Pharmaceutical Chemistry Journal, Vol. 46, No. 8, November, 2012 (Russian Original Vol. 46, No. 8, August, 2012)

STRUCTURE OF CHEMICAL COMPOUNDS, METHODS OF ANALYSIS AND PROCESS CONTROL CONFIGURATION OF CENTRAL METALS IN POLYURONATES STUDIED BY EPR SPECTROSCOPY N. Sh. Kajsheva1 Translated from Khimiko-Farmatsevticheskii Zhurnal, Vol. 46, No. 8, pp. 51 – 53, August, 2012. Original article submitted December 6, 2010.

Configurations of polyvalent central metals in pectinates and alginates were established as octahedral with a coordination number of six for the central metal in Cu(II) alginate; square-planar, tetrahedral, and pyramidal with coordination number 4 or 5 (for Cr) in Cu(II) pectinate and Cr(III), Mn(II), Co(II), and Ni(II) polyuronates. The polyuronates were formed by interchain dinuclear complexes. The metal atoms were coordinated to O atoms of polymer carboxyls and hydroxyls and H2O molecules. The results provided justification for the mechanism of action of polyuronates as antidotes. Key words: polyuronates, EPR spectroscopy, pectins, alginates

lution resulted in the formation of a neutral reaction medium that did not destroy the polyuronides.

Detrimental ecological circumstances prompted a search for novel enteric sorbents for heavy and toxic metals, among which pectins and alginates were especially interesting in view of their preferred functional properties [1]. Despite the focus of attention on the structures of the coordination complexes (CC) formed by polyuronides with metal cations (polyuronates) [2, 3], the issue of the configuration of the central metals in the polyuronates remains unresolved. We first used EPR spectroscopy for this purpose. EPR provides greater possibilities for obtaining information owing to the resonant absorption or emission of electromagnetic energy that accompanies transitions between split energy states in a paramagnetic substance [4, 5]. The goal of the investigation was to study the configurations of central metals and the nature of the metal-ion—ligand bond in solid polyuronates using EPR spectroscopy. The starting compounds for preparing the metal polyuronates were inorganic salts and polyuronides. Acetates of Cr(III), Mn(II), Co(II), Ni(II), and Cu(II) were always used because hydrolysis of the cation and anion in so-

EXPERIMENTAL PART All salts of Cr3+, Mn2+, Co2+, Ni2+, and Cu2+ were analytically pure acetates. Alginic acid (Arkhangelsk Pilot Algae Complex) complying with quality requirements of GOST 26185–84 “Marine algae, marine grasses, and products of their processing. Analytical methods” was used. Pectate isolation method Beet pectin was obtained from beet pressings by successive operations [6]. First, the pressings were treated with EtOH (96%) in a mass:volume ratio of 1:4 at 60°C for 2 h in order to remove pigments and other impurities. Next, the extract was decanted. This operation was repeated until a colorless extract was obtained (3´). Then, the pressings were rinsed with H2O to remove EtOH. Finally, the pressings were treated with HCl solution (0.03 M) at a mass:volume ratio of 1:4 at 60°C for 1 h. The acidic extract was filtered through red-ribbon filter paper and concentrated in vacuo to one third

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Pyatigorsk State Pharmaceutical Academy of Rozdrav, Pyatigorsk, Russia. 516

0091-150X/12/4608-0516 © 2012 Springer Science+Business Media New York

Configuration of Central Metals

of the initial volume. The concentrate was treated with EtOH (96%) in a 1:2 ratio. The resulting precipitate (pectin) was separated by centrifugation, purified by reprecipitation from aqueous EtOH (96%), and dried at 45°C in a drying cabinet to obtain a dry powder. General method for preparing metal polyuronates Solid metal polyuronates were prepared in the following manner [7]. Cu(II) polyuronate was prepared by mixing aqueous Cu(OAc)2 (0.1 M) with aqueous polyuronide solution (25%) in a 1:10 volume ratio, stirring vigorously, and storing at room temperature for 3 h. The resulting gelatinous precipitate was centrifuged, treated with H2O in a 1:4 ratio for 15 min to rinse the precipitate, and centrifuged again. This operation was repeated three times. The precipitate was dried at 60°C in a drying cabinet to obtain a dry powder. The other metal polyuronates were prepared analogously but with different reagent ratios: Cr(III) polyuronates: aqueous Cr(OAc)3 (0.06 M) and aqueous polyuronide (0.3%) in volume ratio 1:7; Mn(II) polyuronates: aqueous Mn(OAc)2 (0.1 M) and aqueous polyuronide (0.3%) in volume ratio 1:3.5; Co(II) polyuronates: aqueous Co(OAc)2 (0.1 M) and aqueous polyuronide (0.3%) in volume ratio 1:4; Ni(II) polyuronates: aqueous Ni(OAc)2 (0.2 M) and aqueous polyuronide (0.3%) in volume ratio 1:6. EPR spectra of the polyuronates were recorded and spectral parameters were measured on a CMS 8400 EPR spectrometer (Adani, Belarus) in the operating VHF frequency range 9.2 – 9.5 GHz, absolute resolving power 0.006 mT, sensitivity 1015 spin/T, relative uncertainty of the polarizing magnetic field scan amplitude 1.5%, and scan time range 12 – 2400 sec. The studies were carried out at 290 and 77 K (liquid N2 temperature). EPR parameters of the CC such as the position, intensity, width, and shape of symmetric lines in the magnetic field were analyzed. RESULTS AND DISCUSSION Polyuronates of Cr(III), Mn(II), and Cu(II) were most suitable for the study. The spin degeneracy of them was removed in order to observe paramagnetic resonance by regulating the magnetic field strength of the EPR spectrometer. The seven-fold degenerate orbital level of the free Cr(III) ion was split into two triplets and one singlet in an octahedrally symmetric crystal field with the principal state becoming an orbital singlet. The shift of the g-factor for Cr(III) relative to g0 was negative (2.0023), like for other ions in which the 3d-electron shell is less than half full. The shift of the g-factor relative to g0 for Mn(II) was zero to a first approximation. EPR spectra of the Cu(II) CC lacked fine structure. The spectral lines were narrow, which indicated that the closest environment had a symmetry less than octahedral. Hyperfine structure due to coupling of unpaired electron spin with the

517 magnetic moments of 63Cu and 65Cu nuclei (I = 3/2) was observed in spectra of magnetically dilute Cu(II) compounds. The proximity of Cu atoms in undiluted compounds led to the disappearance of hyperfine structure. The shift of the g-factor in Cu(II) CC was positive relative to g0. EPR spectroscopy turned out to be less informative for Co(II) and Ni(II) CC because the crystal field split the spin levels by amounts in the absence of a magnetic field that were significantly greater than the energy of the radio-frequency quantum. Conditions for observing paramagnetic resonance were not satisfied so that the resonance became unobservable. Resonant absorption of energy did not occur for the Co(II) ion. The seven-fold degenerate orbital level of the free high-spin Co(II) ion was split into a singlet and two triplets in a crystal field of octahedral symmetry so that the lower level was a triplet. The degenerate electronic state resulted in strong coupling of the electron shell with vibrations of the nuclear framework and to large line broadening. With this, the spin–lattice or longitudinal relaxation time is very short. Table 1 presents results from an analysis of the EPR spectra (n = 7). The strongest paramagnetic absorption in the studied CC was observed in Mn(II) alginate and pectinate. The spectra consisted of one broad line with a g-factor close to the g0-factor of a free electron. Lowering the temperature from 290 to 77 K increased the signal intensity according to the Curie law [5]. However, it did not decrease the line width. These data confirmed that the Mn(II) ion had the 3d5 state. The broad line width was explained by unresolved fine structure of the spectrum with a closest environment symmetry lower than octahedral.

TABLE 1. EPR Spectral Parameters for Metal Polyuronates T = 290 K Compound

T = 77 K

g

DHe

g

DHe

ACu2+ PCu2+ ACr3+ PCr3+ AMn2+ PMn2+ ACo2+ PCo2+

2.120 1.989 1.978 2.008 2.013 2.119

250 490 520 290 560 704

2.124 1.990 1.979 2.002 2.000

260 590 890 350 550 50; 5

PNi2+

2.140

2.095; 2.007 2.129

280

gII

AII ´ 104, cm– 1

g^

2.383

153

2.078

280

Note: A, alginate; P, pectinate; g, effective electron magnetic moment; gII, effective magnetic moment in the magnetic field direction; g^, magnetic moment in the plane perpendicular to the field direction; DHe, resonance line broadening for effective magnetic field; AII, hyperfine coupling tensor in the magnetic field direction.

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Spectra of Cu(II) alginate and pectinate were slightly different from each other. For Cu(II) alginate, an axial spectrum characteristic of most Cu(II) CC was observed [4, 5, 8]. Hyperfine splitting of lines into four components (m = 4) due to coupling of unpaired electron spin with spins of 63Cu and 65 Cu nuclei was detected near gII, where molecules in which the axial axis is aligned with the direction of the magnetic field vector absorb. Lowering the temperature increased the intensity and decreased the line width so that the spectral lines were resolved better at liquid N2 temperature. The gII and AII values were characteristic of Cu(II) CC with a first-coordination sphere consisting of O atoms. Hyperfine structure was not observed for Cu(II) pectinate at 290 or 77 K. This could have been related to closer contact of Cu atoms with each other in this compound. Energy was absorbed weakly in these complexes. Apparently not all Cu(II) atoms were located in states that gave EPR spectra. Cr(III) alginate and pectinate did not absorb strongly. A broad line with a g-factor below the value of 2.0023 that should occur for Cr(III) compounds was characteristic [5]. The large line width in the spectra could be explained, like for Mn(II) CC, by unresolved fine structure with a first-coordination sphere symmetry lower than octahedral. Like for the Cu(II) compounds, apparently only a small fraction of the Cr(III) CC gave an EPR spectrum. Weak absorption was observed in Co(II) and Ni(II) polyuronates only in Co(II) and Ni(II) pectinates. Two resonances were observed in Co(II) pectinates. One had a width of the order of 50 Oe and g » 2.09; the other, width ~5 Oe and g = 2.007. Apparently, these resonances belonged to paramagnetic impurities; the latter resonance, to a free-radical state. A weak line in the spectrum of Ni(II) pectinate had parameters similar to a line in the spectrum of Cu(II) pectinate. Therefore, it could be assigned to an impurity of Ni(I) (3d9), which is isoelectronic with Cu(II) compounds. EPR spectroscopy has limitations with respect to certain metals (Hg, Fe) for studying configurations of central metals in CC. Weak absorption was observed in EPR spectra of Co(II) and Ni(II) CC. Stronger absorption occurred in spectra of Cu(II), Cr(III), and Mn(II) CC. However, it was also weak. This indicated that not all metal atoms in the CC resonated. This was explained by the formation of interchain dinuclear carboxylate-bridged CC (coordination through carboxylates):

Polymer chain

A similar interchain complexation probably occurred in ordinary CC because they were insoluble in water.

The central metal configuration in all studied polyuronates except for Cu(II) alginate was not octahedral but had a lower symmetry such as square-planar, tetrahedral, or pyramidal with distorted configurations. The coordination number (CN) of the central metal in the studied CC was four except for Cr(III) polyuronates (CN 5) and Cu(II) alginate (CN 6). Another three O atoms from neighboring hydroxyls of the polymer and H2O in ordinary CC with structures differing from the acetate-bridged ones were coordinated to the metal atom in polyuronates in addition to the carboxylate. Coordinated water rather than water of crystallization was probably more likely to occur in the obtained metal complexes. The bond of the metal atoms to inner-sphere water molecules in Cu(II) pectinate was confirmed by thermal analysis results [1]. A distorted square-planar configuration was most likely for Cu(II) pectinate. For Cr(III) CC, a tetragonal pyramidal configuration was probable: O O O

O Cr O

O

The most probably configuration was Mn(II) CC was tetrahedral: O

Mn O

O O

The results provided chemical justification for the mechanism of action of polyuronides as antidotes. REFERENCES 1. N. Sh. Kaisheva, Scientific Principles of Application of Polyuronides in Pharmacy [in Russian], PGFA, Pyatigorsk (2003), pp. 37 – 59, 68 – 74. 2. A. I. Usov, Usp. Khim., 68(11), 1051 – 1061 (1999). 3. S. Deiana, L. Erre, G. Micera, and P. Piu, Inorg. Chim. Acta, 46(6), 249 – 253 (1980). 4. O. M. Petrukhin, I. N. Marov, V. V. Zhukov, et al., Zh. Neorg. Khim., 17(7), 1876 – 1885 (1972). 5. I. N. Marov and N. A. Kostromina, EPR and NMR in the Chemistry of Coordination Compounds [in Russian], Nauka, Moscow (1979), pp. 165 – 177, 213 – 218. 6. G. V. Lazur?evskii, I. V. Terent?eva, and A. A. Shamshurin, Practical Studies in the Chemistry of Natural Compounds [in Russian], Vysshaya Shkola, Moscow (1966), pp. 84 – 86. 7. USSR Pat. No. 886,750 (1979); Byull. Izobret., 4, C07H23 / 00 (1981). 8. P. Debongnie, M. Mestdagh, and M. Rinaudo, Carbohydr. Res., 170(5), 137 – 148 (1987).