Bull. SOC. Chlm. Belg. vd. 102 / no 8 / 1993 0037-9648 / 92 / $2.00 + 0.00

0 1993 Comlt6 van Beheer van het Bulletin V.Z.W.

PROTON NMR RELAXATION STUDIES IN AQUEOUS SOLUTIONS OF SUGARS WITH ALKALI HALIDES

S. Sattiacoumar and V. Arulmozhi* N M R Research Group, Raman School of Physics, Pondicherry Central University, R.V. Nagar,

Pondicherly-605014 (India) *Present address : Laboratoire de RMN, Faculte de MBdecine, Universite de Rennes I , France

Aecehred : 19/09/1993 -Accepted : 28/09/1993

ABSTRACT

Proton Magnetic Resonance (PMR) relaxation time measurements were performed in aqueous solutions of maltose and galactose as a function of sugar and alkali halide concentration. The experimental data unambiguously show that the presence of alkali halides highly influences the PMR relaxation process. The nature of molecular interaction in this ternary system is explained on the basis of two state fast exchange model and structure ordering/disordering of alkali halides.

KEY WORDS

Nuclear magnetic resonance, Relaxation times, maltose, galactose, sodium, Potassium , hydrogen bonding.

1. INTRODUCTION

In the past few years Nuclear Magnetic Resonance Relaxation studies in aqueous sugar solutions (1-4) have been carried out to have a clear understanding on the nature of molecular interaction in these systems. An investigation in this nature is interesting from a biological and clinical point of view, as many sugars like glucose, fructose and sucrose are administered to recuperating patients in hospitals in the form of dilute aqueous solutions. Moreover, the role of water in sugar solutions is important from a scientific point of view for a better understanding of concepts like 'Water structure' or definition like 'hydrogen bonded' systems.

Recent sound velocity measurements (5-8) and proton magnetic resonance relaxation studies (4,9) in aqueous solutions of sugars containing alkali halides show molecular interactions leading to the formation of complexes in these systems and thereby altering the flow characteristics of these solutions.

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An important reason of interest in these ternary systems is their direct impact in clinical applications (5). In this framework, the study of a monosaccharide (galactose) and its disaccharide (maltose) aqueous solutions in the presence of alkali halides like sodium chloride and potassium chloride appears very interesting. As is well known in fact (lo), that sodium chloride is a water structure maker while potassium chloride is a water structure breaker. The object of the present study is essentially to throw some light on the nature of molecular interaction in these sugar solutions in the presence of an alkali halide at different concentrations. And to investigate the influence of the alkali ions on the sugar water structure.

Although several techniques (5,11,12) have been used to study the structure of these liquids, only few investigations have been reported by NMR relaxation measurements to obtain information about the interaction in these systems. As NMR relaxation time is very sensitive to molecular environment (13), we have tried to show that it represents a good tool to investigate a complex ternary system like sugar- water-alkali halide. Where in, the alkali halide influence the relaxation process of the system.

2 . EXPERIMENTAL

Proton spin-lattice relaxation time (TI) and spin-spin relaxation time (T2) were measured using a low resolution pulsed Bruker PC 120 NMR process analyser working at a RF frequency of 20 MHz and at a temperature of 37°C. The inversion recovery technique (180 -T- 90) is used for the measurement of Ti. Carr-Purcell- Meiboom-Gill (CPMG) sequences were used for the measurement of T2 (14). The decay of magnetisation in these solutions were found to be mono-exponential. The accuracy in the measurment of relaxation times are of the order of 5%. The solutions were prepared using sugars of AR/BDH quality and double distilled water so as to obtain solutions in the concentration range (21, 5%, 0.5, 1.0, 1.5, 2.0 M). The three component systems were prepared by adding NaCl and KCl in the range 0.1 to 0.6 molar (M) for high concentration sugar solutions and 1 to 4 M in low concentration sugar solutions.

3. RESULTS AND DISCUSSION

3.1 High concentration sugar solutions containing low concentrations of Alkali Halides

The observed values of spin-lattice relaxation time T i are plotted as a function of concentration of NaCl and KCl seperately for the solutions of (0.5-2.0 M) maltose and galactose are shown in figures 1 and 2. It can be seen from figure l(a) that, in 0.5 M maltose solution the addition of NaCl at first decreases the relaxation time but on further addition it increases to a maximum at 0.2 M of NaCl, then it gradually decreases with further increase of NaCl concentration. But the maximum is shifted to 0.1 M concentration of NaCl in 1M maltose solution. Excepting these significant points the relaxation time T1 gradually decreases with increase of NaCl concentration.

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The addition of KCl too brings about some changes in the PMR relaxation time at low concentrations of sugars. There is a maximum observed in the T1 value at 0.4 M concentration of KCI in 0.5 M aqueous maltose solutions which seems to be shifted to 0.1 M concentration of KCl in 1.0 M maltose solutions. In 1.5 and 2 M aqueous maltose solutions the PMR relaxation time generally decreases with increase in KCI concentration as can be seen in figure l(b). It can be seen from figure 2(a) that addition of NaCl to 0.5 M aqueous solution of galactose shows a minimum at 0.3 M NaCl concentration in the T1 value. The relaxation time generally decreases with increase in alkali halide concentration in 1.0, 1.5 and 2.0 M aqueous solutions of galactose.

The relaxation time T1 shows two maxima in 0.5 M galactose solution at 0.2 and 0.5 M concentration of KCI molecules figure 2(b). For higher concentration of aqueous galactose solutions, the addition of KCl generally results in an increase in the T1 value for small KCl concentration. It can further seen from figure 2(b) that in aqueous galactose solutions of 1.0, 1.5 and 2.0 M T1 generally decreases at high KCI concentration.

The above results can be explained on the basis of two state fast exchange model (15) and structrue ordering/disordering effect proposed by Samilov (10). This model assumes the sodium ions to be structure making in the sence that their effect on the water molecules in solutions is restriction of their overall motional freedom. This results in a decrease in the value of T1 in aqueous solutions containg NaCl. The potassium ions are termed as structure breaking implying that their effect in aqueous solutions is to increase the freedom of movement of water molecules which often results in an increase in TI value. Srinivasa Rao et a1 (4) have clearly established the use of two state fast exchange model in these systems. The addition of Na+ ions restricts the overall motional freedom of solute water molecules resulting in a decrease in the bulk water fraction which may be responsible for the decrease in T1 value. While the'presence of K+ ions, increase the mobility of water molecules and thereby increasing the bulk water fraction resulting in an increase in Ti value.

3.1.1 Aqueous maltose solutions containing NaCl

The maximum observed at 0.2 M concentration of NaCl in 0.5 M maltose solution is found to shift to a lower concentration in 1.0 M maltose solution (figure la). This result could be explained as follows. At 0.5 M maltose solution, the macromolecules of maltose takes up monomer water molecules and form a complex like structure. Small addition of alkali halides (NaCI) may disturb slightly the structure of the maltose+water complex and i t is possible that some water molecules are detached from the complex. T h s may be the reason for the observed maximum in T1 values. If the sugar concentration is increased the T1 maximum first shifts to lower concentration of NaCl and then completely disappears at higher sugar concentration. This is to be expected because at higher concentrations of sugars, the probability of complex formation between water and sugar molecules is large. In addition to the above, the structure making property of Na+ may also be the reason for the decrease in the value of Ti at higher sugar concentration.

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3.15 Aqueous maltose solutions containing KCl

From figure 2b, it can be seen that in 0.5 M aqueous maltose solution containing KCl molecules, the PMR spin-lattice relaxation time T1 generally increases with increase of KCl concentration and shows a maximum at 0.4 M concentration. This increase is due to the structure breaking effect of K+ ions. The decrease observed in the value of Ti when KCl exceeds 0.4 M may be expalined by postulating a weak hydration between K+ ions and water molecules which reduces the bulk water fraction. Here too, as in the case of NaCl, the maximum is shifted to lower concentration of KCl when the sugar concentration is increased. At higher relaxation process, the spin-lattice (TI) and spin-spin (T2) relaxation times were measured in maltose and galactose solutions. The measurements were repeated by adding NaCl and KCl in the concentration range 1 to 4 molar. It is worthwhde to note that the sugar solutions were of low concentration as compared to the earlier study (figure 1 and 2) while the alkali halide concentration has been increased ten fold. The variation of TI and T2 with sugar and alkali halide concentrations are more ionic by further addition of Na+ ions, the structure making effect influences the relaxation process in a way so as to build up a structure stronger than that existing in pure sugar solutions.

As can be seen in figures 3b and 4b the interaction of KCl with the water-sugar system is in precise demonstration of its structure disordering behaviour. But it seems to reverse at 3 molar concentration by a decrease in the relaxation value. In fact, by further addition of KCl there is generally an increase in the degree of motional freedom of the solute water molecules. Therefore at one stage, the free water availability becomes large enough to promote a dominating ionic hydration. This results in a decrease in the relaxation values (4).

It is very interesting to note here that the measurement of spin-spin relaxation time (T2) exhibits the same behaviour as spin-lattice relaxation time (TI) which was difficult to obtain in sugar solutions containing low concentration of alkali halides.

4. CONCLUSION

The present paper reports the results of PMR relaxation time measurements performed on aqueous sugar solution contains alkali halides at various solute concentrations. This technique is shown to be a good tool to investigate the nature of molecular interaction in such solutions. The behaviour of PMR spin-lattice relaxation times (TI) as a function of concentration of various sugars has been studied extensively in our earlier reports (4,15), wherein we have attributed the decrease in the relaxation time (TI) to the water structuring effects caused by sugar molecules. The types of hydration and proton exchange in these macromolecular solutions present a very interesting aspect which we plan to investigate in near future. But from our present results, we have unambiguously shown with solutions containing sugar and alkali halide, that the structure making or breaking effects of alkali halides either in a monosaccharide or in its disaccharide is dominant only at low sugar concentrations whereas at higher concentrations their effect is predominated by the stronger association of water and sugar molecules.

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In addition, we have shown that structure ordering/disordering of alkali halides in a three component system is substantially dependent on the alkali halide concentration. This represents, a very interesting feature in solutions containing KCl where at 3 molar concentration of KU, the disordering effect is large enough to induce the ionic hydration may be modifying the hydrogen bonded complex build up by the interaction of the water structure with the sugar molecules. The well established model of two state fast exchange of protons has proved its usefulness in elucidating some aspects of the complex molecular interaction occuring in these ternary systems.

ACKNOWLEDGEMENT

The authors wish to thank Prof A.Srinivasa Rao, Raman School of Physics, Pondicherry central University, Pondicheny, India for helpful discussions. This work was partially supported by the grant of Council of Scientific and Industrial Research, India.

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