Thermodynamic behaviour of adenosine in water-dmso mixtures in the presence of K' and Ca2' ions at 25, 30, 35, and 40 C

Transcription

1 Thermodynamic behaviour of adenosine in water-dmso mixtures in the presence of K' and Ca2' ions at 25, 30, 35, and 40 C ANIL K. PURI' Department of Chemistry, University of Allahabad, Allahabad , India Received April 6, 1984 ANIL K. PURI. Can. J. Chem. 63, 1180 (1985). Partial molar volume (&), partial molar compressibility (44, Jones-Dole viscosity B coefficient, and solute activation parameters of adenosine in water-dmso mixtures in the presence of Ca2' and K' ions have been calculated from ultrasonic, volumetric, and viscometric studies at 25, 30, 35, and 40 C (*O.OI C). The results are discussed in terms of the Jones-Dole viscosity B coefficients and the transition state parameters for viscous flow. ANIL K. PuRI. Can. J. Chem. 63, 1180 (1985). Utilisant les rtsultats d'ttudes ultrasoniques, volumttriques et viscosimctriques effectutes k 25, 30, 35 et 40 C (?0.0I0C), on a calculc les volumes molaires partiels (+!), les compressibilitcs molaires partielles (+:, les coefficients de viscositt B de Jones-Dole et les paramttres d'activation de solution de I'adCnosine dans des mclanges eau/dmso, en prcsence d'ions Ca2' et K'. On discute des rksultats en fonction des coefficients de viscostc B de Jones-Dole et des paramttres de 1'Ctat de transition de I'Ccoulement visqueux. [Traduit par le journal] Introduction Adenosine is one of the most important nucleosides in the study of molecular biology (1). Conformational properties of adenosine, the interaction of its different functional groups with solvent molecules in solution and temperature dependence of these interactions play an important role in the understanding of the thermodynamic behaviour of biochemical processes in living cells. The ultrasonic velocity in solution can be used to study the solute-solvent and solute-solute interactions, which are determined by the chemical structure of the solute and solvent molecules. Ultrasonic velocity measurements are mostly limited to obtaining the hydration numbers in aqueous (2-5) and non aqueous solution (6). Hemmes et al. (1) made a successful attempt to study the dependence of solute-solvent interactions on the chemical structure of nucleic acid derivatives in an aqueous solution. lnteraction studies of bivalent metal ions with uracil, thymine, and cytosine have been investigated potentiometrically (7-10). Many attempts have been made to study the solution properties of DMSO-water mixtures (1 1-14), since DMSO is particularly suitable for biophysico chemical processes. Excess thermodynamic properties of DMSO-water mixtures at 20, 30, 40, and 60 C have been discussed in terms of molecular interactions by Pandey and Tiwari (15). The interaction of DMSO with N-benzoyl amino acids has been recently shown by Fong and Grant (16) using the 'H nmr technique. Results show that the intermolecular H-bonding between DMSO and the amino-hydrogen atom increases the effective steric size of the amino hydrogen. Recently kinetic studies of ligand substitution have been carried out in the DMSO-water system by Coetzee and Karakatsanis (17). Relative viscosity of solution of sodium and potassium bromides and iodides in DMSO-water at 25, 35, and 45 C have been reported by Feakins and Lawrence (Hi-20). Results are discussed in terms of Jones-Dole viscosity B coefficients and transition state treatment. The present article deals with the results of ultrasonic, volumetric, and viscosity studies of adenosine in DMSO-water mixtures in the presence of K+ and Ca2+ ion at 25,30, 35, and 40 C (?O.Ol C). Results have been discussed in terms of par- tial molar volume (+:), partial molar compressibility ($4, Jones-Dole viscosity B coefficient, and transition state theory. Experimental Adenosine (supplied by Sigma chemicals, U.S.A.) was of extra pure "Grade-Reagent" quality. Purity was checked by tlc and paper chromatography according to well known methods (32). Dimethyl sulphoxide (DMSO) was distilled in a stream of dry nitrogen over sodium hydroxide at a pressure of 2.6 kpa using a 50 cm column packed with Finskihelices. A conductivity cell was attached to the still head and solvent collected when the electrolytic conductivity (k) of distillate had fallen to 4 x S m-i. The solvent was stored, and all manipulations on the solutions were made in a nitrogen filled dry box. KC1 was of AR grade while hydrated CaCI2.2H20 was dried at 110 C and stored in dessicator. A solution of adenosine (0.01 M) was prepared in DMSO solvent. Stock solutions of 0. I M KC1 and 0. I M CaC12 were prepared in double distilled water. Two sets of ten solutions each of adenosine-(water-dms0)- Ca2' and adenosine-(water-dms0)-k' designated as system A and system B were prepared in varying composition from 10-75% by weight of DMSO. The densities of solutions were determined with the help of a calibrated bicapillary pyknometer at 25, 30, 35, and 40 C (?O.Ol C), respectively. The estimated error in the density measurements was of the order of 20.05%. Ultrasonic velocity (u) in different solutions were measured using variable temperature, "ltrasonic interferometry with a quartz crystal having a freauencv.. of 2 MHz. The estimated error was?0.01%. viscosity measurements were carried out using a calibrated Ostwold viscometer at 25, 30, 35, 40 C, respectively, with maintained temperature of?0.0i0c. The time of flow for water was selected in the range of s, while for dilute solutions at least 30 s or more was used. The estimated errors in viscosity measurements were of the order of %. Results Density data for adenosine in the mixed solutions of electrolytes have been used for the calculation of apparent molar volume ($:) as a function of total molality of co-solute and water-dmso solvent composition. The corresponding equation in terms of densities is 1000(;iO-d) M +" = + z- 'Present address: 2125 McKesson Drive, Richmond, VA 23235, U.S.A. m, 2' d do

2 PURl 1181 TABLE I. (a) Apparent molar volume at infinite dilution (4;) for adenosine-water-dmso in presence of ca2' (system A); adenosine = 0.01 M, CaCI, = 0.1 M ("c) - 4; X lo-' ml mol-i when [DMSO] is I0 wt.% 20 wt.% 30 wt.% 40 wt.% 50 wt.% 55 wt.% 60 wt.% 65 wt.% 70 wt.% 75 wt.% (b) Apparent molar volume at infinite dilution (+:) for adenosine-water-dmso in presence of K' (system B); adenosine = 0.01 M, KC1 = 0.1 M ("c) - +; X lo-' ml mol-' when [DMSO] is I0 wt.% 20 wt.% 30 wt.% 40 wt.% 50 wt.% 55 wt.% 60 wt.% 65 wt.% 70 wt.% 75 wt.% TABLE 2. Experimental slope (S,) for adenosine-water-dmso in TABLE 4. Viscosity B coefficient for adenosine-dmso-water in the presence of Ca2' (system A) and K+ (system B); adenosine = 0.01 the presence of K+ and Ca2+ ions M, CaCl2 = 0. I M, and KC1 = 0.1 M B coefficient (mol-' cm-') S.. -. c"c) System A System B ("c) System A System B [31 P, = ;i;, TABLE 3. Apaprent molar compressibility at infinite dilution (4:) for adenosine-water-dmso in the presence of K+ and Ca2+ ion; adenosine = 0.01 M, CaC12 = 0.1 M, KC1 = 0.1 M -+: (crd mol-' bar-' x lo-') solute was obtained from the relation ("c) System A System B [4] +k = where do and d are the densities of the DMSO-water solvent and adenosine solution res~ectivelv. The value of do was determined from d = Zdji whkre y, &resents weight fraction, m, is the total molality of the co-solute in DMSO-water solvent and can be determined by m, = Zm, where m, is the molarity of ith component. The mean molecular weight of the two solutes (M) was obtained from M = ZMiyi, here Mi is the molecular weight of component i. Apparent molar volume at infinite dilution (+:) has been evaluated from the equation. [21 4" = +: + s v G where S, is the experimental slope of +, vs. 6, plot. Values of partial molar volume (+:) at different DMSO compositions and different temperatures are recorded in Table 1. S, values are given in Table 2 for systems A and B. From density and ultrasonic velocity the isentropic compressibility (P,) of solution has been calculated from the relationship where u is the ultrasonic velocity and d is the density of the medium. The isentropic aparent molar compressibility (+k) of m,f where p, and Po are the compressibilites of solution and sol- vent. Apparent molar compressibility at infinite dilution (+:) Graphically estimated values of +: are given in Table 3. The experimental viscosity data were analysed in terms of the Jones-Dole equation (21) below which accounts for both solute-solvent and solute-solute interactions. q0 [5] -= 1 + AG + Bm, T Here qo/q is the relative viscosity of solution. The constants A and B are evaluated from the intercept and slope of the (qo/ q - 1 ) / 6 vs. 6 plot. The coefficient A represents the contribution from interionic electrolytic forces (22) and the coefficient B measures the order or disorder introduced by ions into the solvent structure. This constant is specific and approximates the additive property of electrolytes at a given temperature (23) although no satisfactory theoretical treatment has yet been given. B coefficient values for systems A and B are given in Table 4. The viscosity and density data have been utilized for the calculation of activation parameter (24). The free energy of 92 has been extrapolated from the linear plot of +, vs 6.

3 1182 CAN. J. CHEM. VOL TABLE 5. Activation parameters for adenosine in DMSO-water mixtures in the presence of K' Weight % Ap* -AS* -AH* of solvent ("c) (kcal/mol) eu (kcal/mol) TABLE 6. Activation parameters for adenosine in DMSO-water mixtures in the presence of Ca2+ Weight % Ap* -AS+ -AH* solvent ("c) (kcal/mol) (eu) (kcal/mol) dissociates and n2 is the number of moles of solute per litre of solution. The number of moles nl of the solvent per litre of solution is given by activation for viscous flow (25, 33) is given by eq. [6], F] Ap* = RT1n hn where h is Planck constant and N is the Avogadro number, is the viscosity of solution, and V may be regarded as the volume of 1 mol of solution particles and is given by where v is the number of species into which a solute molecule where and are the molecular weights of co-solvent DMSO-water and co-solute adenosine and electrolytes, respectively. Furthermore, by measuring the B coefficient at different temperatures, enthalpies (AH*), and entropies (AS*) of activation can be obtained by using equations (24) [7] AS* = -d(ap*)/dt [8] AH* = Ap* + AS* Calculated values of Ap*, AH*, and AS* are recorded in Tables 5 and 6 for sets A and B. Discussion The results in Table 1 reveal that the values of & (partial molar volume) gradually decrease with increase in temperature

4 from 30 to 40 C for the system A and increase with temperature for system B for the whole range of solvent composition ( wt. %). At the same time the positive S, slopes for both the systems A and B gradually decrease with increasing temperature. It may therefore be concluded that $1' values correspond to a maximum for system B, thereby suggesting that solute-solvent interactions are stronger than solute-solute interactions in the temperature range C. The increase in values of partial molal volume 4: for system B with increase in temperature are due to the dehydration of adenosine. S, values are decreased with increase in temperature. It can be argued that solute-solute interactions can be affected by the influence of K' (system A) on the structure of DMSO as interionic attractions are stronger in DMSO (dipolar aprotic solvent) than in water because of the lower dielectric constant value of DMSO than water. The occurrence of maximum +: values and lower S, values in the case of system B is in agreement with the above inference. According to Kaulgud and Patil (26), negative +: values are associated with the "promotion" of solvent structure, i.e., addition of adenosine in the presence of an electrolyte in DMSO-water may lead to an incipient structure that enhances the water structure^. This result might be due to the accommodation of adenosine in the cavity created by the displacement of water molecule into the interstices. Apparent partial molar compressibilities (+:) (Table 3) increase with increasing temperature for system A for solvent compositions varying from wt. %. The same trend in +: values observed for system B except at 40 C, where the value of +: is significantly lower. It is evident from Table 3 that both systems A and B the values of (+:) are negative at all temperatures. It can be interpreted as due to the loss of structural compressibility of water on account of population increase of four bonded water molecules in the vicinity of adenosine molecules. Critical evaluation for all the available physicochemical studies of DMSO-water mixtures by Frank and Ives (27) led to the conclusion that there must be reinforcement of the water structure in the neighborhood of DMSO molecules in dilute solution. There is also a greater loss of structural compressibility of water employing a greater ordering effect by ions on solvent structure as in the case of systems A and B except at 40 C in system B. Viscosity B coefficients of the Jones-Dole equation are positive for both systems. The B values gradually decrease with increasing temperature. Positive B values indicate strong alignment of solvent molecules with ions, i.e., strong DMSO interaction with Kt or Ca" ions and with amino site of adenosine, which reveals the "structure-making" behaviour of solvent structure. The B values decrease with increasing temperature indicating that ion-solvent interactions are being influenced in the temperature range from 20 to 40 C. A number of factors are believed to contribute to the B values in solutions of Kt and Ca2+ ions in DMSO-water mixture. Dimethylsulfphoxide, a dipolar aprotic solvent (19), is pyramidal in shape with two C-atoms, the 0-atom, and S-atom at the vertices (28). The bonding characteristics of the sulphur atom have been explained by Burg (29). Both the sulphur and the oxygen atoms possess a lone pair of electrons and for sulphur the lone pair is tetrahedrally disposed to the methyl groups and oxygen atom. A strong electrostatic interaction between the positive charge on the cation and the lone pair on the oxygen of each solvent molecule is envisaged when a cation solvent complex is formed as shown by the following mechanism. SCHEME 1. Possible mechanism: addition of metal ions with adenosine Adenosine The cation-solvent complex of Kt and Ca2' with adenosine in the presence of DMSO-water co-solvent are represented by I and 11, respectively (Scheme 1). It is evident that DMSO interacts strongly with the cations and this is reflected by the values of B coefficent shown in Table 4. The analysis of solute-activation parameters reveals (Tables 5 and 6) that AS* and AH* are negative for systems A and B and increase gradually with increasing composition of DMSO-water from wt.%. This fact indicates that in the solvent mixture the average transition state is associated with bond making and an increase in the order. The system B on the other hand with an increase in the percentage of DMSO from 55 to 75 wt. % in the mixture, the values of AS* and AH* are comparatively lower. In the latter case it can be assumed that the transition state for viscous flow is accompanied by the breaking and distortion of intermolecular bonds. Moreover, the smooth progression of the values of AS* and AH* found in system A and B, Tables 5 and 6 confirm the absence of structural enhancement of water by DMSO. In fact, when the water structure is increased by the addition of co-solvent, maxima or minima in the values of activation parameters of viscous flow were obtained for electrolytes in water-ethanol (30) and water-acetone (31) systems. It is evident from Table 5 that free energy of activation (Ak*) is positive for all proportions of solvent composition and at all temperatures studied in the present investigation. The results show that adenosine is higher in free energy state, i.e. formation of the transition state is less favourable in

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