Partial molar volumes of transfer of some disaccharides from water to aqueous guanidine hydrochloride solutions at K

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1 Indian Journal of Chemistry Vol. 4 1A, June 2002, pp Partial molar volumes of transfer of some disaccharides from water to aqueous guanidine hydrochloride solutions at K T S Banipal*, Damanjit Kaur, Gagandeep Singh, B S Larkt & P K Banipal t Department of Pharmaceutical Sciences, Guru Nanak Dev University, Amritsar , India Received 8 October 2001; revised I January 2002 Apparent molar volumes (<I>v) of some disaccharides [D(+)-maltose monohydrate, D(+)-cellobiose, D(+)-Iactose monohydrate and D(+)-trehalose dihydrate] have been determined in water and in aqueous guanidine hydrochloride solutions (0.5, 1.0, 4.0, 6.0 and 8.0 m) at K from density measurements using a vibrating-tube digital densitometer. Partial molar volumes at infinite dilution (\1'\ ) determined from <l>v values have been utilised to estimate partial molar volumes of transfer (11"2.,,) for various disaccharides from water to aqueous guanidine hydrochloride solutions. The 11"2." values have been found to be positive for all the disaccharides and increase with the increase in the concentration of the cosolute which suggest that overall structural order is enhanced in aqueous guanidine hydrochloride solutions. The 11"2." values have been rationalized in terms of solute-co-solute interactions using co-sphere overlap hydration model. Pair, triplet and quartet interaction coefficients have also been calculated using McMillan-Mayer approach and their sign and magnitude have also been discussed. The study of carbohydrates/saccharides has become a subject of increasing interest because of the multidimensional physical, biochemical, and industrially useful properties of these compounds l. 5. In addition to their importance to the food, pharmaceutical and chemical industries, the simple saccharides have received considerable attention for their ability to protect biological macromolecules 6. Sugars and polyols are well known stabilizing agents for the native state of proteins/enzymes 7. 8 because of their ability to enhance the structure of watel Various thermodynamic and spectroscopicl2.13a studies have shown that the hydration of saccharides depends upon the number of hydroxy groupsl3, the potential hydrogen-bonding sites and relative position of the next nearest-neighbour hydroxy groups within the carbohydrate molecule 1o. 14. The denaturation of certain proteins in the presence of denaturants like urea and guanidine hydrochloride is reduced in the presence of sugars or polyhydroxy alcohols I However, the mechanism is not clearly understood by which this renaturating effect on protein structure is induced either by direct binding of urea and guanidine hydrochloride with protein molecules/polyhydroxy compounds or through the alteration of water structure. Earlier we have reported l7 the partial molar heat capacities (C'p.2) and t Department of Chemistry, Guru Nanak Dev University, Amritsar , Indi a volumes (V<>2) of some saccharides in water at different temperatures and observed that the magnitude of C'p.2 and V<>2 increases sharply from mono- to di- to tri-saccharides with the increase in temperature. We have also reported l8 the positive partial molar heat capacities (C"p.2. If) and volumes (V<>2.lf) of transfer of some saccharides from water to aqueous urea solutions which increase with increase in the concentration of the cosolute (urea) suggesting that the overall structural order is enhanced in aqueous urea solutions. Higher C"p.2. V<>2.lf and pair interaction coefficient values for maltose than for cellobiose have suggested that folding is more important in case of maltose than for cellobiose and thus its interaction with urea will be stronger than with cellobiose. As guanidine hydrochloride is stronger denaturating agent than urea, therefore, the study of interactions between guanidine hydrochloride (Gu.HCI) and saccharides will throw further light on the hydration characteristics of the saccharides which will help in understanding the mechanism of protein stabilization. Thus, in continuation of our work on saccharides and polyhydroxy compounds, we report in this paper the apparent molar volumes ($.) of disaccharides [D( + )-maltose monohydrate, D( +) cellobiose, D( + )-Iactose monohydrate and D( +) trehalose dihydrate] in water and in aqueous GU.HCI solutions at K.

2 1132 INDIAN J CHEM, SEC A, JUNE 2002 Materials and Methods D( + )-Maltose monohydrate (M 1971, Sigma Chemical Co.), D( + )-lactose monohydrate (AR, SRL), D( + )-cellobiose and D( + )-trehalose dihydrate (LR, CDH) and guanidine hydrochloride (AR, Lancaster) of highest purity were dried over P 2 0 S in a vacuum desiccator and were used without further purification. All the solutions were prepared using doubly distilled deionised water having specific conductance less than 10-6 ohm-i cm- I which was degassed before use. The solutions were prepared by weight using a Mettler Balance having an accuracy of ± 0.01 mg. Densities of the solutions were measured by using vibrating-tube digital densitometer (Model DMA 60/602, Anton Paar, Austria). The details of its principle and working have been described elsewhere '9. The temperature of the water flowing around the densitometer cell was controlled within O.OIK using an efficient temperature bath (Heto BirkerodlDenmark). The calibration of the densitometer was checked by measuring the densities of aqueous sodium chloride solutions and an excellent agreement was found with the literature values Results and Discussion Densities of solutions of disaccharides in water and in aqueous GU.HCl (0.5, 1.0, 4.0, 6.0, and 8.0 m) solutions at K as a function of molality are given in Table 1. <Pv values of a solute in a given solvent was calculated from the following equation: <Pv=M/d- [(d-do)loooi mddol... (1) where M is molecular mass of the solute, m is molality of the solution, d and do, are the densities of solution and solvent respectively. The apparent molar volumes at infinite dilution (<p v=v'2) were determined by the least-square fitting of the following equation: <Pv=V'2+Svm... (2) where Sv is the slope 21. The V'2 and Sv values have been summarized in Table 2 along with the standard deviations. The literature values have also been included in Table 2. The good agreement between the experimental and literature values "'? has been observed in case of water. The partial molar volumes of transfer (V'2.lf) of disaccharides from water to aqueous GU.HCl solution at infinite dilution have been estimated as follows: V'2,lf = V'2 ( in aqueous Gu.HCl) - V'2 ( in water) The values for V'2,lf are summarised in Table 3 and these are positive for all disaccharides which increase with the increase in the concentration of cosolute. The plots of V'2.lf values vs molality (Gu.HCl) are Table 1-Densities (d) and apparent molar volumes (</lv) of some disaccharides in water and in aqueous guanidine hydrochloride (Gu.HCI) solutions at K m (mol kg-i) d (kg m 3) </l v x I 0 6 (m 3 morl) m (mol kg-i) d (kg m- 3 ) </l v X 10 6 (m 3 mor l ) D( + )-Maltose monohydrate 4.0 m aqueous GU.HCI solution Water m aqueous GU.HCI solution 6.0 m aqueous GU.HCI solution m aqueous GU.HCI solution 1.0 m aqueous GU.HCI solution (Contd)

3 BANIPAL et al.: PARTIAL MOLAR VOLUMES OF TRANSFER OF SOME DISACCHARIDES 1133 Table 1-Densities (d) and apparent molar volumes (<1>.) of some disaccharides in water and ', '\queous guanidine hydrochloride (Gu.HCI) solutions at K-Contd 111 (mol kg l ) d (kg m 3) <1>. x I0 6 (m (mol kg' d (kg m 3) <1>. x l0 6 (m 3 mor l ) I) mor l ) D( + )-Cellobiose 0.5 m aqueous GU.HCI solution Water m aqueous GU.HCI solution m aqueous GU.HCI solution m aqueous GU.HCI solution m aqueous GU.HCI solution m aqueous GU.HCI solution m aqueous GU.HCI solution m aqueous GU.HCI solution 6.0 m aqueous GU.HCI solution D( +)-Trehalose dihydrate 8.0 m aqueous GU.HCI solution Water D( + )-Lactose monohydrate 0.5 m aqueous Gu.HCI solution Water (Contd)

4 1134 INDIAN J CHEM, SEC A, JUNE 2002 Table 1-Densities (d) and apparent molar volumes (<I>v) of some disaccharides in water and in aqueous guanidine hydrochloride (Gu.HCI) solutions at K-(Contd) III (mol kg l ) d (kg m 3) <I> v X 10 6 (m 3 mor 111 (mol kg d (kg m 3) <I> v x l0 6 (m 3 I) I) mor l ) D( + ) Cellobiose 1.0 m aqueous GU.HCI solution 6.0 m aqueous GU.HCI solution m aqueous GU.HCI solution 8.0 m aqueous GU.HCI solution Table 2-Partial mol ar volumes at infinite dilution (V 2 0) of some di saccharides in water and in aqueous guanidine hydrochloride solutions at K Compound 11'2 X 106(m 3 mor l ) Water 0.5 m 1.0 m 4.0 m 6.0 m 8.0 m D( + )-Maltose monohydrate (1.84 ± 0.01)* (0.83 ± 0.0 I) (1.77 ± (0.99 ± 0.01) (2.74 ± 0.0 1) ( 1.50 ± 0.01 ) " 0.01) D( + )-Cellobiose (4.40 ± 0.01) (3.53 ± 0.02) (4.77 ± (I 1.17 ±0.01) (9.38 ± 0.0 I) (6.70 ± 0.01) h 0.0 1) D( + )-Lac tose monohydrate ( 1.04 ± 0.0 I) ( 1.24 ± 0.0 I) ( 1.56 ± ( 1.82 ± 0.0 1) ( 1.67 ± 0.0 1) ( 1.78 ± 0.0 1) " 0.02) D( +)-Trehalose dihydrate (4.37 ± 0.02) (6.63±0.0 1) (5.55 ± (6.16 ± 0.0 1) (8. 19 ± 0.01 ) (5. 15 ± 0.0 1) ' 0.01) ' Ref. 17, bref. 35, *Parenthesis contain Sv values along with standard deviati ons. Table 3-Partial molar volumes of transfer at infinite di lution (V2~b) of some disacehrides from water to aq ueous guanidi ne hydrochloride sol utions at K Compound 11'2 "x10 6 (m 3 mor l ) 0.5 m 1.0m 4.0 m 6.0 m 8.0 m D(+)-Maltose monohydrate D( + )-Celiobiose D(+)-Lactose monohydrate D( +)-Trehalose uihydrate

5 BANIPAL et al.: PARTIAL MOLAR VOLUMES OF TRANSFER OF SOME D1SACCHARIDES 1135 illustrated in Fig. 1. The V0 2. lr values of studied disaccharides increase sharply upto 1.0 m and increase is continuous with less slope upto = 4.0 m but these values tend to level off in all cases at higher concentration which indicates the level of saturation of the interactions between these solutes and GU.HCI. Similar trends have been observed in case of urea as reported earlier'8 and the V0 2. lr values tend to level off at about 3.0 m urea concentration in almost all the cases which suggests that the saturation of interactions takes place at lower concentration. Ahluwalia et al?2 have reported the negative enthalpies of transfer (/)"H lr ) and positive heat capacities of transfer (Cp'lr) values for glucose, sucrose, maltose and cellobiose from water to aqueous urea solutions. These values have been attributed to hydrogen-bonding interactions between sugar and urea molecules because both urea and sugars possess potential sites for hydrogen bonding. Banipal et al.' 8 from partial molar heat capacities and volumes of some saccharides in aqueous urea solutions have reported that the positive values of C'p,2.tr and V0 2. tr, indicate an increase in the structural order of the solvent system. This increase in the structural order has been attributed to a complex formation between sugar and urea molecules through hydrogen bonding. The positive V0 2. lr values may be explained by analysing the effect of solute and cosolute on the structure of water as well as interactions between them using different models According to Franks et al. 23, partial molar volume V0 2 of a non-electrolyte at infinite dilution is a combination of the intrinsic volume (V inl ) of the non-electrolyte and volume due to its interaction with solvent (V s ) which was further modified by Shahidi et al. 24 as shown below: V02 = V. w + V Void - Vshrinkage... (3) where V v.w is the van der Waals' volume, V. oid is the associated void 25 or empty volume and V shrinkage is volume of shrinkage. It has been assumed that V. w and V, oid have the same magnitude in water and in aqueous GU.HCl solutions. Therefore, the positive V\ lr values from water to aqueous GU.HCl solution can be attributed to the decrease in the volume of shrinkage because of stronger interactions between GU.HCl and hydroxyl groups (-OH) of saccharides. Furthermore, these interactions will reduce the structure breaking effect of GU.HCI on water. In other words, more water is released as bulk water in the presence of sugars. Since bulk water has higher volume contribution than structure-broken water, 7 '06.., E5 E4 '" ~_ 2 ~1 0 0 ~~ ~,~=, ~ m,rmlkg 8 10 Fig. I-Partial molar volume of transfer of some disaccharides from water to aqueous GU.HCI solution of different molal ities at K. [D( + )-maltose monohydratcs, +, D( + )-trehalose dihydrate 0, D(+)-lactose monohydrate 11, D(+)-cellobiose. 1. therefore this factor will also contribute to positi ve values of V0 2. lr observed in our work. The hi gher magnitude of V0 2,Ir observed for maltose than for cellobiose reflects the stronger interactions between maltose and GU.HCl than between cellobiose and GU.HCI. The values of V0 2,Ir can be further rationalised by cosphere overlap model developed by Gume/ 6. The properties of water molecules in the hydration cosphere depend on the nature of solute species According to this model when the solute molecules approach each other, their hydration cospheres overlap and some of the cosphere material is di splaced resulting in a change in the thermodynamic parameters GU.HCI exists in the ionic form 3 ' and thus overlap comes into play because of the following types of interactions occur between solute (saccharides) and cosolute (Gu.HCl) molecules. (l) Interactions between the ions of GU.HCl and hydrophilic -OH sites of saccharides. (2) Interactions between the ions of GU.HCl and hydrophobic parts/groups of saccharides. The first type of interactions contribute positively, whereas second type of interactions contribute negatively to V\lr values Therefore, presently obtained positive values of V0 2. lr for the studied disaccharides over the entire concentration range of GU.HCl indicate that the ionic-hydrophilic interactions are dominating over the ionic-hydrophobic interactions. Therefore the mutual overlap of the hydration cospheres of solute and cosolute molecules will lead to an increase in the magnitude of hydrogen bonding interactions between -NH groups of GU.HCI and -OH groups of saccharide molecules. Since Gu.HCI, in

6 1136 INDIAN J CHEM, SEC A, JUNE 2002 addition to having some of the structural features of urea, also possesses ionic character of an electrolyte like NaCl and thus transfer volumes of disaccharides in GU.HCl are intermediate between those in NaCl and ureal Positive y 02'lr values of disaccharides in the presence of urea and NaCl have been attributed to the dominance of hydrophilic-hydrophilic interactions and to the dominance of ionic-hydrophilic interactions respectively. Therefore, both hydrophilic-hydrophilic and ionic-hydrophilic type of interactions are operating between GU.HCl and studied disaccharides. Kozak 33 et al. have proposed a formalism based on the McMillan-Mayer theory of solutions which permits the formal separation of the effects due to interactions between pairs of solute molecules and those due to interactions between three or more solute molecules. This approach has further been discussed by Friedmann and Krishnan27 and Franks et at. 28 in order to include the solute-cosolute interactions in the solvation spheres. According to this treatment, a thermodynamic transfer function at infinite dilution V'2.lr can be expressed as : V'2.tr =2 VAB mb+3 VABB mb 2 +4 VABBB mb (4) where A stands for saccharide, B stands for GU.HCl and mb is the molality of cosolute, VAB, V ABB and VABBB are the pair, triplet, and quartet intermolecular interaction coefficients. The V'Vr data have been fitted into the above equation to obtained VAB, VABB and VAB13B interaction coefficients which are given in Table 4. The values of VAB. VABB and VABBB are positive, negative and positive respectively for all the studied disaccharides. Large positive VAB values indicate an increase in the volume as the hydrogen bonding which is due to the ionic-hydrophilic interactions, take place between disaccharides and cosolute molecules and this is in line with the view that ionichydrophilic interactions are dominating over the others. The relative weightage of the variou<; coefficients may be judged from their contribution to V'2.lr at various molalities of GU.HCl and accordingly Table 4-lnteraction coefficients of some disaccharides in aq ueous guanidi ne hydrochloride soluti ons calculated by using equation (4) at K Compound V A13 V AB B V ABB B D( + )-Maltose monohydrate D( + )-cellobiose D( +)-Lactose monohydrate D(+)-Trehalose dihydrate these contributions have been illustrated in Figs 2-5. The pair interaction coefficient VAB is positive and increases linearly throughout the concentration range of cosolute studied in all cases. The VABB interaction coefficients are negative and almost zero upto 1.0 m concentration and the negative magnitude increases sharply thereafter. VABBB interaction coefficients are positive throughout the concentration range of cosolute and increases very slowly upto = 4.0 m and increase is faster after this (magnitude is almost zero upto =-4.0 m in case of cellobiose). Similar trends of interaction coefficients for studied saccharides in aqueous urea solutions have been reported earlier l8. The VAB values are positive for studied saccharides, VABB values are negative and Y ABBB are positive for all saccharides except for maltose. It has been observed that upto 1.0 m urea, transfer parameters arise from pair solute-cosolute interactions in almost all the cases and the triplet and quartet type interactions at molality higher than 1.0 m contribute effectively. Similar behaviour of various interaction coefficients has been observed in case of GU.HCI. Presently, volumes of transfer arise mainly from pair solute-cosolute interactions upto 1.0 m in almost all cases. Triplet and quartet interaction coefficients contribute only above 1.0 m GU.HCI. Inspite of various experimental and theoreticai attempts being made to explore the correlation between the hydration characteristics of saccharides, their stereochemistry still remains unresolved. Galema et al have proposed a modified stereospecific hydration model in which extent of hydration is determined by the position OH(4) in conjunction with the rel ative position of OH(2) and best compatibility is found when a carbohydrate has an axial OH(4) and an axial OH(2) group. In another paper, Galema and Hoiland 9b on the 30 ""io 20 E M 10 E <D ' o X -10!o ON' -20 > m, rool kg- 1 Fig. 2-Contri bution of various interaction coefficients to partial molar volumes of transfer of D(+)-maltose monohydrate at various molalities of GU.HCl [V AI3., V ABB -, V ABBB t.].

7 BANIPAL et al.: PARTIAL MOLAR VOLUMES OF TRANSFER OF SOME DISACCHARIDES ' ō E E 5 "'0'" 0 ~F4... =:::::---=~=---,-- -,- -, ~ x , s... ~- -10 ~D E 10 'E... 0 ~ x o~ -20 > -30 m, rrol kg"1 Fig. 3-Contribution of various interaction coefficients to partial molar volumes of transfer of D( + )-cellobiose at various molalities of GU.HCI [V AB., V ABB 0, V ABBB t.]. 20 'ō E 10 M E x!=, > -20 Fig. 4-Contribution of various interaction coefficients to partial molar volumes of transfer of D( + )-Iactose monohydrate at various molalities of GU.HCI [V AB., V ABB 0, V ABBB t.]. basis of compressibility data have suggested that 0- arabinose among pentoses (D-ribose, D-xylose and 0- Iyxose) and D-galactose among the hexoses (0- mannose and D-glucose) are the least compatible with water structure. The disaccharides consisting of a glucose and a fructose units (sucrose, turanose and palatinose) are most compatible with water structure, consisting of two glucose subunits (maltose and cellobiose) are intermediately compatible with water and consisting of glucose and galactose subunits (melibiose, lactose and lactulose) are least compatible with three-dimensional hydrogen bonded structure of water. They have reported that 0(+ )-maltose monohydrate and D( + )-cellobiose have similar hydration characteristics due to same isentropic partial molar compressibility, although the type of linkage is different. Neal and Goring35 have reported that folding of 0 (+ )-maltose monohydrate is more than in 0(+) cellobiose due to hydrophobic interactions on the basis of expansibility data. Higher ("'p.2.lr and V0 2,Ir values for 0(+ )-maltose monohydrate than for 0(+) cellobiose for transfer from water to aqueous urea 10 Fig. 5-Contribution of various interaction coefficients to partial molar volumes of transfer of D(+)-trehalose dihydrate at various molalities of GU.HCI [V AB., V ABB 0, V ABBB t.]. solutions as reported earlier's indicate stronger interactions between maltose and urea than between cellobiose and urea which may also be due to more flexible a 1-4 linkage present in maltose than ~ 1-4 linkage present in cellobiose. In the light of above, the presently obtained higher values of V O 2,Ir for 0(+)- maltose monohydrate than in 0(+ )-cellobiose again contradict the observation made by Galema and Hoiland 9 and supports the view of Neal and Goring35 and Banipal et al.'s that for maltose folding is more important than in cellobiose and its interactions with GU.HCl will be stronger than those with cellobiose, which results in higher V\lr values. The higher pair interaction coefficient values [ (cm 3 mol") (mol kg" ).,] for maltose than for cellobiose [ (cm 3 mol") (mol kg" r'] also support the above view. D( + )-trehalose dihydrate has higher V\lr values than cellobiose although both having same subunits present in them (two glucose units). This again support the view that trehalose has stronger interactions with GU.HCI than cellobiose due to different type of linkages present between them. Similarly 0(+ )-Iactose monohydrate has higher V \lr values than 0(+ )-cellobiose although both having similar ~ 1-4 linkage present between them which supports the view'o that sugars (lactose) which are least compatible with water has more interactions with cosolute (Gu.HCI) and independent of the type of linkage between the two subunits. Thus, the present study shows that hydration behaviour of saccharides does not depend only on constituting subunits but also on the linkage between them. These observations show that hydration characteristics of saccharides are very much peculiar and extensive studies are required to rationalise these.

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