916 Acta Chim. Slov. 2010, 57, 916-921 Short communication Chemical Speciation of Binary Complexes of Embelin With Some Biologically Important Metal Ions Vaddadi Usha Rani,1 Ghanta Jyothi,2 Gollapalli Nageswara Rao1 and Budati Bala Venkata Sailaja1* 1 Department of Inorganic and Analytical Chemistry, Andhra University, Visakhapatnam-530 003, India 2 St. Theresa's College for women, Eluru, India * Corresponding author: E-mail: sailaja_bbv@yahoo.co.in Received: 09-01-2010 Abstract Protonation and complexation equilibria of embelin were investigated in 86% v/v dimethyl sulphoxide - water mixture electrometrically at an ionic strength of 0.17 mol dm-3 and at a temperature of 303K. The best fit chemical models were arrived at based on statistical grounds employing crystallographic R factor, %2, skewness and kurtosis. The existence of binary complex species with Co(II), Ni(II), Cu(II) and Zn(II) were established from modeling studies using the computer program MINIQUAD75. Distribution diagrams of various species of the complex in relation to pH are presented. Keywords: Embelin, speciation, DMSO, binary complexes mals. Vitamin B12 is synthesized only by micro-organisms, in particular anaerobic bacteria. Nickel is associated with several enzymes21-23 and any variation in its concentration leads to metabolic disorders.24 Copper is largely rejected from the biological cells but outside the cell, it is essential for the metabolism of many hormones and connective tissue. The biological functions include electron transfer, dioxygen transport, oxygenation, oxidation, reduction and disproportionate.25,26 Zinc is the second most abundant essential trace metal after iron and it plays vital roles in biological systems.27-30 Based on solubility considerations speciation studies of embelin with some essential metal ions such as Co, Ni, Cu and Zn in Dimethyl sulphoxide(DMSO)-water mixture have been reported in this paper. 2. Experimental 2. 1. Materials A solution of 0.05 mol dm3 of embelin was prepared in DMSO. Aqueous solutions of Co(II), Ni(II), Cu(II) and Zn(II) chlorides (0.05 mol dm-3) were prepared in 0.05 mol dm-3 HCl, to suppress the hydrolysis of the metal salts. Sodium chloride was used to maintain the ionic strength in the titrand. The strengths of alkali and mineral 1. Introduction Embelin is produced by plants of the family myrsi-naceae. The myrsinaceae are shrubs or climbers comprising about 30 genera and 1000 species, with high eth-nomedical application in the East and Central African geographical region and the Asian continent. The dried fruits have been in use in India for a long time in the treatment of ringworm and other skin diseases and as an anti-helmintic,1-5 antimicrobial6-9 and antifertility.10-12 The seeds of embelia were used in mordant dyeing.13 The myrsinaceae are chemotaxonomically associated with long alkyl side chain benzoquinones which are found to constitute 10% of the dry weight of their berries and 7% of the root bark.14 It has also been used as an analytical reagent.15-17 Synthesis and characterization of copper(II) complexes of embelin in the cavities of zeolite Y have been conducted18 and found to have good catalytic activity for the reduction of molecular oxygen. Embelin complexes of different metals were also studied.19,20 Speciation studies of essential metal ion complexes of embelin are useful for the understanding of the role played by active site cavities in biological molecules and the binding behavior of protein residues with the metal ions. Cobalt in the form of vitamin B12 is essential for ani- Rani et al.: Chemical Speciation of Binary Complexes of Embelin ... 917 Acta Chim. Slov. 2010, 57, 916-921 acid were determined using the Gran plot method.31 To assess the errors that might have crept into the determination of the concentrations, the data were subjected to analysis of variance of one-way classification(ANOVA). 2. 2. Apparatus The titrimetric data were obtained using a calibrated CONTROL DYNAMICS pH-meter (readability 0.01). The glass electrode was equilibrated in a well-stirred 86% v/v DMSO-water mixture containing an inert electrolyte. All the titrations were performed at 303.0 ± 0.1 K in aqueous medium containing 86.0 % v/v DMSO, maintaining an ionic strength of 0.17 mol dm-3 with sodium chloride. The effect of variations in asymmetry, liquid junction potential, activity coefficient, sodium ion error and dissolved CO2 on the response of glass electrode was taken into account in the form of a correction factor.32 pH Figure 1: Plots of nH versus pH of embelin: (□)0.199, (0)0.299, and (A)0.399 mmol 2. 3. Procedure and Modeling Strategy For the determination of the protonation constants of embelin and stability constants of the binary metal-lig-and species, initially titrations of a strong acid with alkali were performed at regular intervals to check whether complete equilibrium had been achieved. Then the calomel electrode was refilled with 86%v/v DMSO-water mixture. In each of the titrations, the titrand consisted of approximately 1 mmol mineral acid in a total volume of 50 cm3. Titrations with different ratios of metal to ligand were performed with 0.20 mol dm-3 sodium hydroxide. The computer program SCPHD33 was used to calculate the correction factor. The binary stability constants were calculated from pH-metric titration data using the computer program MINIQUAD7534 which exploits the advantage of a constrained least- squares method in the initial refinement and reliable convergence of the Marquardt algorithm. During the refinement of the binary systems, the correction factor and the protonation constants of embelin were fixed. 3. Results and Discussion 3. 1. Protonation Constants Secondary formation functions like average number of protons bound per mole of ligand (nH) and number of moles of alkali consumed per mole of ligand (a) are useful to detect the number of equilibria. Plots of nH versus pH for different concentrations of the ligand should overlap if there is no formation of polymeric species. Overlapping formation curves for embelin (Figure 1) rule out the polymerization of the ligand molecules. The pH values at half integral values of nH correspond to the protonation constants of the ligand shown in Figure 1 which can detect the number of equilibria. A plot of a versus pH is given in Figure 2. The negative values of a correspond to the number of moles of free acid present in the titrand and the number of associa-ble protons. The positive values of a indicate the number of dissociable protons in the ligand molecule. The maximum value of a in Figure 2 is two, which clearly infers that embelin has two dissociable protons. Figure 2: Variation of a with pH of embelin Embelin has two dissociable protons and its various forms are LH2, and LH- in the pH ranges 2.0-4.0 and 6.0-8.0, respectively. The species distribution diagram of embelin shown in Figure 3 represents the acido-basic equilibria which are active in the pH range 2.0-10.0. The protonation equilibria of embelin are as follows L + H+ o LH- LH- + H+ o LH (1) (2) Rani et al.: Chemical Speciation of Binary Complexes of Embelin ... 918 Acta Chim. Slov. 2010, 57, 916-921 w 100 80 ■ 60 40 20 \ FL —A/ \ / ~ •A/ / \ 'V, nH / \ *" ~— » . / \ J \ / \ r 1 1 ' / i 1 pH Figure 3: Formation function (•) and species distribution diagram of embelin The best fit models that contain the type of species and overall protonation constants of embelin along with some important statistical parameters are given below in Table 1. A very low standard deviation in log P values indicates the precision of these parameters. The small values of Ucorr (the sum of the squares of deviations in concentrations of ligand and hydrogen ion at all experimental points) corrected for degrees of freedom, indicate that the experimental data can be represented by the model. Small values of mean, standard deviation and mean deviation for the system corroborate that the residuals are around a zero mean with little dispersion. For an ideal normal distribution, the values of kurtosis and skewness should be three and zero, respectively. The kurtosis values in the present study indicate that the residuals form leptokurtic patterns. The value of skewness given in Table 1 evinces that the residuals form a part of normal distribution; hence, least squares method can be applied to the present data. The sufficiency of the model is further evident from the low Figure 4: Simulated (o) and experimental (solid line) alkalimetric titration curves in 86.0% DMSO-water mixture of embelin: (a) 0.199, (b) 0.299 and (c) 0.399 mmol. crystallographic R-values. The statistical parameters thus show that the best fit models portray the acido-basic equi-llibria of embelin in DMSO-water mixtures. The primary alkalimetric titration data are simulated and compared with the experimental alkalimetric titration data, to verify the sufficiency of the model. The overlap of the typical experimental and simulated titrations data given in Figure 4 indicates that the proposed models represent the experimental data. 3. 2. Speciation of Binary Complexes The overall stability constants of binary complexes of embelin with Co(II), Ni(II), Cu(II) and Zn(II) are given in Table 2. Table 1: Best fit chemical model of acido-basic equilibria of embelin in 86% v/v DMSO- water mixture Log P (SD) pH-range NP Ucorr X2 Skewness kurtosis R - factor LH LH2 9.06(2) 13.89(4) 3.5-10.0 63 10.015 22.57 -1.85 1094 0.031761 Table 2: Best fit chemical models of Metal-Ligand complexes in 86% v/v DMSO- water mixture Metal MLH Log p (SD) ml2h ML2H2 pH-range NP Ucorr X2 Skewness kurtosis R -factor Co 13.93(1) 25.06(1) 26.88(1) 1.8-2.0 45 2.788 5.50 0.58 4.66 0.0068 Ni 14.37(2) - 29.33(1) 1.6-1.9 68 3.976 13.65 -0.04 4.39 0.0066 Cu 7.64(1) 30.56(1) 32.85(1) 1.6-1.9 86 9.8 33.60 -0.21 4.83 0.0117 Zn 15.26(2) - 28.35(1) 1.7-2.0 75 6.696 17.60 -0.03 4.51 0.0097 UcOrr = U/ (NP-m) X 108; NP = Number of points m = number of constants; SD = Standard deviation Rani et al.: Chemical Speciation of Binary Complexes of Embelin ... 919 Acta Chim. Slov. 2010, 57, 916-921 3. 3. Residual Analysis35 In data analysis with least squares methods, the residuals (the differences between the experimental data and the data simulated based on the model parameters) are assumed to follow Gaussian or normal distribution. When the data are fit into the models, the residuals should be ideally equal to zero. Further, a model is considered adequate only if the residuals do not show any trend. Respecting the hypothesis of the least squares analysis, the residuals are tested for normal distribution. Such tests are %2, skewness, kurtosis and R-factor. These statistical parameters of the present data show that the best fit models portray title systems, as discussed below. 3. 3. 1. X2 test X2 is a special case of gamma distribution whose probability density function is an asymmetric function. This distribution measures the probability of residuals c) CO o o d) Q. CO 4D- 23- 0- WUjH -^MJH FM / forming a part of standard normal distribution with zero mean and unit standard deviation. If the %2 calculated is less than the table value, the model is accepted. 3. 3. 2. Crystallographic R-test Hamilton's R factor ratio test is applied in complex equilibria to decide whether inclusion of more species in the model is necessary or not. In pH-metric method the readability of pH meter is taken as the Rlimit which represents the upper boundary of R beyond which the model bears no significance. When there are different numbers of species the models whose values are greater than R-table are rejected. The low crystallographic R-values given in Table 2 indicate the sufficiency of the model. 3. 3. 3. Skewness It is a dimensionless quantity indicating the shape of the error distribution profile. A value of zero for skewness b) 80-