Hydrochloric acid (hcl)

FeCl₂·4H₂O, FeCl₃·6H₂O, KMnO₄, NaNO₃, concentrated H₂SO₄, 25 wt% NH₃ and 30 wt% H₂O₂ solution were of analytical grade (Molar Chemicals Ltd., Halásztelek, Hungary) and used without further purification. Reduction was carried out by NaBH₄ and L-ascorbic acid (Spektrum 3D, Debrecen, Hungary). Constant electrolyte concentration and pH were set and maintained using NaOH, HCl and NaCl in analytical purity (Reanal, Budapest, Hungary). Hydroxylamine, ammonium acetate, glacial acetic acid, FeSO₄·7H₂O and 1,10-phenantroline used for spectrophotometric determination of dissolved iron were purchased from Sigma-Aldrich. Ultrapure water produced by a Zeener Power RO&UP system was used as dispersion medium and the experiments were carried out mainly at room temperature (25 °C). Magnetic nanoparticles were synthesized by a traditional co-precipitation method using Fe(II) and Fe(III) salts under strongly alkaline conditions and purified by dialysis against dilute (0.001 M) HCl, as was described in detail elsewhere [4 (link),33 (link)]. Graphite oxide was prepared by the Hummers–Offeman method, using KMnO₄, and NaNO₃ for oxidation of graphite flakes (SGA20 graphite powder, Kropfmühl GmbH, Germany) under highly acidic circumstances (cc. H₂SO₄) [33 (link),43 (link)]. The product was purified by dialysis against water to eliminate the excess of salts originating from synthesis.

4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 2-(N-morpholino)ethanesulfonic acid (MES), ascorbic acid (AA), GSH, and tetrabutylammonium nitrate (TBAN) were purchased from Sigma-Aldrich and used without further purification. KOH, KCl, HCl, ethylenediaminetetraacetic acid (EDTA), and KH-phthalate were obtained from Reanal (Hungary) in puriss quality. Cu(II), Zn(II), and Fe(III) stock solutions were prepared by dissolving CuCl₂, ZnCl₂, and FeCl₃ in water (or in a known amount of HCl solution for Zn(II) and Fe(III)) and complexometry with EDTA was used to determine the exact concentrations. To prepare the Fe(II) stock solution, iron powder was dissolved in an HCl solution under an oxygen-free argon atmosphere. After the dissolution of iron, the solution was filtered, stored, and used under anaerobic conditions. The exact concentration of the Fe(II) stock solution was determined by permanganometric titrations. Milli-Q water was used for sample preparation. For EDTA competition measurements, pH 6 was adjusted with 50 mM MES buffer; for the redox reactions with GSH and AA, the pH of the solutions was set to 7.4 with 50 mM of HEPES. All stock solutions of the reducing agents were prepared freshly before each series of measurements.

HPLC grade acetic and formic acid, myricetin-3-O-rhamnoside, quercetin-3-O-rhamnoside, hirsutenone, oregonin, caffeine and rutin standards, dimethyl sulfoxide-d₆ (99.8 atom% D with 0.03 vol.% TMS) and methanol-d₄ (99.8 atom% D), phosphatidylcholine, cholesterol and the porcine polar brain lipid extract were purchased from Merck (Darmstadt, Germany). Ethyl acetate, n-hexane and methanol of reagent grade, HPLC-MS grade acetonitrile, methanol, n-dodecane, dimethyl sulfoxide, NaCl, HCl, Na₂HPO₄·7H₂O and NaH₂PO₄·H₂O were obtained from Reanal-Ker (Budapest, Hungary). HPLC-grade water was prepared with a Millipore Direct Q5 water purification system (Bedford, MA, USA). All aqueous eluents for HPLC were filtered through MF-Millipore membrane filters (0.45 μm, mixed cellulose esters) (Billerica, MA, USA) and degassed in an ultrasonic bath before use.

8-Hydroxy-2-quinolinecarboxaldehyde,
1-(2-aminoethyl)-piperidine, 4-(2-aminoethyl)morpholine, and 3-morpholinopropylamine
were purchased from Sigma-Aldrich. 4-(2-Hydroxy ethyl)-1-piperazine
ethanesulfonic acid (HEPES) was obtained from Sigma-Aldrich, and KCl,
KOH, HCl, ethylenediamine tetraacetic acid (EDTA), and potassium hydrogen
phthalate were obtained from Reanal (Hungary). All products were used
without further purification. ZnCl₂ and CuCl₂ were purchased from Thermo Scientific and BDH Chemicals, respectively.
CuCl₂ and ZnCl₂ stock solutions were prepared
by the dissolution of anhydrous CuCl₂ and ZnCl₂ in water; their concentrations were determined by complexometry
with EDTA. The water used in all biological studies was double-deionized
in a Milli-Q system (Millipore). All of the remaining chemicals used
were of analytical grade.

2-Formylpyridine, 2-acetylpyridine, and CuCl₂·2H₂O were purchased from commercial suppliers
and used without further purification. 3-(tert-Butoxycarbonyl)amino-2-formylpyridine
and 4-(4-hydroxy-3,5-dimethylphenyl)thiosemicarbazide were synthesized
as reported previously.^{39 (link),40 (link)} KCl, KOH, HCl, and
dimethyl sulfoxide (DMSO) were obtained from Reanal. GSH, 2-morpholinoethanesulfonic
acid (MES), and 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic
acid (HEPES) were purchased from Sigma-Aldrich and used without further
purification. Copper(II) stock solution was prepared by the dissolution
of CuCl₂ in water, and its concentration was determined
by complexometry with ethylenediaminetetraacetic acid (EDTA). The
stock solutions of HL¹–HL³ in DMSO were prepared on a weight-in-volume
basis.