By derivatography in insoluble pectins Cu 2+ (РCu 2+) and Pb 2+ (РPb 2+), the presence of “a high-temperature component” (150°C - 165°C ) is established. During potentiometric alkalimetric titration of РCu 2+ and РPb 2+, endpoints are established at рН accordingly 4.87 and 4.95, proving acid properties of PM. Obtained data show the presence of water in the internal sphere of PM. Considering the loss of this water and the known ratio of metal cations and monomers of pectin (L - ), the simplest formulas of pectins are established: [Cu(L -) 2(H 2O) 2], [Pb(L -) 2(H 2O) 4].
Effect of heavy metals (HM) on a human is carried out in the natural circulation in the biosphere under conditions of environmental and plant pollution (air, water, soil) and plants. Even herbal medicines are contaminated by impurities of HM by an average of 27% [
The problem of HM enrichment is solved with the help of decontamination protocols based on the principles of: termination of toxic poisoning of a body, of support of detoxification organs and drainage, of stimulating the elimination of toxins, of the increase of susceptibility of patients to detoxification [
・ Antidotes form strong compounds with HM to take away the active center of enzymes and to get out from a body; antidotes with 5-6-membered rings, which can form coordination compounds (CC) with metals and which have several electron-donating groups, preferably chromophores providing strong, practically complete compound of TM, satisfy this requirement;
・ The ability of antidotes and their CC with HM to pass through a cell membrane, for which they should be electrically neutral or bear a small charge to dissolve in the lipid membranes;
・ Nontoxicity of antidotes and CC with HM formed by them;
・ Selective compound of HM and biogenic metals (BM) by antidotes due to various stability of formed CC: formed CC must be more stable with HM than with BM to avoid the elimination BM from biological systems.
Considering these requirements for the different mechanisms of functions of chemical antidotes there are the following advantages and disadvantages.
1) Chelators binding HM in a little dissociated and easily soluble CC in water (chelates) [
Advantages: a wide spectrum of detoxication action for a number of HM (Pb, Cd, V, Cr, Hg, Cs, U, Y, Ce, Th, Ni, Cu, Pu, Rb, Zr, Nb), even bound with enzymes; high CC durability with HM (logarithm of the durability constant (lg β) 14.0 - 19.0); rapid renal elimination.
Disadvantages: forming with BM (Ca, Mg, Co, Fe, Zn, Mn) very strong CC (lg β 5.0 - 11.0), leading to a decrease of hemoglobin composition, Fe, vitamin В12, Ca in blood, P (phosphorus) in bones and blood; easy absorption of water- soluble CC from a gastrointestinal tract and strengthening of effects of toxic nephrosis.
2) The antidotes containing sulfhydryl (mercapto), and easily forming with HM-soluble compounds [
Advantages: a wide spectrum of detoxication action for a number of HM (Hg, Bi, Cu, Au, Ni, Cr, Ag, Pb, Cd), even bound with enzymes; high CC durability with HM (lg β 14.0 - 19.0); rapid renal elimination.
Disadvantages: forming with BM (Zn, Fe) very durable compounds (lg β 5.0 - 11.0), the consequences of which are similar to those complexes described above.
3) The antidotes absorbed HM [
Advantages: elimination of HM; high sorption capacity.
Disadvantages: elimination of BM (Fe), vitamins, hormones, lipids, proteins; HM desorption from the surface of adsorbents, which requires the prescrition of laxatives; detoxification only in the gastrointestinal tract.
4) The antidotes accelerating biotransformation of HM to form insoluble and non-toxic metabolites [
Advantages: formation of durable compounds with HM.
Disadvantages: formation of durable compounds with BM.
5) The antidotes, enhancing neutralizing function of a liver [
Advantages: formation of durable compounds with HM.
Disadvantages: formation of durable compounds with BM.
6) Pectins occupy a special position because of the specific structural and physicochemical properties [
・ in their molecules there are more coordinating groups (carboxyl, hydroxyl groups, the glycosidic bond, oxygen atom of the pyranose cycle) than it is required for the binding of HM, which can select the group to form a more durable CC;
・ the tendency to the formation of CC less than conventional chelators have, (and, consequently, the durability of products) due to the rigidity of the circuit, limiting the freedom of its bending and twisting;
・ solubility or insolubility of formed by pectins CC with metals depends on the degree of polymerization and the concentration of pectin, therefore, pectins can act in a gastrointestinal tract and in body fluids.
The consequence of these features is that the lg β of CC of BM with pectins (1.2 - 2.4) is much lower than with amino acids, nucleotides and enzymes in a human body (for Mg 4.0 - 4.8, Mn 4.5 - 6.1, Fe 6.5 - 8.5, Co 7.2 - 10.2, Zn 8.1 - 10.2 [
The detoxication action of pectin enhances by their adsorption, hepatoprotective and water retaining properties. Pectin is practically non-toxic and biologically compatible with a human body [
Despite of the study of the structure of such CC as pectinates of metals (PM) [
To determine water in PM there are analytical methods: polarography [
The durability of bonds depends on the position of water molecules: the most durable bond is connection of the water molecules with metal ions in the inner sphere of CC, less solid―connection of water molecules in the outer sphere of CC, the weakest―connection of the adsorption water. [
The goal of the study is to determine the presence, position and number of water molecules in setting of the molar composition of insoluble pectinates Cu2+ (PCu2+) and Pb2+ (PPb2+) by methods of derivatography and potentiometry.
The object of the study is a beet pectin (satisfying the requirements of short-term certified pharmacopeial description 42-3433-99 “Pectin”) with an average molar mass of 3200 kg/mol and a dissociation constant in water 3.2 × 10−4 and it contains 14.4% of free carboxyl groups, 9.2% of the methylated carboxyl groups [
The study of the composition PCu2+ and PPb2+ compared with pectin and AM2+ is carried out in stages. Using variants of thermal analysis: differential thermal (DTA), differential thermogravimetric (DTGA) and thermogravimetric (TGA) on a derivatograph “Q-1500” (Hungary, “MOM”) in the temperature range 20˚C - 1000˚C in a dynamic atmosphere of air at a heating rate of substances 10 deg./min, at the speed of paper movement 5 mm/min, at using aluminium oxide as a standard, the presence of “high-temperature” component in solid PM weighing about 0.5 - 0.6 g (accurately weighed amount) was determined. Selection of high heating rate is conditioned by the need to prevent PM structural change in the course of writing of thermal curves: water molecules transition in sphere of the coordination ion [
Analysis of the composition PCu2+. Comparative analysis of pectin thermograms (
Effect of DTA (Т1-Т2), ˚С | Nature of the effect | Effect of DTGA (Т1 - Т2), ˚С | Total weight loss, % |
---|---|---|---|
Pectin | |||
100 - 115 (max 113) | ¯ desolvation | 80 - 105 (max 105) | 98.0 |
190 - 210 (max 200) | ¯ destruction of carboxyl groups | 210 - 230 (max 230) | |
230 - 260 (max 240) | ¯ destruction for 1,4-glycosidic bonds | 255 - 270 (max 265) | |
420 - 450 | ¯ destruction | 410 - 415 (max 415) | |
АCu2+ | |||
115 - 120 (max 118) | ¯ desolvation | 110 - 115 (max 115) | 68.0 |
300 - 430 (max 400) | ¯ destruction with melting | 320 - 450 (max 390) | |
PCu2+ | |||
115 - 120 (max 120) | ¯ desolvation | 90 - 115 (max 110) | 75.0 |
155 - 160 (max 160) | ¯ desolvatin | 150 - 165 (max 165) | |
200 - 220 (max 215) | ¯ destruction of carboxyl groups | 215 - 230 (max 225) | |
240 - 260 (max 255) | ¯ destruction for 1,4-glycosidic bonds | 250 - 265 (max 260) | |
470 - 500 | ¯ destruction | 460 - 480 (max 475) |
Note here and in
The first heat effect (endothermic), observed for all substances, refers to the temperature range 80˚C - 115˚C (DTGA), 100˚C - 120˚C (DTA). The results of the quantitative determination of water by drying (120˚C, 8 hours): mass reduction of pectin from 0.60802 g to 0.50344 g (water loss 17.2%), АCu2+―from 0.52315 g to 0.47607 g (water loss 9.0%), PCu2+―from 0.58683 g to 0.54340 g (water loss 7.4%) shows that the effect of the first heat loss is associated with loss of capillary connected (adsorption) water. Unlike pectin (
Assuming that the “high temperature” component in PCu2+ are molecules of intracoordination water, PCu2+ , practically completely dehydrated by adsorption water at a temperature of 120˚C (upper limit of the endothermic effect) for 8 hours, was alkalimetrically titrated in comparison with pectin and АCu2+.
If during pectin titration (
Of all the substances only PCu2+ is characterized by the endpoint in an acidic media having pH significantly below pH and pectin and АCu2+. The obtained data suggest that occurring of acidic PCu2+ properties when dealing with alkali, and it is possible only due to water molecules, the acidic properties of which are increased as a result of coordination with ions of Cu2+.
Thus, determination of “high-temperature” component (150˚C - 165˚C) in PCu2+ and demonstration of its acidic properties (pH 4.87) proves the existence of water molecules in the internal sphere of PCu2+ and it is not typical for reagents.
The calculated according TGA amount of water removed from the decomposed substances are given in
Unlike pectin (
The mass difference of the PCu2+ ( 0.54340 g ) and intracoordination water ( 0.04356 g ) aquacomplex showed a mass of anhydrous PCu2+ ( 0.49984 g ). Considering the molar ratio in PCu2+ of Cu2+ ions and galacturonic acid residues (monomers of pectin, L) 1:2 (15.46 wt%:84.54 wt%) [
Т, ˚С | Amount of removed water | |||||
---|---|---|---|---|---|---|
Pectin | АCu2+ | PCu2+ | ||||
mg | mmol | mg | mmol | mg | mmol | |
70 | 8.4 | 0.452 | 26.68 | 1.482 | 22.47 | 1.248 |
80 | 9.40 | 0.522 | 28.28 | 1.571 | 23.90 | 1.328 |
90 | 11.59 | 0.644 | 30.28 | 1.682 | 25.33 | 1.407 |
100 | 14.78 | 0.821 | 31.72 | 1.762 | 28.73 | 1.596 |
110 | 17.51 | 0.973 | 33.23 | 1.846 | 31.34 | 1.741 |
120 | 20.59 | 1.144 | 34.96 | 1.942 | 33.46 | 1.859 |
130 | 23.81 | 1.323 | 37.15 | 2.064 | 35.85 | 1.992 |
140 | 26.66 | 1.481 | 38.93 | 2.163 | 38.54 | 2.141 |
150 | 29.20 | 1.622 | 41.49 | 2.305 | 40.32 | 2.240 |
160 | 30.96 | 1.720 | 44.14 | 2.452 | 41.04 | 2.280 |
170 | 33.16 | 1.842 | 46.46 | 2.581 | 47.80 | 2.656 |
180 | 33.89 | 1.883 | 48.31 | 2.684 | 52.27 | 2.904 |
i.e. PCu2+ composition is expressed by the simplest formula [Cu(L−)2(H2O)2]. These results suggest that when dealing of pectin with Cu2+ ions there is a partial replacement of water molecules in the hydration shell of Cu2+ ions to L−.
Analysis of the PPb2+ composition. Feature comparison of pectin thermograms АPb2+ and PPb2+ (
The conclusion about the loss of adsorption water is confirmed by the results of the quantitative determination of substances mass after drying (120˚C, 8 hours): reduction of pectin weight is 17.2% (from 0.57942 g to 0.47976 g ), АPb2+― 13.5% (from 0.53274 g to 0.46082 g ), PPb2+―9.4% (from 0.56358 g to 0.51060 g ). Unlike reagents for PPb2+ a “high temperature” component (150˚C - 160˚C) is discovered.
Comparative analysis of the curves of alkalimetric titration shows that if during pectin titration (
Thus, the occurring of a “high-temperature” component (150˚C - 160˚C) in PPb2+ and the demonstration of its acidic properties by reacting with alkali (pH 4.95) proves the existence of water molecules in the internal sphere of CC, which is not observed for reagents.
Effect of DTA (Т1-Т2), ˚С | Nature of the effect | Effect of DTGA (Т1-Т2), ˚С | Total weight loss, % |
---|---|---|---|
Pectin | |||
100 - 115 (max 113) | ¯ desolvation | 80 - 105 (max 105) | 98.0 |
190 - 210 (max 200) | ¯ destruction of carboxyl groups | 210 - 230 (max 230) | |
230 - 260 (max 240) | ¯ destruction for 1,4-glycosidic bonds | 255 - 270 (max 265) | |
420 - 450 | ¯ destruction | 410 - 415 (max 415) | |
АPb2+ | |||
100 - 110 (max 105) | ¯ desolvation | 100 - 115 (max 110) | 70.4 |
275 - 320 (max 280) | ¯ destruction with melting | 300 - 350 (max 310) | |
PPb2+ | |||
110 - 120 (max 110) | ¯ desolvation | 80 - 110 (max 110) | 73.0 |
150 - 158 (max 155) | ¯ desolvation | 150 - 160 (max 157) | |
190 - 220 (max 220) | ¯ destruction of carboxyl groups | 215 - 235 (max 235) | |
245 - 250 (max 250) | ¯ destruction for 1,4-glycosidic bonds | 250 - 255 (max 250) | |
340 - 530 | ¯ destruction | 350 - 500 (max 380) |
As follows from TGA the presence of intracoordination water is not observed neither for pectin (
Thus, the composition PPb2+, released from adsorption water, is expressed by the following ratios of Pb2+ ions, L− and molecules of coordination water: by mass (g)―0.16814 : 0.28396 : 0.05850; by the number (mmol)―0.811:1.623:3.25, or 1:2:4, i.e. PPb2+ composition is expressed by the simplest formula [Pb(L−)2(H2O)4]. The relative error of determining is 3.1% - 4.8%.
The presence of intarcoordination water in the composition of PCu2+ and PPb2+ is proven by the method of derivatography by the endothermic effect exceeding 150˚C (respectively, as 150˚C - 165˚C and 150˚C - 160˚C), and by potentiometric alkalimetric titration by formation of hydroxycomplexes in a weakly acidic medium (respectively as pH at the equivalence points is 4.87 and 4.95). By thermogravimetrically established quantitative loss of intracoordination water mass, based on the known ratio of the metal cations and L−, the CC compositions are determined and expressed by the formula: [Cu(L−)2(H2O)2], [Pb(L−)2(H2O)4]. The results are needed to determine the minimum and therapeutic doses of pectins, as an antidote for poisoning compounds of Cu2+ and Pb2+.
Kajsheva, N.S., Kajshev, A.S., Samoryadova, B.A., Smolen- skaya, K.V. and Smolenskaya, G.V. (2017) Identification and Quantification of Intracoordination Water in Insoluble Pectinates Cu2+ and Pb2+. American Journal of Analy- tical Chemistry, 8, 210-224. https://doi.org/10.4236/ajac.2017.83017