Spectrophotometric Method for the Determination of Caffeic Acid Complexation and Thermodynamic Properties

Transcription

1 International Journal of Biophysics 22, 2(2): 2-7 DOI:.5923/j.biophysics Spectrophotometric Metho for the Determination of Caffeic Aci Complexation an Thermoynamic Properties Abebe Belay Physics Department, School of Natural Sciences, Aama Science an Technology University, P.O.Box 888, Aama, Ethiopia Abstract The concentration epenent self-association of caffeic aci an its complexation with soium ions have been stuie in aqueous solutions at room temperature or 298 K. The self-association an hetero-association have etermine by imer moel an Benesi-Hilebran approach respectively. The imerzation constants of caffeic aci in water an acetonitrile solutions are 2947 an 758 M - respectively. The equilibrium constant for the complexation of caffeic aci with soium ions at the wavelength of 343 nm was 686 M -. The thermoynamic properties of self-association of caffeic aci were estimate from Van t Hoff s equation. The values of change in Gibb s free energy, enthalpy an entropy are calculate at temperature of 328 K. The imerization of caffeic aci are ue to intermolecular hyrogen boning by carboxylic aci groups of the compoun an exothermic which characterize by relatively large negative of change in enthalpy. The intermolecular hyrogen boning of such kin of compoun is very important force in the formation of supramolecular liqui crystals. Keywors Dimer Moel, Benesi-Hilebran Approach, Caffeic Aci, Thermoynamic Parameters, Non-Linear Curve Fitting. Introuction Caffeic aci, trans-3- (3, 4-ihyroxycinnamic aci) prophenic aci is one of the most common phenolic acis frequently occurre in fruits, vegetables, cereals, legumes an in beverage of plant such as wine, tea an coffee for human consumption as simple esters with quinic aci[-2]. The compoun has attracte attention of researchers ue to health promoting attributes, such as lowering the risks of cariovascular isease, cancer, iabetes, an other conitions associate with aging. The biological mechanisms behin these effects are protection against free raicals, free raical meiate, inflammation an viral infection[-3]. Caffeic aci is a multifunctional naturally available organic aci substance which plays a significant role in bining metal ions from natural environment, foo substances an beverage such as coca cola, mineral water etc. The ligan has two complexing sites in competition: the catechol group (ihyroxybenzene) an the carboxylic function. Several coworkers have reporte the complexation * Corresponing author: abebebelay96@gmail.com (Abebe Belay) Publishe online at Copyright 22 Scientific & Acaemic Publishing. All Rights Reserve of this compoun with ifferent metal ions in aqueous solutions, Al (III)[4-6], copper (II), Ni (II), Zn (II) an Co (II)[6] an iron (III)[7]. In aition, the complexation of caffeic aci with polyphenol an aromatic compouns were also investigate by spectroscopic an computational methos to esign avance an controllable carriers of rugs an foo components[8-9]. The investigation of self-associate molecules by spectrophotometric has a limitation ue to ifficulty in obtaining the spectra of highly concentrate solutions an the technique nees of careful observation of changes in the apparent molar absorptivity over a wie range of concentrations[]. The formation of imer complicate the use of Beer-Lambrt slaw. The values of molar absorptivity an shape of the absorption ban of the fraction of imerize molecules are usually unknown an that often leas to ifficulty in the interpretation of experiment[]. Investigation of the self-association an hetero-association of the solute is an important phenomenon. This is accounte in analyzing an interpreting the spectroscopy, photophysics an photochemistry of the system[-]. Moreover, in orer to esign a more avance an controllable carries of rugs or foo components it is necessary to know the association mechanisms to control the processes. Despite, there were various research reports on the complexation of caffeic aci with toxic metal ions an other

2 International Journal of Biophysics 22, 2(2): molecular compouns; however, currently there are no report as far as the author knowlege concerns on the concentration epenent self-association of caffeic aci, its thermoynamic properties an the complexation this compoun with soium ions, Therefore, the objectives of this research are investigating the self-association, hetero-association an thermoynamic properties of caffeic aci using spectrophotometric. The technique is the most suitable for quantitative stuy of the aggregation properties of the molecules as a function of concentration. 2. Materials an Methos The stuie compoun in this work is caffeic aci, bought from Sgima-Alrich. The absorption spectra were recore by Perkin-Elmer Lama 9 spectrophotometer with ouble monochromator, which offers the avantage of a low level of stray light that is significant for measure of high value of absorbance using cm cuvette. The spectra were recore with a spectral ban with of 2 nm an scan spee 24 nm min -. The acquisition of ata were performe by 4.3 UVCSS soft ware, using a suitable computer on line with the spectrophotometer. The electronic absorption spectrum of the compoun was measure ranges of 2-5 nm. The analyze spectra were obtaine by subtracting the spectrum of pure solvent (water) from that of the solution containing caffeic aci. The experimental metho implemente to stuy the self-association of caffeic aci is similar with metho applie by[2], for stuying 5-caffeoylquinic aci. The change in the shape of the spectrum, shift of maximum absorbance an isobestic points at three wavelengths inicating the existence of two ifferent species. The self-association of the compoun was stuie over the concentrations ranges of (.67-) x -5 an ( ) x -5 M in acetonitrile an water solutions respectively. The self-association can be etecte over a wie range of concentration an nee a careful examination of the changes in the apparent molar absorptivity. The absorbance as a function of concentration were measure at 286 an 38 nm wavelengths in acetonitrile an water respectively. On the other han for stuying the complexation of caffeic aci with soium ions, the aqueous solution of soium hyroxie ( ) x -3 M an caffeic aci 5.83x -5 M solution were prepare in water. The electronic absorption spectra of the complex solution recore at various concentrations of soium hyroxie solutions. The thermoynamic parameters of caffeic aci self-association in water solution were calculate at temperature of 328 K. For numerical analysis the molar absorptivity an equilibrium constants, imer moel for self-association of caffeic aci an Benesi-Hilebran approach for the complexation with soium ions were use. Numerical proceure of fitting the imer moel to experimental ata was carrie out by non-linear curve fitting base on Levenberg- Marquart algorithm. The molar absorptivity an equilibrium constants were use as searching parameters in orer to achieve the minimum iscrepancy between the experimental ata an theoretical expression. 3. Results an Discussion 3.. Concentration Depenent Self-Association of Caffeic Aci Fig shows the absorption spectra of caffeic aci in acetonitrile which has lack of hyroxyl group. In the concentration ranges of (.67-) x -5 M caffeic aci has three isobestics at the wavelength of 254, 282 an 374 nm respectively. In aition, shift of peak maximum, increase an ecrease the intensity spectra at ifferent absorption regions were observe relevant to the concentration of caffeic aci Wavelength/nm Figure. the electronic absorption spectra of caffeic aci in acetonitrile in the concentration ranges of (.67-) x -5 M Absorption spectra of caffeic aci measure in istille water for two ifferent concentrations at room temperature. The ot shows the absorption spectrum of caffeic aci at concentration of 23.9x -5 M. In this concentration, the main absorption bans of caffeic aci are ouble ban at 294 an 39 nm an other locate at 27 nm. On the other han the line shows the caffeic spectrum at the concentration of.49x -5 M; in this case the peaks were observe at 287, 32 an 25 nm respectively. Like that in acetonitrile shift of peak maxima, intensity variation at peaks an four isobestic points at (225, 255, 283 an 374 nm) were observe relevant to the concentration. When the concentrations of caffeic aci are greater than 5.3x -5 M, the peak intensity at the wavelength (~39 nm) greater than the two peak ban; however; for concentration less than 5.3x -5 M, the peak intensity at the wavelength (~25 nm) greater than the two peaks foun at higher wavelength. The shift of maximum absorbance, intensity variation an isobestic points, clearly suggests the existence of self-association of caffeic aci ue to hyrogen boning of carboxylic groups. This has also been reporte by[3-4], that the imer of caffeic is ue to hyrogen bon of carboxylic groups.

3 4 Abebe Belay: Spectrophotometric Metho for the Determination of Caffeic Aci Complexation an Thermoynamic Properties The quantitative analysis of caffeic aci self-association was carrie out using the concentration epenence of molar absorptivity of the molecule. The values of molar absorptivity of caffeic aci in water an acetonitrile increase an ecrease as the concentration increases respectively. The eviation from Beer-Lambert s law an epens on concentrations, suggests the existence of self-association process. These effects are plotte in fig 3a-b, where the points show the experimental ata an the lines are the imer moel fitte to the experimental ata. For numerical analysis of the molar absorptivity of monomer, imer an imirazation constant the imer moel were erive as follows. The moel was erive by consiering the following molecular equilibrium in solutions Where, B an 2 B B +B B () K B 2 K is the equilibrium imerzation constant, B are monomers an imers of caffeic aci respectively. The over all concentration of the issolve molecules in solution is given by the mass conservation law, [B ]=[B ]+2[B 2] (2) Where, [B ] is the total concentration of caffeic aci, 2 [B ] is the monomer concentration an [B ]=K [B ] 2 B is the concentration of the imers. The contribution of the monomer an imer to the molar absorptivity, ε of the solution is commonly consiere to be aitive, ε=ε f +ε f (3) m m Where, ε m, ε are absorptivity of caffeic aci monomers an imers respectively. f m=[ B ] [B ] is the equilibrium mole fraction of the molecules in the monomer 2 an f =2K B[ B ] [B ] is a mole fraction of the molecules with in the imer concentration. Concentration [B ] can be etermine from the solution of the mass conservation law of equation (2). By substituting the solution of equation (2) in to equation (3) the following imer moel obtaine, Molar absorptivity/m - cm - Molar absorptivity/m - cm -.8x 4.8x 4.7x 4.6x 4.5x 4.5x 4.8x 4.65x 4.5x 4.35x 4.2x 4 5.x -5.x -4.5x -4 2.x x -4 a.concentration of caffeic aci/m in water 2.x -5 4.x -5 6.x -5 8.x -5.x -4 b. Concentration of caffeic aci/m in acetonitrile Fiure 3. a) the molar absorptivity versus concentration of caffeic aci in water at maximum wavelength ( λmax=38 nm) an b) the molar absorptivity versus concentration of caffeic aci in acetonitrile at maximum wavelength ( λmax= 286 nm) Fraction of Monomer Molecules x -5.x -4.5x -4 2.x x -4 Concentration of Caffeic Aci/M Figure 4. the mole fraction of monomer an imer versus total concentration of caffeic aci in water solution - 8[B ]K B+ ε=ε + (ε-ε m) (4) 4[B ]KB In equation (4) there are three unknown parameters, ε m, ε an K B which can be obtaine from numerical analysis of experimental concentration epenence of the molar absorptivity of caffeic aci in acetonitrile an water solutions, fig 3 an 4.The values of these three quantities were compute by nonlinear regression base on the Lavenberg-Marquart algorithm by origin software. The three quantities are serving as a search parameters being ajuste in orer to achieve the minimum iscrepancy between the experimental ata an equation (4). By this metho the values of ε m&ε, K B were calculate for caffeic aci in acetonitrile an water solutions an these values are shown in table. The obtaine values are quite reasonable an comparable with the results obtaine by other workers in the UV region of the spectrum for other molecules. The values of ε m&ε etermine for lauric aci in liqui of CCl 4 using FTIR were 2.8x 4 an 8.66x 3 L mol - cm - respectively[5]. Furthermore, the self-association constant an monomer molar absorptivity etermine for ADP stuie in aqueous solution at high ph value woul be x 4 M - an.5x 4 M - cm - respectively[6]. Moreover, the calculate imerzation constant of caffeic in water in this research is quite similar with the imerzation constant calculate for 5-caffeoylquinic aci an reporte by[2] Table. the molar absoptivity of monomer, imer an imerzation constant of caffeic aci in acetonitrile an water solutions ε Solution m/l mol - cm ε - /L mol cm K /M Acetonitrile 9.69x4 3.89x4 7.58x2 Water.4x x x3 The fraction of monomer an imer molecules versus total caffeic aci concentration were also etermine in water solution an shown in fig 4. The formations of imer molecules are favore by high concentration, in which the Fraction of Dimer Molecules

4 International Journal of Biophysics 22, 2(2): fraction of monomer an imer molecules are ecrease an increase respectively The Complexation of Soium ions with Caffeic Aci Wavelength/nm Figure 2. the electronic absorption spectra of caffeic aci in water for two ifferent concentrations. The ot shows the spectrum of caffeic aci at C= 23.9x-5 M an line the spectrum at C=.49x-5 M Wavelength/nm Figure 5. The UV-Visible absorption spectra for titration of caffeic aci (5.83x-5 M) with aqueous of soium hyroxie (7.59x-3M) caffeic aci]/naoh] Figure 6. Absorbance versus mole ratio ([caffeic aci]/[soium hyroxie]) at 343 nm The electronic absorption spectrum of free caffeic aci in water solution shown in figure 2 has mainly characterize by ouble absorption ban at 38 an 294 nm an other ban C D locate at 26 nm. The aition of anhyrous of soium hyroxie to caffeic aci solution in water result in spectral moification an apparition of a new ban at higher wavelength (343 nm). In aition, a ecrease in absorbance is obsorbe upon aition of increasing quantities of soium ions to caffeic aci solution as shown in fig 5. These changes coul be attribute to the complexation between the ligan (caffeic aci) an soium ions. The mole ratio metho was use to etermine the stoichiometry of complexe forms of caffeic aci with soium ions. The absorbance versus molar ratio plot at the wavelength of 343 nm fig 6 the stoichiometry Na ion:caffeic aci=:3. This result is similar with the report by[5] for the complexation of caffeic aci with aluminuim ions. The complexation of Na+ with caffeic aci coorinating at carboxylic an catechol sites by releasing the H+ from oxygen. Many literatures are also escribe the strong bin of metal ions with oxygen-onar ligans an the chealting power of this compoun at catechol an carboxylic groups with Al (III) an Pb (II) were also stuie by[4-5] previously. Fig 5 shows peak absorbance vs various concentration of soium hyroxie in caffeic aci solutions or ([NaOH]:[Caffeic aci] for the ratio of 7-87) at peak 343 nm. Furthermore, the isobestic points in the absorption spectra, which are also the inicative of the complex formation between soium ions an caffeic aci The quantitative analysis of the complexation of soium ions with caffeic aci was accomplishe using Benesi-Hilebran approaches[7], uner the conition of[b] >>[A]. A caffeic aci solution of ([CA] =[A] =5.83x-5 M=constant) an ifferent concentrations of soium hyroxie ([NaOH] =[B] = ( ) x-3 M were use to calculate the equilibrium constant an molar extinction coefficient of the complex formation. The equilibrium constant for the complex formation K, erive as follows, K AB (5) A+B The equilibrium constant for the complex formation, K can be efine as K [AB] [A][B] = (6) Where,[A],[B], an[ab], are the equilibrium concentration of caffeic aci, soium hyroxie an association of caffeic aci with soium respectively. If the initial concentration of caffeic aci an soium hyroxie esignate as[a ] =[A] +[AB] an[b ] =[B] +[AB], substituting these in to Eq (6) it gives, [AB] K= (7) ([A ]-[AB])([B ]-[AB]) For[B]>>[A], the[b ]-[AB] approximately becomes[b ], so Eq (7) can be written as [AB] K= ([A ]-[AB])([B ]) (8) After re-arranging Eq (8) it gives

5 6 Abebe Belay: Spectrophotometric Metho for the Determination of Caffeic Aci Complexation an Thermoynamic Properties K[A ][B ] [AB]= (9) +K[B ] The absorbance (A) for concentration[ab] accoring to Beer's law is A=[AB]εl=εl () +K[B ] Eq () can be re-arrange as follow K[A ][B ] [A ] +K[B ] = A Kεl[B ] () When the path length l=cm, Eq () can be written in the form of Benesi-Hilebran equation [A ] = ( )( ) A ε [B ] εk + (2) [A ] The plot of A vs. [B ] (R=.99) with y-intercept ε an slope gives a straight line. By this εk metho the equilibrium constant for the complex formation an molar extinction coefficient calculate at peak position were K=6.86x 3 M - an.4x 4 M - cm - respectively. The equilibrium constant for the complex formation with soium is with in the range of the imerzation constants of caffeic aci Thermoynamic Properties of the Self-association of Caffeic Aci ln K /T/ K - Figure 7. the Van t Hoff plots for self-association of caffeic aci at concentration (C=5.83x-5 M) Heating the aqueous solution of caffeic aci shows that the absorption spectra of the molecules are strongly epenent on the temperature in the range of ( K). As temperature increases, the absorption intensity increases which inicate a issociation of the molecular associate forms in the solutions. Fig 7 shows the graph of ln K B versus f(/t),of caffeic aci, which is linear. The magnitue of the enthalpy was estimate from the slope of the approximating line accoring to van't Hoff's equation: ln(k B) ΔH =- (3) ( ) R T Where, H is the molar enthalpy change, R= 8.3 J mol - K -, the universal gas constant an temperature in Kelvin. The entropy was erive from Gibb's free energy an enthalpy. The Gibb's free energy can be expresse as G=-RTln K B (4) The entropy becomes ( G- H) S=- (5) T The calculate values of Gibb s free energy, enthalpy an entropy of caffeic aci self-associations are -34.6, k J J K mol respectively. These values are in mol an goo agreement with the results reporte by[9] using fluormertic an quantum chemical methos for : stoichimetry molecular association between caffeic aci an malviin3-o-glucosie. From obtaine results the self-association of caffeic aci are exothermic which are characterize by relatively large negative of change in enthalpy 4. Conclusions The concentration epenent self-association of caffeic an its hetero-association with soium hyroxie were reporte in water solution by UV-Vis spectroscopy at room temperature. A shift of maximum absorbance an isobestic points relevant to the concentration of caffeic aci inicates the self -association of molecules ue to carboxylic or catechol groups. Similarly the appearance of new ban at higher wavelength, ecrease in relative absorbance an the existence of two isobestic points in the mixe solution of caffeic aci an soium hyroxie show the hetero-association of the molecules. Unerstaning the mechanism of self-association of caffeic aci is useful in orer to esign the avance an controllable carriers of rugs an foo components. On the other han the results obtaine from complexation of the compoun with soium ions are great applications in extraction alkali metals ions selectively from natural environment. REFERENCES [] RW Jiang, KM Lau, P M Hona, T CW Mak, K S Woo, K P Fung, Current Meicinal chemistry (25). [2] J Iglesias, M Pazos, M L Anersen. L H Skibste, I Meina, Agriculture an Foo Chemistry, 57, (29).

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