untitled ISSN 2 Thermo ions with Ebrahim G Chemistry Depar * Corresponding Tel.: +98.056.322 ARTICLE INF DOI: 10.5155/eu Received: 12 July Received in revis Accepted: 07 Oct Published online Printed: 31 Dece KEYWORDS Kinetics Tryptophan Stability constan Thermodynamic Metal ion‐compl Dioxane‐water m 1. Introducti The seco biochemical f by noncovale and with vari between met protein react understand evaluation of for determini potentiometr tion of amino used as a m constants. T Bjerrum [2] a been utilized solvent and t quite number L‐tryptop important ro essential am precursor of biomolecules the extent of 2153‐2249 (Prin dynamic h tryptop Ghiamati * an rtment, University o author at: Chemist 202065. Fax: +98.05 FORMATION urjchem.8.4.333-3 y 2017 sed form: 03 Octob tober 2017 e: 31 December 20 ember 2017   nt cs lex mixture ion ondary, tertiary function of pep ent interaction ious metal or o tal ions and a tions is an inte how well the their stability c ing the stability ry has its own o acids in the p method for me his technique and later modif extensively by temperature ef r of researchers phan (Trp) is a ole in many mino acid for hormones, neu . The level of T f hepatic diseas Eu t) / ISSN 2153‐2 ht Europ and kinet phan nd Zahra Ab of Birjand, Birjand, try Department, Un 56.32202065. E‐ma 338.1613 ber 2017 017 y and quatern ptides and prot n among consti organic cations amino acids wh eresting phenom ese interaction constants. Amo y constant of m advantages. P presence of me easuring meta which first fied by Calvin a y numerous res ffect have also s [19‐34]. an oxidizable a biochemical p human and urotransmitters Trp in plasma i se [35]. The pr uropean Journal Europe 2257 (Online)  2 ttp://dx.doi.org/10 pean Jo Journal web tic studies bazari 971‐743‐4765, Ira niversity of Birjand, ail address: eghiam ABSTRACT Amino acid of study its intera examine the th these tasks, the and Pb(II) at t utilizing modifi water:dioxane complexes incr negative values conveying the contributing m of natural loga the stability co to estimation o was concluded kf Pb(II)‐Trp < kf Ni( aforementione Ea Fe(III)‐Trp < Ea N Cite this: Eur. J nary structures teins are contr ituent amino a [1]. The intera hich mimics m menon. One w ns could occu ong various met metal ion compl Potentiometric tal ions is gene l complex sta was described and Wilson [3,4 searches [5‐18] been examine amino acid play processes. It i herbivores as s and other rele is closely relat esent study foc of Chemistry 8 ( ean Journal of Ch 2017 Atlanta Pub 0.5155/eurjchem ournal bpage: www. s on inter an , Birjand, 971‐743‐ mat@birjand.ac.ir (E tryptophan (Trp action with som hermodynamic e stability consta temperatures of ied Bjerrum’s m mixture (50:50 reased as the di s of ΔG° are indi complex forma more to spontane rithm of the sta nstant of the com of rate constant that the order o (II)‐Trp < kf Cu(II)‐Trp d complexes in Ni(II)‐trp < Ea Co(II)‐trp J. Chem. 2017, 8( s and rolled acids, action metal‐ ay to ur is thods lexes, titra‐ erally bility d by 4] has . The ed by ys an is an s the evant ed to cuses on t of tr Pb(I natio stab ΔH°, rate 2. Ex 2.1. H reag Merc disti cont 1.69 was pour statt then Duri pH w incre (4) (2017) 333‐3 hemistry lishing House LLC m.8.4.333-338.16 of Che eurjchem.com raction of 4765, Iran. (E. Ghiamati). p) was chosen a me transition me and kinetic phe ants of Trp com f 25, 30, 35, an ethod. Potentiom , v:v). Our findin ioxane content w cation of sponta ation is an endo eity, causing rea ability constants mplexes at any t and activation e of increasing stab < kf Fe (III)‐ Trp. Fu water‐dioxane m p < Ea Cu(II)‐trp < Ea P (4), 333‐338 he i) Determin ryptophan with II) metal ions on of the natu ility, (iii) Evalu , and ΔS° and f constant, k and xperimental Materials and Highly pure Tr gent grade, dio ck, Germany. A illed water (DD taining 5.000× 90×10‐2 M HClO added to adj red into a dou ted to desired n titrated with ing the experim was corrected a ements of 0.05 338 C ‐ All rights rese 13 emistry m f some tra as a drug. A syst etal ions, and q enomena on thi mplexes with Fe( nd 40 °C were d metric titrations ngs showed tha was raised or t aneity of the pro othermic proces action favoring a versus 1/T are temperature. Mo energy for each bility of the com urthermore the a mixture obeyed Pb(II)‐trp. ation of the sta h Fe(III), Co(II in water and ure of solvent uation of thermo finally estimatio d activation ene d procedure rp, nitrate salt oxane and HClO All of the solut DW). A 25.00 mL 10‐3 M Trp, 3 O4 . The sufficie ust the ionic uble walled gla temperatures an accurately s ments the react and recorded af mL. Each titrat rved ‐ Printed in y ansition m tematic approac qualitatively and is model drug. III), Co(II), Ni(II determined pote s were carried ou at the stability c emperature was cesses. ΔH° valu ss and ΔS° valu and disordering. linear leading t oreover, kinetic complex forma mplexes is: kf Co(II) activation energy the following tr ability constant I), Ni (II), Zn(I dioxane mixtu t and tempera odynamic para on of kinetic pa ergy, Ea. of the respect O4 all were pu tions were mad L solution mixt .000×10‐3 M m ent amount of 0 strength. The ass reactor; it of 25, 30, 35 a standardized N tor was purged fter each additio tion consumed the USA metal ch was made to d quantitatively To accomplish I), Cu(II), Zn(II) entiometrically, ut in water, and onstants of the s elevated. The ues are positive, es are positive The variations to evaluation of study gave rise ation process. It ‐Trp  kf Zn(II)‐Trp < y values for the rend Ea Zn(II)‐trp < t of complexes II), Cu(II) and ure, ii) Exami‐ ature on their meters of ΔG°, arameters, i.e., tive metals as urchased from de by doubled ture was made metal ion and 0.10 M NaNO3 solution was was thermos‐ and 40 °C and NaOH solution. d with N2. The on of titrant in 6‐8 mL of the 334 Ghiamati and Abazari / European Journal of Chemistry 8 (4) (2017) 333‐338 titrant, and five replicate measurements were conducted in order to check the reproducibility of the data. The calculation of the stability constants of the complexes were performed using our developed computer programme. Spectrophoto‐ metric measurements were done with a UV‐Vis Specord 210 plus with a GDU computer and using thermostated matched 10 mm quartz cells. IR spectra were acquired as KBr disc by Avatar, USA 370 FT‐IR. 2.2. Calibration of the glass electrode All pH titrations were performed using a Metrohm 794 basic automatic titrator (Titrino), coupled with a thermo‐ stating bath Hero (±0.1°C). The pH meter was calibrated using Merck standard buffer solutions with pH of 4.0, 7.0 and 9.0. A combination of calomel and glass electrode was used. To compensate for acid error (pH = 2‐4) of the electrode, 30 mL solution containing KCl, 0.1 M was titrated with standardized perchloric acid, following a plot of calculated pH versus read pH, and producing the equation for pH correction. To account for alkaline error (pH = 10‐12) the same volume of KCl, 0.1 M solution was titrated with standardized sodium hydroxide and the pH correction equation was acquired [36]. 2.3. The method for determination of stability constant The method for determination of stability constant was established by J. Bjerrum [2]. He measured the stability of metal amines using the concentration of metal free ligand and the total ligand concentration. The key to his method was the use of the then recently developed glass electrode and pH meter. Later M. Calvin and K.W. Wilson [3] modified the method so that pH measurements made during titration of a solution containing chelating agent in the presence and in the absence of a metal ion, with a base could be used to calculate the amount of hydrogen ions released in the reaction vessel, the free ligand exponent, the degree of formation of the system, , and the stability of the metal‐ligand complex. If we assume the presence of the reacting species H2L+ as protonated amino acid, HL as the amino acid, and L‐ the anion of amino acid, then the following reactions may occur in the solution. H L ⇌ HL H (1) K (2) HL ⇌ L H (3) K (4) M HL ⇌ ML H (5) K (6) ML L ⇌ ML (7) K (8) Here Kf1 and Kf2 are the first and the second stability constants of the complex. We define as: n (9) The concentration of free ligand is the sum of concent‐ ration of contained ligand species at different form, i.e. L H L HL L (10) The bound ligand concentration is then estimated as: C C C (11) After rearrangement and substitutions we have: n (12) where TH2L+ is the total concentration of H2L+. Then: n (13) We know that: T M ML ML (14) T HL ML 2 ML (15) Perchloric acid was added to titrand solution in excess to prevent the hydrolysis of metal ion. ClO T 2 T (16) T is total concentration of added perchloric acid to titrand. ML 2 ML Na T H (17) n (18) Finally, we will have: HL (19) From plot of pHL versus , the stability constant could be calculated. K (20) K (21) All our calculations in this work were executed by GRCβeta computer‐program developed in our lab. The software inputs are a) Initial volume of solution containing the amino acid, metal ion, and perchloric acid, b) The concentration of perchloric acid, c) The concentration of sodium hydroxide, d) The concentration of amino acid, and e) pKa1 and pKa2 of the amino acid in the specified medium and at desired ionic strength which was found in the literature [37]. In water:dioxane mixture (50:50, v:v) and 0.1 M NaNO3, Trp has pKa1 = 3.47 and pKa2 = 10.22. After insertion of the pertinent values, the software plots the corrected pH of the titrand solution versus the concentration of added standar‐ dized NaOH, plus drawing two curves, one for a = 0.5 and the other for = 1.5. The intersections of the potentiometric titration curve with these two curves produce two points (Figure 1) whose corresponding pHs will be used to evaluate the respective stability constants of the metal ion‐amino acid complexes. Table 1. Compa Log Kf log Kf1 log Kf2 Additiona second deriv points. Thermod ΔS° values. T calculated fro ∆ ° RT ln The plot slop equals to Slope ∆ . Using the For calculatin ∆ ° ∆ ° Regarding depends on th reaction of: aA + bB → pro The rate l Rate = k×[A]n where n and depend on th reaction, the i Regarding initial concen t A By taking we have: log t log The outpu slope of (1‐n amino acid is of the reacti concentration the obtained concentration correspondin half of this c recorded. Th time required half. By findi repeated unt versus log t1/2 Using exp By substituti G rison between our Co (II)‐Trp 4.41 (3.86) 7.98 (‐) ally the softwa vative of d‐pH ynamic study i The Gibb’s fre om the equation n of log Kf versu o: ° e Equation (23), ng ΔS° we have: T∆ ° g kinetics stud he concentratio oduct law could be de n×[B]m m are the orde he detailed rea integrated rate n 1 k g half‐life defin ntration we hav A g logarithm from 1 n ut of plotting lo n) and intercep allowed to rea on is measure n of one compo curve an arbitr n is regarded ng time is initial oncentration is e discrepancy d to reduce the ing other point til enough data 2. perimental data ng n in the Eq hiamati and Aba r obtained logarith Ni (II)‐T 5.81 (5.7 9.98 (10 re is capable o versus d‐VNAOH involved evalu ee energy chan n below: us 1/T produce , enthalpy chan dy, the rate of ons of the react escribed as: rs of the reacta ction mechanis equation could t nition, i.e., as [A e: m both sides o log A og t1/2 versus lo pt of log act with metal i ed at different onent is drawn rary point could as initial con l time. Then the s chosen and i between these concentration o ts on the cure a to be produ a the value of n quation (28), th azari / European hm stability constan Trp 76) .20) of plotting first H to clarify the ating ΔG°, ΔH° nge, ΔG°, coul es straight line ges were comp a chemical rea ting species. Fo ants and their v sm. For the n o d be represente A] reaches to h f the Equation g [A] is a linear . In practice on and the prog times. The pl against the tim d be selected an ncentration an e point equivale ts pertinent tim e two values i of the compone e this task coul ced to plot log could be calcul he rate consta n Journal of Chem nts of Trp complex Fe(III)‐Trp 8.79 (9.00) 17.21 (‐) t and e end , and ld be (22) with (23) puted. (24) action or the (25) (26) alues order ed as: (27) alf of (28) (28), (29) r with e, the gress lot of me. On nd its d its ent to me is s the ent to ld be g [A] lated. nt, k, coul and equa k whe 8.31 the E ln k A slop 3. Re F (II)‐ been betw ion‐T they Tabl resp temp Figur stand T kf Co( mistry 8 (4) (2017 xes in water at 25 ° Zn (I 4.85 8.75 ld be estimated activation en ation: Ae ere Ea is the act 145.J/K.mol. Ta Equation (30), y k A plot of ln(k) v e of ‐Ea /R, or E esults and disc Figure 1 illustr Trp complex in n shown on F ween the stabil Trp complexes y are in a good le 2 demonstr pective complex peratures of 25 re 1. A plot of dardized NaOH solu F The order of inc (II)‐Trp  kf Zn(II)‐Trp … kf Cu(II)‐Trp < kf 7) 333‐338 °C with literature ( II)‐Trp (5.01) (8.20) d. The relation nergy has bee tivation energy aking the natur yields: ln A versus 1/T pro Ea = ‐R (slope) a cussion rates a potentio n water at 25 Figure 2. Tabl lity constant va in water at 25 agreement with rates the stabi xes in water‐dio 5‐40 °C. corrected pH ve ution for Zn(II)‐Tr N H H O Figure 2. The stru creasing stabili p < kf Pb(II)‐Trp < k Fe (III)‐ Trp (in parenthesis) re Cu (II)‐Trp 8.37 (7.80) 15.83 (15.90) n between the n shown by y in J/mol, R ha ral logarithm o duces a straigh nd R = 8.3145 J ometric titration °C. The structu le 1 exhibits alues of the pe 5 °C. The results h the works in ility constant oxane mixture rsus the concent rp in water at 25 °C N H O H O ucture of Trp. ity is as follows kf Ni(II)‐Trp < … 335 eference [14]. ) rate constant the following (30) as the value of f each side of (31) ht line with the J/K.mol. n curve for Zn ure of Trp has a comparison ertinent metal s indicate that the literature. values of the (50:50, v:v) at tration of added C. : (32) 336 Table 2. Logarit Complex Fe(III)‐Trp Co(II)‐Trp Ni(II)‐Trp Cu(II)‐Trp Zn(II)‐Trp Pb(II)‐Trp Table 3. ‐ΔG°1 v Complex Fe(III)‐Trp Co(II)‐Trp Ni(II)‐Trp Cu(II)‐Trp Zn(II)‐Trp Pb(II)‐Trp Table 4. ΔS°1 an Complex Fe(III)‐Trp Co(II)‐Trp Ni(II)‐Trp Cu(II)‐Trp Zn(II)‐Trp Pb(II)‐Trp It was fou first formatio process (i.e. a and L‐). Her between ligan the stepwise illustrates tha which is the Regarding m tryptophan, d the side chai center to me formula for M Figure 3. The m possible configu unpublished res The varia complex of P that the stab evaluated at a Table 3 s the solvent m these compl values were order of incre thm of the first and values for the corre nd ΔH°1 values for t und that in gen on complex pr attraction betw re electrostatic nds are not im formation cons at the most stab e outcome of q metal ion‐Trp does not contain in does not po etal ions. Acco M(Try)2(H2O)2 c most stable config urations according sults). ation of natura Pb(II)‐Trp has b bility constant any temperatur hows ΔG1° valu mixture. The n exation proces increased by easing ΔG1° is as Ghiama d second stability c kf1 & kf2 kf1 kf2 kf1 kf2 kf1 kf2 kf1 kf2 kf1 kf2 kf1 kf2 esponding Trp com ‐ΔG°1 (KJ/mol) 25 °C 6.48 5.21 5.69 6.30 5.21 5.61 the related Trp com ΔS°1 (J/mol) 25 °C 33.15 47.55 27.92 30.65 45.59 29.51 neral Kf2 > Kf1. T rocess is weake ween M2+ and L‐ c repulsion an portant factors stant value of th ble configuratio quantum mech complexation, n any readily io ossess any cle ordingly, the su complex is show uration of Trp in g to quantum mech al logarithm of been shown in for all the c re. ues for the resp negative ΔG1° v sses are spon enhancing the s follows: ati and Abazari / constants of the res T (°C) 25 9.86 19.02 6.12 10.99 7.34 13.46 9.33 17.31 6.15 10.98 7.20 12.92 mplexes in water:di 30 °C 6.63 5.44 5.84 6.41 5.44 5.78 mplexes in water:d 30 °C 33.07 47.51 27.95 30.51 44.87 29.65 This means tha er than the se is weaker than nd steric hindr s for the increa he complex. Fig on of Trp, in sol hanical calculat Indolyl grou onizable proton early defined d uggested struc wed in Figure 4. liquid phase, amo hanical calculation f kf1 versus 1/T n Figure 5, mea omplexes coul pective complex values indicate ntaneous, and e temperature. / European Journ spective Trp comp ioxane mixture (50 dioxane mixture (5 at the econd n ML+ rance ase in ure 3 ution tions. up of n, and donor ctural . ong 22 ns (our T for aning ld be xes in e that ΔG1° . The ΔG1° Figur water T valu chan com incre al of Chemistry 8 plexes in water:dio 30 9.99 19.25 6.50 11.55 7.51 13.73 9.35 17.73 6.35 11.24 7.32 13.39 0:50, v:v). 35 °C 6.81 5.60 5.96 6.59 5.60 5.92 50:50, v:v). 35 °C 33.13 47.26 27.88 30.60 44.73 29.65 ° Zn (II)‐Trp  ΔG1° … ΔG1° Ni(II)‐Trp < HN Figure 4. The p re 5. A plot of lo r:dioxane mixture. The data in Tab ues, for those c nge enforces th mpletion of the eased. 8 (4) (2017) 333 xane mixture (50: 35 10.24 19.91 6.58 12.04 7.60 13.80 9.43 18.26 6.57 11.67 7.55 13.53 40 °C 6.96 5.96 6.12 6.75 5.96 6.06 40 °C 33.08 47.65 27.95 30.62 45.14 29.51 Co (II)‐Trp < ΔG1° P < ΔG1° Cu(II)‐Trp < Δ NH2 HO O M H H proposed structure og Kf1 vs. 1/T for . ble 4 shows tha complexes at 2 e spontaneity o e reaction bec ‐338 50, v:v). 40 10.35 20.23 7.26 13.44 7.72 14.24 9.72 18.43 7.16 13.13 7.62 13.79 ΔH°1 3.40 9.52 2.63 3.07 8.22 3.18 b (II)‐trp < … ΔG1°Fe (III)‐ Trp NH2 OHO Mn+ H2O H2O e of metal ion‐Trp Pb(II)‐Trp comple at all ΔS1° posse 25‐40 °C. Here of the reaction a cause disorder 5 3 4 4 3 3 9 (kJ/mol) (33) NH complex. ex in 50:50 (v:v) essing positive e the entropy and favors the ing has been Ghiamati and Abazari / European Journal of Chemistry 8 (4) (2017) 333‐338 337 Table 5. Activation energy (Ea) and the rate constant (k) values for the pertinent Trp complexes in water:dioxane mixture (50:50, v:v). Complex Ea (J/mol) k (1/s) 25 °C 30 °C 35 °C 40 °C Fe(III)‐Trp 9.56×104 2.84×10‐7 5.12×10‐7 9.15×10‐7 1.83×10‐6 Co(II)‐Trp 1.09×105 1.76×10‐7 3.18×10‐7 7.05×10‐7 1.41×10‐6 Ni(II)‐Trp 1.08×105 2.20×10‐7 4.24×10‐7 8.36×10‐7 1.79×10‐6 Cu(II)‐Trp 1.13×105 2.41×10‐7 4.93×10‐7 9.75×10‐7 2.18×10‐6 Zn(II)‐Trp 9.52×104 1.82×10‐7 3.40×10‐7 6.93×10‐7 1.11×10‐6 Pb(II)‐Trp 1.29×105 2.00×10‐7 3.70×10‐7 8.11×10‐7 2.50×10‐6 ΔH1° values are positive and independent of temperature, also the temperature increase affects the complexation. Because M+2 ions exert solvation effect and L‐ and M+2 ions with opposite charge could break, causing H° and S° to become positive. Although the process of Trp complex formation with a metal ion has an endothermic nature as a result of the positive ΔH° value, it could be concluded that the second formation step of complex is highly spontaneous compared to the first formation step. Positive H° and S° means that the reactions become spontaneous at higher temperatures. Regarding using dioxane as a co‐solvent, 1,4‐dioxane‐ water mixture is a well‐defined solvent. Changes in stability constants upon addition of 1,4‐dioxane to aqueous solution is due to increase in ion‐ion interactions resulting from both the decreasing dielectric constant and the change in solvent‐ion and solvent‐solvent interactions. As the dielectric constant decreases, the ion interaction involving the proton and anionic oxygen on the amino acid decreases to a greater extent than the ion dipole interaction between the proton and the solvent molecules. Co‐solvent influences the protonation‐deproto‐ nation equilibria in solution by changing the dielectric constant of the medium, which varies the relative contri‐ butions of electrostatic and non‐electrostatic interacttions. Our outcomes demonstrate that Cu(II) complexes are more stable than the other metal ion complexes as a result of the larger ratio of valance/radius and the Jahn‐Teller effect. Figure 6 shows a potential energy diagram for an endothermic reaction. The energy difference between the reactants and complex transition state is called activation energy. Higher activation energy translates lower rate of as reaction and more dissimilarities between the reactants and the transition states. More discrepancy between bonds in the two states leads to higher activation energy. Table 5 shows the rate constant and activation energy values for the aforementioned complexes in water‐dioxane mixture. The following trend was acquired. Ea Zn(II)‐trp < Ea Fe(III)‐Trp < Ea Ni(II)‐trp < Ea Co(II)‐trp < … … Ea Cu(II)‐trp < Ea Pb(II)‐trp (34) Figure 6. Potential energy diagram for an endothermic reaction. 4. Conclusions In this study, a systematic approach was made to examine the interaction of some transition metals ions with tryptophan in water and water:dioxane mixture (50:50, v:v) at various temperatures. As was expected, cobalt and zinc complexes had the lowest stability and copper and iron the highest. Our results show that as the temperature rises, or on addition of dioxane to water, the stability constant increases, The Gibbs free energy change is negative correlating to the spontaneity of the complex formation. ΔH° and ΔS° values were positive The variations in logarithm of stability constants versus 1/T is linear evidencing that ΔH°s are independent of temperature in temperature range of the study, and increase in entropy enforces the formation of these complexes. 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