Scientific paper pH Metric, Spectroscopic and Thermodynamic Study of Complexation Behavior of 2-aminobenzothiazole with Ni (II) in Presence of Amino Acids M. A. Neelakantan,1* S. S. Mariappan,1 J. Dharmaraja1 and K. Muthukumaran2 1 Chemistry Research Centre, National Engineering College, K. R. Nagar, Kovilpatti - 628 503, Thoothukudi District, Tamil Nadu, India 2 Department of Chemistry, Government College of Engineering, Tirunelveli - 627 007, Tamil Nadu, India * Corresponding author: E-mail: maneels@rediffmail.com; drmaneelakantan@gmail.com Telephone.: +91 9442505839, Fax.: +91 4632232749 Received: 07-08-2009 Abstract The complexation of 2-aminobenzothiazole (2abt) [A] with Ni(II) in presence of amino acids viz., glycine (gly), L-ala-nine (ala), L-valine (val) and L-phenylalanine (pal) [B] in 50% (v/v) water-ethanol mixture containing NaClO4 (0.15 M) has been studied by pH metrically at various temperatures (300, 310, 320 and 330 ± 0.1 K). Mixed ligand complexes of types NiAB and NiAB2 were observed and their stability constants were determined. The stabilization of mixed ligand complexes over binary analogues has been derived from A log K, log X' and log X values. The thermodynamic parameters AG, AH and AS were derived from the temperature dependence of the equilibrium constants. The complexation behavior has also been studied by means of electronic spectra. On the basis of stability constants and electronic spectra, it is revealed that the mixed ligand complexes have six-coordinated octahedral structure. The binary and mixed ligand complexes were screened for their microbial activities in vitro on common bacteria, fungi and yeast. The DNA cleaving activities were studied by electrophoresis method. Keywords: Mixed ligand complexes, 2-aminobenzothiazole, amino acids, stability constants and biological screening. 1. Introduction In recent years, the importance of chemical equilibrium modeling has been developed from an empirical qualitative tool to a sophisticated quantitative tool for chemists1. Thiazoles are a crucial part of vitamin B1 and found in cocarboxylase coenzyme2. Benzothiazoles are used in pharmacology and cancer biology3. Mixed ligand complexes of amino acids are involved in the exchange and transport mechanism of trace metal ions in the human body4. Much attention has been paid to the study of mixed ligand complexes of transition metals with ligands of biological and pharmaceutical interest5,6. In a sequel of continuation, the major goal of the present work is to determine stability constants of various species present in the Ni(II)-2abt (A)-amino acid (B) mixed ligand complexes containing 0.15 M NaClO4 by pH-metrically at different temperatures (300, 310, 320 and 330 ± 0.1 K). The corresponding thermodynamic functions of complexation were evaluated and are discussed. The coordination environment of Ni(II) ion in mixed ligand complexes was determined by electronic spectral measurements at different pH in 50% (v/v) water-ethanol medium. Biological activities of binary and mixed ligand complexes were tested with different microorganisms. DNA cleavage activities of binary and mixed ligand complexes were monitored by gel electrophoresis. 2. Experimental 2. 1. Materials All the chemicals used in this study were analytical grade and were used without further purification. Carbonate free sodium hydroxide solution was prepared and standardized against standard potassium hydrogen phthalate solution. The ionic strength of each solution was adjusted to 0.15 M by the addition of NaClO4 as supporting electrolyte. The metal per chlorate and other reagents were prepared and estimated as described elsewhere.7'8 2. 2. Potentiometric Equilibrium Measurements The pH titrations were carried out in a digital pH meter (Systronics ppH System 361) with a combined glass electrode (accuracy ± 0.01 pH unit). The temperature of the sample solutions was maintained at 300, 310, 320 and 330 ± 0.1 K. In both the acidic and alkaline regions, the electrode system was calibrated in terms of hydrogen ion concentrations instead of activities. The instrument was calibrated using standard buffer solutions9. The stability constants were evaluated with the aid of SCOGS computer program10. In this study the selection of 'Best Fit Model' is based on the factors such as (i) the analysis of formation curves; (ii) standard deviation in titre with 0.02 cc which is with in 0.05% error with respect to total volume of the titre and compares well with ov obtained for ligand protonation constants under similar conditions and (iii) minimum standard deviation in the log P values of the individual species. In addition to these factors the chemical logic as also been taken into consideration in the selection of the 'Best Fit Model'. Oxygen free nitrogen gas was bubbled through the solution before and during titrations. Multiple titrations were carried out for each system. The pH-meter readings in 50% (v/v) water-ethanol mixture were corrected by Van Uitert and Hass relation11. The ion product of water (Kw = [H+] [OH-]) were calculated at constant ionic strength of 0.15 mol L-1 with NaClO4 in 50% (v/v) water-ethanol mixture based on the measurement of [H+] and [OH ] and pH in several experiments. The Kw obtained is 14.42 (±0.03) at 27 °C and in agreement with the literature value12'13. Spectrophotome-tric determinations in the visible region were performed with Jasco 430 UV-visible spectrophotometer in the range of 200-1100 nm at 310 K. 2. 3. Biological Studies The in vitro biological activity of binary and mixed ligand complexes were tested against the bacteria, fungi and yeast by a modified well diffusion method14. Commercially available ampicillin and nystatin were used as antibacterial and antifungal control respectively. The test solutions of Ni(II)-2abt-gly/ala/val/pal (1:1:1) [3 x 10-3 M] were prepared by dissolving the mixed ligand complexes in 50% (v/v) water-ethanol mixture. The bacteria were cultured for 24 h at 37 °C in an incubator. The compounds to be tested were added to a 10 mm diameter well and the plates were then kept at 37 °C in an incubator. The growth of inhibition zones was measured and is compared with control. 2. 4. DNA Cleavage Studies The DNA cleavage activity of binary and mixed ligand complexes was monitored by agarose gel electrophoresis on CT DNA. The gel electrophoresis experiments were performed under aerobic conditions with H2O2 as oxidant by incubation at 37 °C for 2 h as follows: CT DNA 30 pM, 50 pM of each mixed ligand complex, 500 pM of H2O2 in 50 mM Tris-HCl buffer (pH = 7.2). After incubation, 1 pL of loading buffer (bromophenol blue in H2O) was added to each tube and the mixed samples were loaded on 1% agarose gel. The samples were electropho-resed for 2 h at 50 V in Tris-acetic acid - EDTA buffer (pH = 8.3). After electrophoresis, the gel was stained with 1 pg/cm3 ethidiumbromide (EB) for 30 min prior to being photographed under UV light. 3. Results and Discussion 3. 1. Stability and Structure of Binary Species The protonation constant of 2abt and the stability constant of Ni(II)-2abt(A) binary complex were determined by pH-metrically at different temperatures using NaClO4 as supporting electrolyte. The stability constant values are given in Table 1. The pKa value of 3.94 for 2abt compares very well with the value of 3.92 of thiazole15. The log /¡NiA and log /3NiM values obtained in Ni(II)-2abt are 4.09 and 6.44 respectively at 300 K. The pKa value of amino group in 2abt is in the range of 10-11. The amino nitrogen atom coordinates with metal ion only at higher pH. In the present investigation the titration was carried out up to 8 pH, and increasing the pH above 8 leads to precipitation rules out the possibility of involvement NH2 nitrogen in coordination with metal ion. This shows that 2abt binds the metal ion via thiazole ring nitrogen atom. This type of binding has already been establised in Ni(II)-2-aminobenzothiazole derivatives in their solid state16. The stability constants of ami-no acids were redetermined under the present experimental conditions and the values agree well with the reported va-lues17-19. The amino acids bind Ni(II) ion in a bidentate manner through carboxylate oxygen and amino nitrogen atoms forming a 5-membered ring. 3. 2. Stability and Structure of Mixed Ligand Species In Ni(II)-2abt(A)-amino acid(B) systems, NiAB and NiAB2 species have been identified. The log K NAB/log KNNiBiA2 B values obtained at different temperatures in Ni(II)-2abt(A)-amino acid(B) systems compare favorably with Tablel. Stability constants for the proton and parent binary complexes of Ni(II) with 2abt*(A), gly, ala, val and pal (B) in 50% (v/v) water - ethanol mixture at 300, 310, 320 and 330 K, I = 0.15 M (NaClO4) Temp(K) Parameters 2abt* giy ala val pal 300 logßHA log ßH2A log ßNiA log ßNiA2 3.94(3) 4.09(3) 6.44(4) 9.77(2) 12.05(3) 5.73(2) 10.34(3) 10.07(2) 12.38(4) 5.67(3) 10.56(4) 10.11(3) 12.59(4) 5.68(4) 10.20(3) 9.55(3) 11.67(5) 5.52(3) 10.01(3) 310 l0g^HA log ßH2A logft«A logßN1A2 3.80(3) 4.31(4) 6.79(5) 9.57(3) 11.74(4) 5.66(3) 10.06(4) 9.72(2) 12.25(3) 5.55(4) 10.27(5) 9.80(3) 12.45(5) 5.49(5) 10.05(6) 9.36(4) 11.50(6) 5.34(4) 9.79(6) 320 log^HA log ßH2A logft«A logßN1A2 3.67(4) 4.56(3) 7.10(5) 9.20(3) 11.40(5) 5.54(4) 9.72(6) 9.48(2) 11.92(4) 5.41(3) 10.14(5) 9.53(3) 12.17(5) 5.34(4) 9.70(6) 9.19(4) 11.27(5) 5.25(4) 9.47(7) 330 log ßHA log ßH2A logßNA log ßNiA2 3.55(3) 4.82(4) 7.32(5) 8.97(3) 11.19(4) 5.44(3) 9.54(5) 9.30(2) 11.67(3) 5.31(4) 10.02(6) 9.42(4) 11.85(5) 5.27(5) 9.49(7) 8.96(4) 10.82(5) 5.18(5) 9.34(6) * 2abt becomes primary ligand (A) in the mixed ligand systems. log KN^value in Ni(II)-2abt binary system (Table 1). This shows that 2abt in mixed ligand complexes binds the metal ion in a manner similar to its binding in the NiA species. Again, log ^NiAB/log ^Niab? values (Table 2) obtained in NiAB / NiAB2 systems compare favorably with log KNiB/log KNiB values in Ni(II)-amino acid systems. This shows that the binding mode of amino acid (B) ligand in NiAB / NiAB2 mixed ligand species is similar to its binding mode in the corresponding binary systems. Thus, the three coordinating positions in Ni(II)-2abt(A)-amino acid(B) systems would be occupied by the monodentate binding of 2abt and bidentate binding of amino acids. Solvent water molecules would occupy the remaining positions (Figure 1). Figure 1. Proposed structures of NiAB and NiAB2 species in Ni (II)-2abt (A) - amino acid (B) systems (R = H - glycine, CH3 - alanine, (CH3)2 - valine and C6H5 - phenylalanine). The stabilization of mixed ligand complexes over binary analogues can be expressed in terms of A log K (= log P Nab - log KNa - log KNb). The A log Rvalues (Table 2) calculated for all these systems are more positive indicating the marked stabilization of mixed ligand complexes20. The quantitative stabilization of mixed li- gand complexes can be expressed by log X values21 ( = 2log PNab - log PN!a2 - log PNNb2). The log X values (Table 2) are more positive compared to the statistical values22 demonstrate that interligand and electronic interactions are present in the mixed ligand complexes. The log X' (= aog P NNAB + logK NNA) - aog P NA2 + log K NL)) va- lues23 are found to be greater than 0.3, suggesting that NiA and NiB bonds in mixed ligand complexes are stronger than in binary complexes. 3. 3. Effect of Temperature The dissociation constants of the ligands, as well as the stability constants of the complexes with Ni(II) ions in 50% (v/v) water-ethanol mixture have been evaluated at 300, 310, 320 and 330 K (Table 1). From the results, it is clear that the pKHA values of the ligands decrease with increasing temperature. In Ni(II)-2abt binary system, the stability constants log KNNiiA and log KNNiiA values increase with increasing temperature. The enthalpy change (AH) for the dissociation and complexation process was calculated from the slope of Vant Hoff plot, log PNiAB vs. 1/T (Figure 2). The thermodynamic functions (AG, AH and AS) for binary systems are given in Table 3. For binary systems, (i) the AH values are negative (except 2abt) indicating the exothermic nature of reaction. (ii) the negative values of AG show that the driving tendency of the com-plexation reaction is spontaneous process. (iii) the AS values of the complexation process are positive, confirming that the complexation process is entropically favorable. All the thermodynamic parameters of the mixed li-gand complexes are given in Table 4. The log PNiAB increases with increasing temperature and log PNiAB2 decreases with increasing temperature (Figure 2). A negative va- O in oo a\ CO CM t-CM 1/0 o f 1 r ■> 0<) I/O I/o o\ CD CD OO CM « o m s Zt £ « « Ï "3 a » M "m CO ci ^o >0 oo oo^ ci co co CD -tf IN ••o ••o IN 00 ay I/o cK CD « T3 B C3 M Tt oo o oo i/o i/o o ^ o s a o "m C3 « O ^ Is « a J3 aa M "m pi co O CD c^ o^ 00 V(3 I/o o\ CD CD CM IN P^ 1/^ 00 00 c^ P~ C^ 00 I/O o\ CD CD oo CM o 00 IN I/o CD c^ CO CD 00 o^ I/o CD CD CD oo CM IN 00^ O ■0 CD "tf i/o a\ 1/0 CD bjîj cd: ^ Î^ Î^ o o o o o o I/o I/o 1/^ 1/^ c^ CD CD oo CM p~ I/o IN CM CM CD CD CD CO CM cq cq" s s; . S: „ M M X o < o < M i<3 M r(D „ __♦ e>- n --— --"V----- _____ o.ooao ! i o 0032 c.oo^a (1/T) K Figure 2. Vant Hoff plot of log ^NiAB and log ^NiAB vs 1/T. [□] Ni(II)-2abt-gly, [O] Ni(II)-2abt-ala, [A] Ni(II)-2abt-val and [V] Ni(II)-2abt-pal in NiAB species, [■] Ni(II)-2abt-gly, [•] Ni(II)-2abt-ala, [▲] Ni(II)-2abt-val and [▼] Ni(II)-2abt-pal in Ni-AB2 species lue of AG for these complexes suggests that the complexa-tion process is spontaneous. A negative value of AH in NiAB complexes shows that the process is exothermic and favorable at low temperature. But NiAB2 complexes show a positive value indicates that the process is endot-hermic and favorable at higher temperature. All the complexes show positive AS values predict that the complexa-tion of 2abt with Ni(II) in presence of amino acids is en-tropically favorable. 3. 4. Electronic Spectra All the electronic absorption spectra of the complexes were taken at different pH in 50% (v/v) water-ethanol mixture at 3 x 10-3 M concentration and the values are given in Table 5. The spectral bands of both binary and mixed ligand complexes have low molar absorptivity. This shows that hexa coordination is achieved by required number of water molecules. The spectral data (Table 5) confirms NiAB complexes have distorted octahedral geometry with 3A2g as ground state24'25. The values of the ligand field splitting energy (Dq) Racah interelectronic repulsion parameter (B) nephelauxetic ratio C percentage of covalency C°(%) and LFSE for NiAB complexes have been calculated and given in Table 5. The Racah parameter B is less than the free ion value (1041 cm-1) and the C values lies in the range of 0.66 - 0.72 indicating the cova-lent character of the complexes. The values of v2/v1 (1.661.77) and C (28-34%) supports the distorted octahedral geomentry25 around the Ni(II) ion. 3. 5. Species Distribution Diagram Distribution diagram for all the mixed ligand complexes in the present investication has been obtained for different metal to 2abt and amino acid ratio. The formation of NiAB complex starts at pH 5 and it has been found to be maximum in the pH range of 7.0 to 8.0 and accoun- Table 3. Thermodynamic parameters of Ni(II)-binary systems System Species -AG (kJ mol-1) AH (kJ mol-1) AS (J K- 1 mol-1 ) 300 K 310 K 320 K 330 K 300 K 310 K 320 K 330 K Ni(II)-2abt HA 22.63 22.56 22.49 22.43 - 24.65 -6.73 -6.76 -6.77 -6.73 NiA 23.49 25.58 27.94 30.90 49.88 244.57 243.42 243.18 244.78 NiA2 36.99 40.30 43.50 45.49 49.33 287.72 289.12 290.09 287.33 Ni(II)-gly HA 56.12 56.80 56.37 56.68 - 52.44 12.26 14.07 12.27 12.83 H2A 69.22 69.68 69.85 70.70 - 55.43 45.96 45.98 45.06 46.29 NiB 32.92 33.60 33.94 34.37 - 18.72 47.31 47.98 47.57 47.43 NiB2 59.39 59.71 59.56 60.28 - 52.02 24.58 24.82 23.55 25.03 Ni(II)-ala HA 57.84 57.69 58.08 58.76 - 48.50 31.15 29.66 29.95 31.10 H2A 71.11 72.71 73.03 73.74 - 46.47 82.15 84.66 83.02 82.63 NiB 32.57 32.94 33.15 33.55 - 23.13 31.45 31.64 31.29 31.57 NiB2 60.66 60.96 62.13 63.31 - 33.34 91.05 89.08 89.96 90.82 Ni(II)-val HA 58.07 58.20 58.39 59.43 - 44.56 45.06 43.91 43.24 45.35 H2A 72.32 73.90 74.57 74.87 - 47.14 83.93 86.32 85.71 84.04 NiB 32.63 32.59 32.72 33.30 - 26.28 21.15 20.34 20.12 21.26 NiB2 58.59 59.65 59.43 59.96 - 22.93 38.67 40.85 38.89 39.32 Ni(II)-pal HA 54.86 55.56 56.31 56.61 - 36.72 60.45 60.76 61.21 60.28 H2A 67.03 68.26 69.05 68.37 - 52.35 48.95 51.33 52.20 48.54 NiB 31.71 32.29 32.17 32.73 - 22.93 29.26 30.19 28.86 29.70 NiB2 57.50 58.11 58.02 59.01 - 44.24 44.20 44.74 43.08 44.78 Table 4. Thermodynamic parameters of Ni(II)-2abt(A)-amino acid(B) mixed ligand systems Mixed ligand complexes Species 300 K -AG (kJ mol-1) 310 K 320 K 330 K - AH (kJ mol-1) 300 K AS (J K-310 K 1 mol-1 ) 320 K 330 K NiAB 57.97 60.17 64.73 66.91 11.32 230.98 263.48 278.16 289.41 NiAB2 84.33 86.74 89.05 89.30 -36.86 158.25 205.28 190.20 189.79 NiAB2 58.96 61.09 63.96 66.80 20.09 237.08 261.87 278.76 290.97 NiAB2 86.74 88.90 90.74 92.97 -25.16 160.59 205.59 185.26 187.36 NiAB2 57.05 57.63 58.69 57.85 26.40 237.67 262.63 278.15 289.06 NiAB2 84.97 82.59 84.15 81.99 -27.92 163.09 204.93 193.26 187.36 NiAB2 56.17 59.55 61.84 65.06 30.66 218.65 244.88 256.39 272.12 NiAB2 82.94 84.08 85.96 88.16 -26.01 134.31 179.89 163.86 164.08 Ni-2abt-gly Ni-2abt-ala Ni-2abt-val Ni-2abt-pal Table 5. Electronic absorption spectral data of Ni(II)-2abt(A) and Ni(II)-2abt(A)-amino acid(B) mixed ligand complexes: [M] = [A] = [B] = 3 X 10-3 M at 310 K in 50% (v/v) water - ethanol mixture at pH = 8.0 Complex (cm-1) Band assignments Geometry Ligand field parameter 10 Dq B ß ß0 LFSE (cm-1) (cm-1) (%) kJ mol-1 Ni(II)-2abt Ni(II)-2abt-gly Ni(II)-2abt-ala Ni(II)-2abt-val Ni(II)-2abt-pal 13514 23810 10438 17301 24390 10395 18450 24038 10549 18382 23981 10482 18450 24213 3A2g(F) 3A2g(F) -3A2g(F) 3A2g(F) -3A2g(F) 3A2g(F) 3A2g (F) ■ 3A2g(F) 3A2g(F) 33A2g (F) 3A2g(F) 3A2g(F) 33A2g (F) X (F) 3T1g (F) . 3T1g (F) 3 3T1g (p) . X (F) 1g 3T2g (F) 33T1g (p) 33T1g (F) 3T2g (F) 33T1g (p) 33T1g (F) 3T2g (F) 33T1g (p) 33T1g (F) (F) Octahedral Distorted Octahedral Distorted Octahedral Distorted Octahedral Distorted Octahedral 1044 1040 1055 1048 692 0.66 34 149.94 754 0.72 28 149.32 714 0.69 31 151.53 748 0.72 28 150.57 ted ca. 40-80% of the total metal ion. NiAB2 complex has been found to be maximum favored in the pH 7.0 to 7.5 and accounted ca. 10-20% of the total metal in this form. At lower pH NiA and NiA2 complexes are present in con- siderable amount. At higher pH less than 10% of total metal ions are present as NiB and NiB2 complexes. A representative species distribution diagram is given in Figure 3. 3. 6. Biological Activities The investigation of antibacterial and antifungal activities (Table 6) shows that the biological activity is found to be in the following order: Control > mixed ligand complexes > Ni(II)-2abt(A). This enhancement in the activity can be explained on the basis of chelation theory26. Chelation reduces the polarity of the metal ion to a greater extent due to the overlap of the ligand orbital and partial sharing of the positive charge of the metal ion with donor groups. Also, it increases the delocalization of n-electrons over the whole chelate ring and enhances the lipophilicity of the complexes. This enhancement in the activity may also be explained on the basis of their structures by mainly possessing additional electron donor group (-NH2) present in 2abt27. A representative graph is given in Figure 4. Figure 3. Species distribution diagram for (a) Ni(II)-2abt(A)-val(B) and (b) Ni(II)-2abt(A)- pal(B) mixed ligand complex system, at C„ = C = 0.003 and C = 0.006 mol dm-3 at 310 K. B - Salomicine aureus C - Pseudomonas aeruginosa D - Escherichia coli E - Aspergillus niger F - Penicilline species G - Tricoder-ma virida H - Saccharomyces species Figure 4. Biological activities of Ni(II)-2abt(A) and Ni(II)-2abt(A)-amino acid(B) mixed ligand complexes (a) at 24 h and (b) at 48 h by well diffusion method (zone formation in mm) Table 6. Biological activities of binary and mixed ligand complexes by well diffusion method (zone formation in mm) Diameter of inhibition zone in mm for different microbial species Sal Pseudo E. Coli A. Niger Penicilline Triv Saccharo Complexes Time (h) Time (h) Time (h) Time (h) Time (h) Time (h) Time (h) 24 48 24 48 24 48 24 48 24 48 24 48 24 48 Control 15 16 13 14 25 26 13 15 15 18 20 22 25 28 Ni(II)-2abt 6 7 10 11 - - 6 7 - - 8 9 - - Ni(II)-2abt-gly - - 10 11 8 10 - - 11 12 10 11 - - Ni(II)-2abt-ala 8 9 12 13 9 11 10 12 11 11 12 13 7 9 Ni(II)-2abt-val 12 13 14 16 12 13 12 13 14 15 14 16 8 9 Ni(II)-2abt-pal 7 8 8 8 10 10 10 11 11 12 10 11 - - 3. 7. DNA Cleavage Activity In the present study, the oxidative CT DNA cleavage activity of Ni(II)-2abt (A) and Ni(II)-2abt(A)-gly/ala/ val/pal(B) complexes at 37 °C was studied by gel elec-trophoresis in presence of oxidant H2O2 (Figure 5). The DNA cleavage efficiency of the complex was due to the different binding affinity of the complex to DNA. Control experiment using DNA alone (lane 1) does not show any significant cleavage even on longer exposure time. From the observed results, it is clear that the Ni(II)-2abt (lane 2) cleave DNA as compared to control DNA. Probably this may be due to the formation of redox couple of the metal ion and its behavior28. It is also thought that, most cleavage cases are caused by nickel ions reacting with H2O2 to produce the diffusible hydroxyl radical (-OH) or molecular oxygen, which may damage DNA through Fenton type chemistry. Lane (1) DNA alone (2) DNA + Ni(II)-2abt + H2O2 (3) DNA + Ni(II)-2abt-ala + H2O2 (4) DNA + Ni(II)-2abt-val + H2O2 (5) DNA + Ni(II)-2abt-pal + H2O2 (6) DNA + Ni(II)-2abt-gly + H2O2 Figure 5. Changes in agarose gel electrophoretic pattern of calf-thymus DNA induced by mixed ligand complexes in presence of 4. Conclusion In the present work, we determined the stability constants of binary and mixed ligand complexes in varying temperatures. The percentage distribution of various binary and mixed ligand species in solution was evaluated. 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H. Chohan, C. T. Supuran, A. Scozzafava, J. Enz. Inhib. Med. Chem, 2004, 19, 79-84. 27. M.A. Neelakantan, S.S. Mariappan, J. Dharmaraja, T. Jeya-kumar, K. Muthukumaran, Spect. Chim. Acta., 2008, 71A, 628-635. 28. M.A. Neelakantan, F. Rusalraj, J. Dharmaraja, J. Johnsonra-ja, T. Jeyakumar, M. Sankaranaranaya Pillai, Spect. Chim. Acta, 2008, 71A, 1599-1609. Povzetek V tempraturnem območju med 300 in 330 K smo potenciometrično raziskovali kompleksacijo 2-aminobenzotiazola (2abt) [A] z Ni(II) v prisotnosti različnih amino kislin; glycina (gly), L-alanina (ala), L-valina (val) ter L-fenilalanine (pal) [B] v 50 % (v/v) mešanici vode in etanola, ki je vsebovala 0.15 M NaClO4. Predpostavili smo nastanek kompleksov tipa NiAB in NiAB2 ter jim določili konstante stabilnosti. S pomočjo temperaturne odvisnosti konstant stabilnosti smo ocenili termodinamske parameter (AG, AH and AS) za procese nastanka kompleksov. Lastnosti kompleksov smo raziskovali tudi s pomočjo elektronskih spektrov ter ugotovili, da imajo oktahedralno strukturo. Proučevali smo tudi možno mikrobiološko aktivnost in vitro na nekaterih bakterijah, glivah in kvasovkah ter njihov vpliv na cepitev DNA.