1 flow, 400 L⋅h ;conevoltage,60V;conegasflow,50L⋅h ; of amino acid-cobalt solution was determined by UV-Vis collision energy, 20 eV; and multiplier, 650 V.