RAMA RAJU B, SANTHEE DEVI K V, PADMAJA N AND

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J. Chil. Chem. Soc., 56, Nº 4 (2011)PROTONATION EQUILIBRIA OF L-DOPA AND 1,10 PHENONTHROLINE INPROPYLENE GLYCOL-WATER MIXTURESRAMA RAJU B, SANTHEE DEVI K V, PADMAJA N AND NAGESWARA RAO G*Department of Inorganic & Analytical Chemistry, Andhra University,Visakhapatnam-530003, India(Received: September 23, 2011 - Accepted: May 23, 2011)ABSTRACTProtonation equilibria of L-Dopa and 1,10 phenonthroline have been studied in varying concentrations (0-60% v/v) of propylene glycol–water mixturesmaintaining an ionic strength of 0.16 mol dm-3 at 303 K using pH metric method. The protonation constants have been calculated with the computer programMINIQUAD75 and the best fit models are arrived at based on statistical grounds employing crystallographic R factor, χ2, skewness and kurtosis. Dopa has threedissociable protons and one amino group which associate with proton. It exists as LH4 at low pH and gets deprotonated with the formation of LH3, LH-2 and LH2successively with increase in pH. Phen forms LH22 at low pH and gets deprotonated with the formation of LH and L with increase in pH. Secondary formationfunctions confirm the existence of 3 and 2 protonation equilibria for dopa and phen, respectively. The linear increase of log values of protonation constants ofDopa with decreasing dielectric constant of PG-water mixtures indicates the dominance of electrostatic forces in the protonation-deprotonation equilibria. Phenexhibits non-linear trend indicating the dominance of non-electrostatic forces.Keywords: Protonation equilibria, propylene glycol, L-Dopa, 1,10-Phenanthroline1. INTRODUCTIONL-Dopa (L-3,4-dihydroxyphenylalanine) is a naturally occurring dietarysupplement. Its richest natural source is from plant kingdom like the seedsof Mucuna Pruriens.1 Dopa is a popular drug in the treatment of manganesepoisoning and Parkinson’s disease (PD)2 which are accompanied byneurologically similar sequels3. Dopa increases dopamine concentration,since it is capable of crossing the blood brain barrier, where dopamine itselfcannot. Once dopa enters the central nervous system (CNS) it is convertedin to dopamine by the enzyme aromatic L-amino acid decarboxylase, alsoknown as dopa decarboxylase. However, conversion to dopamine also occursin the peripheral tissues, causing adverse effects and decreasing the availabledopamine to the CNS. So it is the standard practice to co-administer aperipheral dopa decarboxylase inhibitor. Compounds containing Dopa werefound to cross-link to proteins4. Protonation reactions of dopa were reported5-13that Hdopa (H4L) , dopa (H3L), dopa- (H2L)- and dopa2- (HL)2- were formed inthe pH range of 1.6-11.0 and dopa3- (L)3- above pH 13.0.1,10 Phenonthroline (phen) or 4,5-diazaphenanthrene is a tricycliccompound. Phen is a metal chelator. As a bidentate ligand in coordinationchemistry, it forms strong complexes with many metal ions through N-atoms14-20.Due to hydrophobicity of aromatic rings of phen, the solubility of the neutralspecies is low in water which remarkably increases in organic solvents andalso in aqua-organic mixtures. The protonation constant of phen were reportedin various aqueous alcohol solutions21. The protonated species Hphen andH2phen2 were reported in the pH range 3.8-5.5 and 1.0, respectively14-16,22,23.1,2-propanediol, also known as propylene glycol (PG) has a dielectricconstant24 of 30.2. The dielectric constant of PG-water mixture decreases withincrease in the mole fraction of PG. Hence this medium is chosen to study theacido-basic equilibria to mimic the physiological conditions where the conceptof equivalent solution dielectric constant25 for active site cavities of proteinis applicable. The effect of dielectric constant on the protonation equlibriaof Dopa and phen in Dioxan-water mixtures has been studied earlier in ourlaboratory.262. EXPERIMENTAL2.1 MaterialsSolutions (0.05 mol L-1) of L-Dopa (Loba, India) and 1,10-phenanthrolinemono hydrate (Finar, India) were prepared in triple-distilled water bymaintaining 0.05 mol L-1 hydrochloric acid concentration to increase thesolubility. 1,2 Propanediol (Finar, India) was used as received. Hydrochloricacid (Qualigens, India) of 0.2 mol L-1 was prepared. Sodium chloride(Qualigens, India) of 2 mol L-1 was prepared to maintain the ionic strength inthe titrand. Sodium hydroxide (Qualigens, India) of 0.4 mol L-1 was prepared.All the solutions were standardized by standard methods. To assess the errors842that might have crept into the determination of the concentrations, the datawere subjected to analysis of variance of one way classification (ANOVA)27.The strengths of alkali and mineral acid were determined using the Gran plotmethod28,29.2.2 Alkalimetric TitrationsAlkalimetric titrations were carried out in media containing varyingcompositions of PG (0-60% v/v) maintaining an ionic strength of 0.16 molL-1 with sodium chloride at 303 0.05K. An Elico LI-120 pH meter was used.Potassium hydrogen phthalate (0.05 mol L-1) and borax (0.01 mol L-1) solutionswere used to calibrate the pH meter. In each titration, the titrand containedapproximately 1 mmol of hydrochloric acid. The initial concentrations ofingredients are given in Table I.Table I: Total initial concentrations of ingredients (in mmol) in protonligand titrations.PG % 60.37280.49710.25010.37510.5002e-mail: gollapallinr@yahoo.com

J. Chil. Chem. Soc., 56, Nº 4 (2011)The glass electrode was equilibrated in a well stirred PG-water mixturecontaining inert electrolyte for several days. At regular intervals titration ofstrong acid was titrated against alkali to check the complete equilibration ofthe glass electrode. The calomel electrode was refilled with PG-water mixtureof equivalent composition as that of the titrand. Alkalimetric titrations wereperformed in media containing 0-60 % v/v PG-water mixtures pH metrically.The details of experimental procedure and titration assembly have beendetailed elsewhere30.2.3 Modeling StrategyThe approximate protonation constants of dopa and phen were calculatedwith the computer program SCPHD31. The best fit chemical model for eachsystem investigated was arrived at using non-linear least-squares computerprogram, MINIQUAD7532, which exploits the advantage of constrained leastsquares method in the initial refinement and reliable convergence of Marquardtalgorithm. The variation of stepwise protonation constants (log K) with thedielectric constant of the medium was analyzed on electrostatic grounds for thesolute-solute and solute-solvent interactions.2.4 Residual Analysis27In data analysis with least squares methods, the residuals (the differencesbetween the experimental data and the data simulated based on the modelparameters) are assumed to follow Gaussian or normal distribution. For anideal normal distribution, the values of kurtosis and skewness should be threeand zero, respectively.χ2 testχ2 is a special case of gamma distribution whose probability densityfunction is an asymmetrical function. This distribution measures the probabilityof residuals forming a part of standard normal distribution with zero mean andunit standard deviation. If the χ2 calculated is less than the table value, themodel is accepted.Crystallographic R-testHamilton’s R factor ratio test is applied in complex equilibria to decidewhether inclusion of more species in the model is necessary or not. In pH metricmethod, the readability of pH meter is taken as the Rlimit which represents theupper boundary of R beyond which the model bears no significance. Whenthese are different numbers of species the models whose values are greater thanR-table are rejected.SkewnessIt is a dimensionless quantity indicating the shape of the error distributionprofile. A value of zero for skewness indicates that the underlying distributionis symmetrical. If the skewness is greater than zero, the peak of the errordistribution curve is to the left of the mean and the peak is to the right of themean if skewness is less than zero.KurtosisIt is a measure of the peakedness of the error distribution near a modelvalue. For an ideal normal distribution kurtosis value should be three(mesokurtic). If the calculated kurtosis is less than three, the peak of the errordistribution curve is flat (platykurtic) and if the kurtosis is greater than three,the distribution shall have sharp peak (leptokurtic).Figure 1: Plots of nH versus pH in 30 % v/v PG-water mixture; (A) dopa( ) 0.29, ( ) 0.44, and ( ) 0.59mmol and (B) phen ( ) 0.25, ( ) 0.38, and ( )0.51mmol, respectively.3. RESULTS AND DISCUSSION3.1 Secondary formation functionsSecondary formation functions like average number of protons bound permole of ligand (nH) and number of moles of alkali consumed per mole ofligand (a) are useful to detect the number of equilibria. Plots of nH versus pH(formation curves) for different concentrations of the ligand should overlap ifthere is no formation of polymeric species. Overlapping formation curves fordopa and phen (Figure 1) rule out the polymerization of the ligand molecules.The pH values at half integral values of nH correspond to the protonationconstants of the ligands. Three half integrals in the case of dopa and one halfintegral in the case of phen (Figure 2) emphasize the presence of three andone protonation-deprotonation equilibria in the pH range of present study. Thenumber of plateaus in the formation curves corresponds to the number of theseequilibria.843

J. Chil. Chem. Soc., 56, Nº 4 (2011)Figure 2: Formation functions ( ) and Species distribution diagrams of(A) Dopa and (B) phen in 30% v/v PG-water mixture.The plots of a versus pH are given in Figure 3. The negative values of acorrespond to the number of moles of free acid present in the titrand and thenumber of associable protons. The positive values of a indicate the number ofdissociable protons in the ligand molecules. The maximum value of a in Figure3(A) is 3, which indicates that dopa has three dissociable (one carboxyl andtwo phenolic) protons. The corresponding value for phen (Figure 3(B)) is zero,which clearly infers that phen has no dissociable protons.844Figure 3: Variation of a with pH in 30 % v/v PG-water mixture: (A) Dopaand (B) phen, respectively.Dopa contains two ionizable phenolic protons (catecholate) in addition tocarboxylic and amino protons. Its neutral ligand form is a tribasic acid, H3L,with four potential co-ordination centers. So Dopa possesses four protonationconstants corresponding to four protons in H4L from. The first proton (aphenolate proton) to coordinate has a very high affinity for the L3- ion (log K 13). The next two protons coordinate to the other phenolate oxygen and theamine nitrogen. These two formation reactions overlap. The fourth proton tocoordinate is the carboxyl proton (log K 2). From spectroscopic evidenceMartin5,6 and Gergely et al7 concluded that the amine group has higher affinity(log KNH3 9.17) for protons than the second phenolate oxygen (log KOH 8.97). Based on linear free energy relationship and kinetic evidence, Jameson8interpreted the phenolate oxygen to protonate first (log KOH 9.76) followed bythe amine nitrogen (log KNH3 8.93). This ambiguity was resolved by Jamesonet al9, in a proton NMR study which identified the second phenolic group ofdopa to be more acidic (log KOH 8.97) than the amino group (log KNH3 9.20).The best fit models containing the type of species and overall formationconstants along with some of the important statistical parameters are givenin Table II. A very low standard deviation (SD) in log β values indicates theprecision of these parameters. The small values of Ucorr (sum of squares ofdeviations in concentrations of ligand and hydrogen ion at all experimentalpoints) corrected for degrees of freedom indicate that the experimental datacan be represented by the model. Small values of mean, standard deviation andmean deviation for the systems corroborate that the residuals are around zeromean with little dispersion.

J. Chil. Chem. Soc., 56, Nº 4 (2011)Table II: Best-fit chemical models of acido-basic equilibria of dopa and phen in PG-water mixtures.% v/vLog β1 (SD)Log β2 (SD)Log β3 (SD)NPUcorrPGDopa (pH ranges 1.6-3.5 & 8.0-11.0)1.78010.11 (14)19.03 (12)21.31 (16)1281.761010.00 (9)19.05 (9)21.51 (14)1501.702010.16 (10)19.26 (9)21.86 (15)1400.303010.14 (8)19.19 (7)21.69 (12)1410.354010.39 (10)19.58 (11)22.29 (18)1320.395010.56 (9)19.86 (11)22.89 (21)1520.396010.48 (9)19.61 (11)22.43 (18)162Phen (pH range 2.5-7.0)05.13 (26)23105.06 (13)20205.09 (14)18304.82 (8)17404.56 (8)17504.50 (17)19604.35 10.570.0144Ucorr U/ (NP-m) X 108;NP Number of pointsm number of protonation constants, SD Standard deviationThe kurtosis values in the present study indicate that residuals formleptokurtic patterns. The values of skewness given in Table II are between 2.6and 7.63. These data evince that the residuals form a part of normal distribution;hence, least squares method can be applied to the present data. The sufficiencyof the model is further evident from the low crystallographic R-values. Thesestatistical parameters thus show that the best fit models portray the acido-basicequilibria of dopa and phen in PG-water mixtures. The low crystallographicR-values given in Table II indicate the sufficiency of the model. The values ofskewness recorded in Table II are between -1.77 and 0.44. These data evincethat the residuals form a part of normal distribution; hence, least-squaresmethod can be applied to the present data. The kurtosis values in the presentstudy indicate that the residuals form leptokurtic pattern in the case of dopa andplatykurtic for phen.Alkalimetric titration data are simulated using the model parametersgiven in Table II. These data are compared with the experimental alkalimetrictitration data, to verify the sufficiency of the models. The overlap of the typicalexperimental and simulated titrations data given in Figure 4 indicates that theproposed models represent the experimental data.Figure 4: Simulated (o) and experimental (solid line) alkalimetric titrationcurves in 20% v/v PG water mixture: (A) Dopa and (B) phen; (a) 0.25, (b) 0.38and (c) 0.50 mmol, respectively.3.2 Effect of systematic errors in best fit modelMINIQUAD75 does not have provision to study the effect of systematicerrors in the influential parameters like the concentration of ingredients andelectrode calibration on the magnitude of protonation constant. In order to relyupon the best chemical model for critical evaluation and application undervaried experimental conditions with different experimental with differentaccuracies of data acquisition, an investigation was made by introducingpessimistic errors in the concentration of alkali, mineral acids and the ligands.The results of a typical system given in Table III emphasize that the errors inthe concentrations of alkali and mineral acid affects the protonation constantsmore than that of the ligand.845

J. Chil. Chem. Soc., 56, Nº 4 (2011)Tabla III: Effect of errors in influential parameters on the protonationconstants in 30%v/v PG-water mixture.Figure 5: Variation of step-wise protonation constant (log K) withreciprocal of dielectric constant (1/D) in PG-water mixture: (A) Dopa and (B)phen ( ) log K1 ( ) log K2 and ( ) log K3.3.3 Effect of solventThe variation of protonation constant or change in free energy withco-solvent content depends upon two factors, viz., electrostatic and nonelectrostatic. Born’s classical treatment holds good in accounting for theelectrostatic contribution to the free energy change33. According to thistreatment, the energy of electrostatic interaction or the logarithm of stepwise protanation constant (log K) should vary linearly as a function of thereciprocal of the dielectric constant (1/D) of the medium. Such linear variationof the protonation constants of dopa (Figure 5) in PG-water mixture showsthe dominance of electrostatic interactions. In the case of some mono- anddi- carboxylic acids and simple phenolic ligands, electrostatic (long-range,non-specific or universal) solute-solvent interactions are predominant in binarymixtures of water with methanol, ethanol, dioxan or acetone as co-solvent34.3.4 Distribution DiagramsTypical distribution plots produced by DISPLOT38 using protonationconstants from the best fit models are shown in Figure 2. A single representativeplot is shown for each system at a particular PG-water concentration. Thezwitterion of dopa, LH3, is present to an extent of 95% in the pH range 2.010.0. The distribution plots show the existence of LH4 , LH3, LH2- and LH2- inthe case of dopa and LH and L in the case of phen in different pH ranges. Thecorresponding protonation-deproonation equilibria are shown in Figure 6.Figure 6: Protonation-deprotonation equilibria of (A) dopa, and (B) phenThe present study is useful to understand (i) the role played by the activesite cavities in biological molecules, (ii) the type of complex formed by themetal ion and (iii) the bonding behavior of the protein residue with the metalion. The species refined and the relative concentrations under the presentexperimental conditions represent the possible forms of these amino acids inthe biological fluids.846

J. Chil. Chem. Soc., 56, Nº 4 (2011)4. CONCLUSIONSDopa has three dissociable protons and one amino group which associatewith proton. It exists as LH4 at low pH and gets deprotonated with theformation of LH3, LH-2 and LH2- successively with increase in pH. Phen formsLH22 at low pH and gets deprotonated with the formation of LH and L withincrease in pH. Secondary formation functions confirm the existence of 3 and2 protonation equilibria for dopa and phen, respectively. The linear increase oflog values of protonation constants of Dopa with decreasing dielectric constantof PG-water mixtures indicates the dominance of electrostatic forces in theprotonation-deprotonation equilibria. Phen exhibits non-linear trend indicatingthe dominance of non-electrostatic forces. The effect of systematic errors in theinfluential parameters shows that the errors in the concentrations of alkali andmineral acids will affect the protonation constants more than that of the M. Damodaran, R. Ramaswamy, Biochem. 31 (1937) 2149O. Horneykiewicz, Wien. Klin. Wschr. 75 (1963) 309J. Mena, J. Court, S. Fuenzalida, P. S. Papavasiliou, G. C. Cotzias, NewEng. J. Med. 282 (1970) 5L. Burdine, T.G. Gillette, H. J. Lin and T. Kodadek, J. Am. Chem. Soc.126 (2004) 11442R. B. Martin, J. Phys. Chem. 75 (1971) 2657R. K. Boggess and R. B. Martin, J. Am. Chem. Soc. 97 (1975) 3076A. Gergely, T. Kiss and G. Deak, Inorg. Chim. Acta 36 (1979) 113R. Jameson, J. Chem. Soc. Dalton Trans (1978) 43R. F. Jameson, G. Hunter and T. Kiss, J. Chem. Soc. Perkin II (1980) 1105D. J. Perkins, J. Biochem. 55 (1953) 649B. Grgas-Kuzner, V. Simeon, O. A. Weber, J. Inorg. Nucl. Chem. 36(1974) 2151M. L. Barr, K. Kustin and S. T. Liu, Inorg. Chem. 12 (1973) 1486A. Gergely and T. Kiss, Inorg. Chim. Acta 16 (1976) 51M. J. Fahsel and C. V. Banks, J. Am. Chem. Soc. 88 (1966) 87815. P. Paoletti, A. Dei and A. Vacca, J. Chem. Soc. (A) (1971) 265616. R. D. Alexander, A. W. L. Dudeney and R. J. Irving, J. Chem. Soc. FaradayTrans 1, 74 (1978) 107517. P. R. Mitchell, J. Chem. Soc. Dalton Trans (1980) 107918. P. G. Daniele, C. Rigano and S. Sammartano, Talanta 32 (1985) 7819. S. Capone, A. D. Robertis, C. D. Stefano and R. Scarcella, Talanta 32(1985) 67520. A. D. Robertis, C. Foti, A. Gianguzza and C. Rigano, J. Solution Chem.25 (1996) 59721. S. Bandyopadhyay, A. K. Mandal, and S. Aditya, J. Indian Chem. Soc.58 (1981) 46722. T. S. Lee, I. M. Kolthoff, and D. L. Leussing, J. Am. Chem. Soc. 70 (1948)234823. A. A. Schilt and W. E Dunbar, Tetrahedron 30 (1974) 40124. R. J. Sengwa, R. Chaudhary, S. C. Mehrotra, Mol. Phy. 21 (2001) 180525. H. Sigel, R. B. Martin, R. Tribolet, U. K. Haring, R. M. Balakrishnan, Eur.J. Biochem. 152 (1985) 18726. K. V. Santhee Devi, B. Rama Raju, G. N. Rao, Acta Chim. Slov. 57 (2010)39827. R. S. Rao, G. N. Rao, Computer Applications in Chemistry, HimalayaPublishing House, Mumbai, 2005, 30228. G. Gran, Analyst 77 (1952) 66129. G. Gran, Anal. Chim. Acta 206 (1988) 11130. N. Padmaja, M. S. Babu, G. N. Rao, R. S. Rao, K. V. Ramana, Polyhedron9 (1990) 249731. G. N. Rao, Ph.D. Thesis, Andhra University, Visakhapatnam, India, 198932. P. Gans, A. Sabatini and A. Vacca, Inorg. Chim. Acta 18 (1976) 23733. M. Born, Z. Phys. 1 (1920) 4534. M. P. Latha, V. M. Rao, T. S. Rao, G. N. Rao, Acta Chim. Slov. 54 (2007)16035. H. Schneider, Top. Curr. Chem. 68 (1976) 10336. M. H. Abraham, J.Liszi, J. Inorg. Nucl. Chem. 43 (1981) 14337. D. Feakins, D. O’ Neille, W. E. Woghonie, J. Chem. Soc. Faraday Trans,(1983) 228938. G. N. Rao, A. R. Babu, S. V. V. Satyanarayana, A. Satyanarayana, R. S.Rao and K. V. Ramana, Acta Cien. Indica, 15 (1989) 321.847

RAMA RAJU B, SANTHEE DEVI K V, PADMAJA N AND NAGESWARA RAO G* Department of Inorganic & Analytical Chemistry, Andhra University, Visakhapatnam-530003, India (Receiv

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