Thermodynamics of interaction between L-cystine and sodium/magnesium chloride in aqueous solution

Transcription

1 Indian Journal of Chemistry Vol. 41A. June pp Thermodynamics of interaction between L-cystine and sodium/magnesium chloride in aqueous solution Dacheng Li, Qingjiang Yu, Yong Wang & Dezhi Sun" Department of Chemistry. Liaocheng Teachers' University. 34 Wenhua Road. Liaocheng. Shandong P R China Received 3 May 200}; revised 27 January 2002 Interactions of L-cystine with sodium chloride and magnesium chloride have been studied by determining the solubility of cystine in aqueous salt solutions of different concentration at varying temperatures. The interaction thermodynamic parameters have been estimated with the solubility data. The results show that both NaCI and MgCI2 in water is beneficial to dissolution of L-cystine. Enthalpy or entropy of interaction between L-cystine and NaCI is negative while that between L cystine and MgCI2 is positive. These interesting results are discussed in the light of static electricity theory as well as hydration of ions. L-Cystine is one of the important amino acids for both human being and animals. It has been extensively used in food industryl.2, feed of livestock 3, as well as in study and manufacture 4 5 of pharmaceuticals. Recently, a study by Wan's group has shown that L cystine affects the fatal process of mobilization of iron from liver into blood 6, Hack et at. have reported that cystine level is very important in the treatment of malignancy and HIV infection 7. This amino acid always coexist with various salts in the body fluid of humans and mammals, hence an investigation of its interaction with these salts is of great significance. In the present work, we have studied interactions of L cystine with sodium chloride and magnesium chloride by determining the solubility of cystine in aqueous salt solutions of different concentrations at various temperatures. Some interesting results are obtained and discussed in the light of static electricity theory as well as hydration of ions. Materials and Methods L-cystine (Aldrich, purity, 99%), was purified by recrystallization from acid-base-water system twice and dried at 90 C under reduced pressure before being used. Sodium chloride and magnesium chloride were both pure grade analytical reagents supplied by Shanghai Chemical Reagent Company (Shanghai, China). The two salts were purified by recrystallization from doubly distilled water, dried at 400 C and stored in a desiccator. The water used in the solubility determination was distilled in the presence of basic potassium permanganate. A set of salt solutions was stored in sealed bottles with excess L-cystine and shaken within a thermostatic water bath at a given temperature (D until equilibrium was established. Then excess L cystine in each solution was removed by filtration at the equilibrium temperature. After that, 1.00 cm 3 of the equilibrium solution which was saturated with L cystine was removed into a cm 3 measuring flask, con~aining 1.00 cm 3 ninhydrin (2%)-stannous chloride (0.08%) aqueous developer solution 8 and 5.00cm 3 acetic acid -sodium acetate buffer solution (PH=8.87). Then the volume was made up to the mask with water at 25 C. After the solution was thoroughly mixed, its absorbance (A) at 570 nm was determined using an UV-365 spectrophotometer (Shimadzu, Japan) with pure water as reference. The concentration (C) of L-cystine in the flask was according to Eq. 1. A = 7.096x103C xlO-4... (1) The saturated concentration of L-cystine (S) was simply S=IOC

2 Li et al : INTERACTION BE1WEEN L-CYSTINE & SODIUMIMAGNESIUM CHLORIDE 1127 Table l--solubility of L- cystine in aqueous solutions of NaCI and MgCI2 at different temperatures Salt soln Solubility (x 10-4 mol kg l ) at temp. (K) (mol kg l ) NaCi ILl ll L L ll L ll L MgCl L L Results and Discussion Solubility of L-cystine in aqueous salt solution Salting in effect has been observed in both NaC! and MgCh solutions. Solubility data of L-cystine in solutions of different concentration of NaCI and MgCh are given in Table L Table 1 shows that MgCh enhances the solubility of L-cystine more effectively than NaCI at the same concentration and temperature. For example, when the concentration of NaC! or MgCh in the aqueous solution is about 0.53 mol kg ), the solubility of L cystine in NaCI solution is only about 60% of that in MgCIz solution. This fact can be partly explained by the static interaction between molecules of L-cystine and ions. L-cystine exist mainly in the electrically charged double dipolar form shown in (I). The positively charged NH3+ group and negatively charged -COO group respectively attract the anion (Cn and cations (Na+ or Mg 2 +). At the same molar concentration, ionic strength of MgCh is higher than that of NaC!. Therefore static electricity interaction of L cystine with dissolved MgCl 2 is stronger than that with NaC! in the solution. (I) Thermodynamics of interaction between L-cystine and salt According to Setschenowe' s experiential formula 9... (2)

3 1128 INDIAN J CHEM, SEC A, JUNE 2002 where f is activity coefficient of non-electrolyte such as L-cystine in salt aqueous solution, SO and S are solubility of the non-electrolyte in the absence and in the presence of salt in water respectively; Ks is salt effect constant which is independent of concentration of the salt, and me molality of the salt. The values of f calculated from solubility of L-cystine, are given in Table 2. Ks obtained with linear simulation are in Table 3 (the correlative coefficients are all greater than 0.998). However, curves of logarithm of activity Table 2-Negative logarithm of activity coefficient (-log/) of L-cystine in aqueous solution of NaCI and M~C12 at different temperatures Salt soln (mol kg l) -Iogfat temp. (K) NaCl MgCl Salt Table 3---Salt effect constant of L-cystine (-K,) in NaCIIMgCI2 aqueous solution at different temperatures and empirical constants -K. at temp.(k) NaCI MgCI Emperical constants of equation Ks=aJT+b+cT Salt aj (kg kmorl) IO s bl (kg mor l ) cl (kg k mor l ) R2 NaCI MgCI

4 Li el al : fnteractlon BETWEEN L-CYSTINE & SODIUMIMAGNESIUM CHLORIDE coefficient ( log[) of L-cystine versus square root of ionic strength (1"2) in the salt solutions at a certain temperature are not very similar (Fig. I); higher the temperature, longer the distance between the two curves. This phenomenon indicates that besides static electric interaction, there might be other factors affecting the solubility of L-cystine in strong electrolyte solutions. Thermodynamic function of interaction between non-electrolyte and salt in aqueous solution can be calculated with the following equations 10. II G E = RTKs/2 v... (3) S EN =-(dgen/dt) p H EN = G EN +TSEN... (4)... (5) where G EN, S EN and H EN are the interaction Gibbs 0.6 O "" bd O j O. 2 O. 1 0 x 4 0 O jj Fi g. I-Logarithm of ac ti vity coefficient of L-cystine (log!) versus square root of ionic strength( Jl ). [I. In NaCI solution at 288.2K 2. In NaCI solution at 308.2K; 3. In MgCI2 solution at 288.2K 4. x In MgCI2 solution at 308.2K] free energy, interaction entropy and interaction enthalpy respectively; v is the number of ions in the chemical formula of salt. For instance, values of v are 2 and 3 in NaCI and MgCh respectively, In order to estimate the above mentioned thermodynamic parameters (G EN, S EN and H EN), we have obtained an empirical equation correlating the salt effect constant Ks and temperature T through non-liner simulation with our own program: Ks=aJT+b+cT... (6) where a, band c are all empirical constants, the constants as well as the correlation coefficient are in Table 3. The thermodynamic parameters calculated from Eqs 3-6 are put into Table 4. The following interesting observations can be made from the data in Table 4. (1) Gibbs free energy of interaction between L-cystine and either NaCI or MgCh is negative. In other word, the presence of both NaCI and MgCl2 in water is beneficial to dissolving process of L-cystine. There may be two causes of the salting-in effect. The first one is the above mentioned static electric interaction of COO- with Na+ or Mg2+ ions and that of NH/ with Cr. The second one is that when a number of cations and anions respectively approach the COO- and - NH3 + ions of L-cystine molecules, the icebergl2 structure around the L-cystine molecule may be partly destroyed, and hence the hydrophobic effect of -CHr S-S-CH2- residue of L cystine might be weakened. (2) Enthalpy or entropy of interaction between L-cystine and NaCI is negative while that between L-cystine and MgCl2 is positive, which indicates that while some energy is released when L-cystine interacts with NaCI in the solution, energy is absorbed when L-cystine interacts with MgCh. The anion of the two salts is the same one (Cn and the difference between the two salts is the structure of sodium ion and magnesium ion. Temp (K ) Table 4--Thermodynamic parameters of interaction between L-cystine and NaCII MgCI2 GEN (1 'mor 2 'kg) SEN(J 'mor 2 'K 1 ) HEN(J ' mor 2. kg) NaCI MgCl 2 NaCI MgC12 NaCI MgCI

5 1130 INDIAN J CHEM, SEC A, JUNE 2002 Hydration of Mg2+ is stronger than that of Na+, since the electric charge of Mg2+ is twice of that of Na+ and the radius of the former is much shorter than that of the latter. According to Huang et at. 13, the first order hydration number of Mg2+ and Na+ is about 3 and 14, respectively. It is understandable that several water molecules attached to Mg2+ are freed when the cation approaches the L-cystine molecule, with absorbtion of some energy, while there is increase in the entropy of the system to a certain extent. (3) The thermodynamic parameters, H E and S EN are more or less independent of temperature. This is in agreement with the experimental results of other non-electrolyte-salt interaction systems 14. Furthermore, H EN - TS EN, is very small in both the systems at the experimental temperatures. This fact shows that the interaction between L-cystine and NaCI or MgCI2 is of the weak interaction, type, which is of great importance in life sciences 15. References I Chion T-K, Lan H-L & Shian C-Y, Zhollgguo Nongye Huaxue Huizhi, 37 (1987) 42 (in Chinese) 2 Luthra G & Sadana B, J food sci Technol, 32 (1995) Moita A, Marcos S & Donzele J L, Rev Soc Bras Zootec, 25 (1996) Wang Y & Tate Suresh S, FEBS Left 368 (1995) Nagano N, Ota, M & Ni shikawa K, FEBS Le ft, 458 (1999) Wan Q, Yang B-S & Kato N, J nutr Sci Vitmninol, 42 (1996) Hack V, Gross A, Kinscherf R, Bockstette M, Fiers W, Berkeg G & Droe W, FASEB J, 10 (1996) (a) Seracu D I, Rev Chim, 39 (1988) 54; (b) Li D-C, Wen L X & Liu D-J, Nat Prod R&D, 9 (1997) Long M, Ch em Rev, 51 (1952) Fiedman H L, J sol Chem, I ( 1972) Perron G & Desnogers J E, Call J Chem, 56 (1978) Desnogers J E & Pelletier G E. Call J Chem. 43 (1965) Huang Z Q, Introduction of theory about electrolyte solution, (Science Press, Beijing China), 1983, pp Li D & Xie W-H, Acta Chim Sin, 52 (1994) Lehn J-M, Science, 260 (1993) 1762.