Semiha Çakir

Crystal structures of mixed ligand complexes containing saccharinate and nicotinamide: [Cu(saccharinato)2 (nicotinamide) (H2O)]⋅(H2O) and [M(nicotinamide)2 (H2O)4]⋅(saccharinate)2 (M = Ni2+, Co2+)

Abstract[M(saccharinato)2(H2O)4] (M = Cu2+, Ni2+, Co2+) react with nicotinamide to form mixed ligand complexes, [Cu(saccharinato)2(nicotinamide)(H2O)]⋅(H2O) (1) and [M(nicotinamide)2(H2O)4]⋅(saccharinate)2 (2: M = Ni2+; 3: M = Co2+), and their crystal structures have been determined by X-ray diffraction. In 1, the Cu2+ atom in an octahedral configuration is coordinated by two monodentate saccharinato ligands in the trans arrangement through the deprotonated ring nitrogens, by two bidentate nicotinamide ligands, one through the pyridyl ring nitrogen and the other through the amide oxygen, and by a water molecule, thus forming a nicotinamide-bridged one-dimensional extended structure. In the isomorphous complexes 2 and 3, the octahedral metal atom, which rides on a crystallographic center of symmetry, is coordinated by two monodentate nicotinamide ligands through the ring nitrogens and four water molecules to form a discrete [M(nicotinamide)2(H2O)4]2 + structural unit, which captures up and down two saccharinate ions, each through three hydrogen bonds: two hydrogen bonds between two water ligands and the ring N and the carbonyl O atoms and one between the amide N of the nicotinamide ligand and the carbonyl O.

Simultaneous square-wave voltammetric determination of riboflavin and folic acid in pharmaceutical preparations

The square-wave voltammetric (SWV) behaviour of riboflavin and folic acid was studied at a static mercury drop electrode by square wave voltammetry. In 0.05M KCl (pH 5.89) a cathodic scan gave peaks at — 0.56 and — 0.87 V vs. Ag/AgCl for riboflavin and folic acid, respectively. The reduction peak currents are linearly dependent on the concentration of vitamins. Both vitamins can be simultaneously determined from the same voltammogram. The method proposed for the determination of riboflavin and folic acid in multivitamin tablets is very simple, rapid and does not involve time-consuming separation steps. The average contents of riboflavin and folic acid were found to be 14.8 ± 1.26% and 1.46 ± 2.66%, for tablet A and 9.86 ±1.40% and 1.47 ± 2.0% for tablet B, respectively.

Binary and ternary complexes of Cu(II) ions with saccharin and cysteine

The square-wave voltammetric behaviour of cysteine and saccharin was studied at a static mercury drop electrode at pH 7.4 in the presence of Cu(II) ions. In the presence of excess Cu(II), cysteine exhibited three reduction peaks for Hg(SR)2 (−0.086 V), free Cu(II) (−0.190 V) and Cu(I)SR (−0.698 V), respectively. Saccharin produced a catalytic hydrogen peak at −1.762 V. In the presence of Cu(II), saccharin gave a new peak (−0.508 V), corresponding to the reduction of Cu(II)–saccharinate, which in the presence of cysteine formed a mixed ligand complex (−0.612 V), CuL2A2 (L=saccharin and A=cysteine). The peak potentials and currents of the obtained complexes were dependent on the ligand concentration and accumulation time. The stoichiometries and overall stability constants of these complexes were determined by Linganes method (voltammetrically) and Job’s method (spectrophotometrically). The mixed ligand complex in the molar ratio 1:2:2 (log β=33.35) turned out to be very much stronger than the 1:1 Cu(I)SR (log β=21.64) and 1:2 Cu(II)–saccharinate (log β=16.68) complexes. Formation of a mixed ligand complex can be considered as a type of synergism.

Synthesis and spectroscopic studies of novel Cu(II), Co(II), Ni(II) and Zn(II) mixed ligand complexes with saccharin and nicotinamide

Abstract Four novel mixed ligand complexes of Cu(II), Co(II), Ni(II) and Zn(II) with saccharin and nicotinamide were synthesised and characterised on the basis of elemental analysis, FT-IR spectroscopic study, UV–Vis spectrometric and magnetic susceptibility data. The structure of the Cu (II) complex is completely different from those of the Co(II), Ni(II) and Zn(II) complexes. From the frequencies of the saccharinato CO and SO 2 modes, it has been proven that the saccharinato ligands in the structure of the Cu complex are coordinated to the metal ion ([Cu(NA) 2 (Sac) 2 (H 2 O)], where NA — nicotinamide, Sac — saccharinato ligand or ion), whilst in the Co(II), Ni(II) and Zn(II) complexes are uncoordinated and exist as ions ([M(NA) 2 (H 2 O) 4 ](Sac) 2 ).

Electrochemical study of cystine in the presence of cadmium and folic acid 1

Abstract The cyclic voltammetric behaviour of cystine (RSSR) was studied on a static mercury drop electrode (SMDE) in the hanging mercury drop elecrode (HMDE) mode at pH 7.4. RSSR gave two peaks at −0.106 V (mercuric cysteine thiolate, RS–Hg–SR) and −0.552 V (mercurous cysteine thiolate, RS–HgHg–SR), respectively. The electrochemical behaviour of complexes of RSSR or folic acid with cadmium(II), and cadmium(II)–cysteinate (Cd(SR) 2 ) complex in the presence of folic acid was also investigated. Experimental results show that the Cd(SR) 2 complex forms a peak at −0.808 V on the mercury electrode surface. Also, the electroreduction of the cadmium(II)–folic acid complex occurs at −0.722 V. These complexes are reduced at a more negative potential than that of the free cadmium ion. When folic acid is added to RSSR solution, it causes shifts of the voltammetric reduction peaks of RS–Hg–SR and RS–HgHg–SR towards positive potentials. The shifts are invariably accompanied by a decrease of peak currents. This is explained as due to the formation of adducts between folic acid and cysteine thiolate (reduction product RS − ) at the electrode surface. Analogous results were observed between folic acid and Cd(SR) 2 complex, and were ascribed to the same cause.

Electrochemical and spectroscopic study of 4-(Phenyldiazenyl)-2-{[tris-(hydroxymethyl)methyl] aminomethylene}cyclohexa-3,5-dien-1(2H)-one: mechanism of the azo and imine electroreduction

Newly synthesized 4-(phenyldiazenyl)-2-{[tris(hydroxymethyl)methyl]aminomethylene} cyclohexa-3,5-dien-1(2H)-one was characterized by elemental analysis, FT-IR, NMR, electronic spectra, voltammetry. Tautomeric equilibrium of 4-(phenyldiazenyl)-2-{[tris(hydroxymethyl) methyl]aminomethylene}cyclohexa-3,5-dien-1(2H)-one in DMSOd is supported by 1H NMR data. The nature of electrochemical process of 4-(phenyldiazenyl)-2-{[tris(hydroxymethyl) methyl]aminomethylene}cyclohexa-3,5-dien-1(2H)-one in Britton-Robinson buffer (pH 2-9) was studied on the HMDE by square-wave (SWV), adsorptive stripping square-wave (AdSWV) and cyclic voltammetry (CV). The electrochemical parameters (Ip/Ep, Ip/v, Ip/pH, Ip/tacc) of the compound were determined.

Synthesis, spectroscopic and voltammetric studies of mixed-ligand copper(II) complexes of amino acids

Two CuII complexes of the type [Cu(L1)(L2)] (where L1 = tryptophanate or phenylalaninate; L2 = cysteine thiolate) have been prepared and characterised, and their spectrophotometric and voltammetric behaviour has been investigated. The results obtained by means of FT-IR, e.s.r., u.v.–vis. spectroscopy and by voltammetry revealed the existence of two different [Cu(L1)(L2)] complexes. A significance decrease in the g|| value and, concomitantly, an increase in the d–d transition energy was observed when a mixed-ligand complex is present. The observed anisotropic g-values indicate the presence of CuII in a tetragonally distorted octahedral geometry. Formation of a mixed-ligand copper complex can be considered as a type of synergism in the presence of cysteine. The redox state CuII or CuI of copper in the Cu(L2) complex depends on the analysing conditions, i.e., cysteine forms a CuII complex under aerobic conditions and a CuI complex in anaerobic media. Tryptophan or phenylalanine is bound to CuII ions in the Cu(L1) complexes.

Electrochemical monitoring of the interaction of doxorubicin with nicotinamide and Fe(III) ions under aerobic and anaerobic conditions

The interaction of doxorubicin with Fe(III) ions and nicotinamide (NA) has been followed by square-wave voltammetry, cyclic voltammetry and UV-VIS. spectroscopy techniques at aerobic and anaerobic conditions. Fe(III)-doxorubicin complex gives a 1-electron reversible step at -0.494 V and a shoulder at 580 nm. Further, the Fe(III)-doxorubicin complex was found to be more stable at aerobic conditions. In the presence of NA, an intermediate (NA-Fe(III)-DQ) forms at -0.462 V under aerobic conditions. Because of the formation of this intermediate, nicotinamide may reduce the cardiotoxic effect of doxorubicin and cause to its detoxification.

Voltammetry of resazurin at a mercury electrode

Electrochemical behavior of resazurin on HMDE in Britton-Robinson (B-R) buffers (pH 2.0–10.0) was studied using the square-wave voltammetry (SWV), square-wave adsorptive stripping voltammetry (SWAdSV), and cyclic voltammetry (CV) techniques. The voltammogram of resazurin in B-R buffer at pH < 4.0 exhibited two cathodic reduction peaks. The voltammetric peaks were obtained at −0.144 V (reversible) and −1.250 V (irreversible) at pH 3.2, and correspond to the reduction of resorufin to dihydroresorufin and to the catalytic hydrogen wave, respectively. At pH > 4.0, a new irreversible cathodic reduction peak, assigned to the protonation of N-oxide on the phenoxazin ring, was observed. Electrochemical parameters (Ip/Ep, Ip/v, Ip/pH, Ip/tacc) of the compound were determined. From the voltammetric data, electrochemical reduction mechanisms for all peaks have been suggested. Maximum peak current for the reversible peak was obtained at pH 4.1. A linear relationship between the current and concentration was determined, and also the lowest detection limit was found as 3.25 × 10−8 mol L−1 and 1.98 × 10−10 mol L−1 for SWV and SWAdSV, respectively.

Electrochemical study of the complexes of aspartame with Cu(II), Ni(II) and Zn(II) ions in the aqueous medium

The voltammetric behaviours of aspartame in the presence of some metal ions (Cu(II), Ni(II), Zn(II)) were investigated. In the presence of aspartame, copper ions reduced at two stages with quasi-reversible one-electron and, with increasing the aspartame (L) concentration, Cu(II)L(2) complex reduces at one-stage with irreversible two-electron reaction (-0.322 V). Zn(II)-aspartame complex (logbeta=3.70) was recognized by a cathodic peak at -1.320 V. Ni(II)-aspartame complex (logbeta=6.52) is reduced at the more positive potential (-0.87 V) than that of the hydrated Ni(II) ions (-1.088 V). In the case of the reduction of Ni(II) ions, aspartame serves as a catalyst. From electronic spectra data of the complexes, their stoichiometries of 1:2 (metal-ligand) in aqueous medium are determined. The greatness of these logarithmic values is agreement with Irwing-Williams series (NiZn).