J . Phys. Chem. 1991,95, 5321-5326 the double-layer capacitance of the R/Me2PEG interface seems 'normal", details of its structure are unknown. There may be special steric effects arising from characteristics of macromolecular phase boundaries, and the dynamics of polymer chain segmental motions at the interface are also not necessarily the same as those of the polymer bulk. Lastly, the present results contain no correction of k, for double-layer 'C#J~= e f f e c t ~ . ~ J ~ , ~ *
5321
Acknowledgment. We thank Dr. M. Rhodes for preparation of the ac voltammetry ASYST program and Dr. Louis Coury of Duke University for insightful discussions. The research was supported in part by grants from the National Science Foundation and the Department of Energy (DE-FG05-87ER13675). (41) Geiger, W. E.; Smith, D.E. J. Electrwnal. Chem. 1974, 50, 31.
Stereochemical Aspects of Hydration of Carbohydrates In Aqueous Solutions. 3.' Density and Ultrasound Measurements Saskia A. Galema' Organic Chemistry Department, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands
and Harald Hailand* Physical Chemistry Department, University of Bergen, Allegaten 41, N5007 Bergen, Norway (Received: December 17, 1990)
Density and ultrasound measurements have been performed in aqueous solutions of pentoses, hexoses, methylpyranosides, and disaccharides as a function of molality of carbohydrate ( 0 . 3 mol kg-'1. Partial molar volumes, partial molar isentropic compressibilities, and hydration numbers have been calculated. The data reveal that the compatibility of a carbohydrate into the three-dimensional hydrogen-bonded structure of water is governed by the stereochemistry of the carbohydrate. The carbohydrates can be divided into three groups with different fit into water, dependent on the relative position of OH(4) in conjunction with OH(2). This is discussed in terms of the fit of the 0 2 - 0 4 distances of the carbohydratemolecule with the 0-0 distances of water. The smaller hydration numbers are found for the carbohydrates which fit the water structure best. The results agree with previously obtained kinetic data.
Introduction In aqueous solutions, a carbohydrateis present in several forms? This complicates the study of the stereochemical aspects of hydration in aqueous solutions. For instance, crystallographicdata or data for carbohydrates in other solvent mixtures than water cannot be used due to the special characteristics of water." The design of the experiments has to be such that one avoids measuring effects due to (complex) mutarotation, which means that the equilibrium composition of the solution studied should not change too much during the measurements. This excludes measurements in large concentration ranges? temperature-dependent studies7 (except when the anomeric p i t i o n is blocked for mutarotation), and measurements of thermodynamic data which refer to the solid state as a reference state.* The hydration of carbohydrates in relation to their stereochemistry has been a subject of study over a long period of time! ~
(1) Part 1: Galema, S.A,; Engbcrts, J. B. F. N.; Blandamer, M.J. J. Am. Chem. Soc. 1990, 112,9665; for part 2 see ref 25. (2) Angyal, S.J. Angew. Chem., In?. Ed. Engl. 1%9,8, 157. ( 3 ) Franks, F.; Lillford, P. J.; Robinson, G. J. Cfiem. Soc., Farday Trans. I 1989.85, 2417. (4) Proly, J. D.; Lemieux, R. U. Can. J. Chem. 1987.65, 213. (5) Suggett, A. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, Chapter 6. (6) Franks, F. Pure Appl. Chem. 1987, 59, 1189. The concentration of a carbohydrate in an aqueous solution also affects the equilibrium. (7) Sec ref 3: the temperature dependence of the equilibrium depends on rimpk or complex mutarotation. (8) Partial molal heat capacities, CO 2, are often obtained by combining ACO, with AC,', the heat capacity of t t e crystalline phase; for an example ace ref 19. However, t b values can also k measured directly, see: Kawaizumi, F.; Nishio, N.; Nomura, H.; Miyahara, Y . 1. Chrm. Thermodyn. 1981, 331. 889.
0022-3654/91/2095-5321$02.50/0
due to their importance in life Concepts such as hydration numbers,lCI7 the anomeric effect,'* and the ratio of axial versus equatorial hydroxy g r ~ u p s ' have ~ . ~ been used to rationalize the hydration characteristics as well as the hydrophobic index,2l the hydrophilic volume of a carbohydrate,= and the extent of compatibility with water structure dependent on the position of the next-nearest-neighbor hydroxy groups within a carbohydrate molecule.aa However the studies have not let to a comprehensive theory. In an earlier study' we investigated the hydration of carbohydrates at very low concentrations by measuring the kinetic medium effect on a water-catalyzed hydrolysis reaction at constant (9) Franks. F. Pure Appl. Chem. 1987,59, 1189 and references therein.
(IO) Blanshard, J. M. V., Mitchell, J. R., Eds. Polysaccharides in Foods:
Butterworths: London, 1979. (11) Sharon, N.; Lh, H.Chem Eng. News 1981,59,21 (issue 13). (12) Lemieux, R. U. Chem. Soc. Rm.1989,18,347. (13) Quiocho, F. A. Annu. Rev. Biochem. 1986,55, 287. (14) Stokes, R. H.;Robinson, R. A. J. Phys. Chem. 1966, 70, 16. (15) Tait, M.I.; Suggett, A.; Franks, F.; Abblett, S.;Quikenden, P. A. J . Solution Chem. 1972, 1, 131. (16) Suggett, A.; Abblett, S.;Lillford. P.J. J . Solution Chem. 1976, 5. 17. (17) Ucdaira, H.;Ucdaira, H.J. Solution Chem. 1985, 14, 7. (18) Kabayama, M. A.; Patterson, D.; Piche, L. Can. J. Chem. 1958,36, 557. (19) Franks, F. Cryobio/ogy 1983, 20, 335. (20) Suggett, A. 1. Solution Chem. 1976, 5, 33. (21) Miyajima, K.;Machida, K.; Nagagaki, M. Bull. Chem. Soc. Jpn. 1985,58, 2595. (22) Walkinsaw, M. D.1. Chem. Soc., Perkin Trans. 2 1987. 1903. (23) Danford, M.D.J. Am. Chem. Soc. 1%2,84. 3965. (24) Warner, D.T. Nature 1962, 196. 1055.
0 1991 American Chemical Society
5322 The Journal of Physical Chemistry, Vol. 95, No. 13. 1991
Galema and Hailand
TABLE I: P 8 d d Mohr Voluws, Imtroplc Partial Molar CompresibiUties, and Hydration Numbers io Aqueous sdutioae at 298 K:
D-ghCOSe Dgalactose D-mannose D-talOSe D-fr UCtOSe L-sorbose D-xylose D-arabinose D-lyXOSe D-riboSe
1e2e3e4e6e le2e3e4a6e I a2a3e4e6e la2a3e4a6e le2a3e4e5a le2a3e4e5e 1 e2e3e4e la2e3e4a 1 a2a3e4e 1e2e3a4e
11 1 .O (0.4) 112.5 (0.1) 110.8 (0.3) 110.7 (0.5) 95.5 (0.2) 93.5 (0.2) 94.3 (0.1) 95.3 (0.1)
11 1 .7' (0.3) 110.2O (0.3) 1 1 1.3" (0.3)
-20.4 (0.4)
1 10.4d (0.4) 110.6d (0.4) 95.4' (0.3) 93.2" (0.3)
-1 1.9 (0.3) -21.7 (0.4) -2 1.8 (0.4) -13.1 (0.2) -19.2 (0.1) -13.1 (0.1) -1 2.4 (0.2)
95.2' (0.3)
-17.6' (0.3) -20.8" (0.5) -16.P (0.5) -21.4' (0.01) -12.9' (0.5) -19.3' (0.5) -12.5' (0.2)
8.4 8.7 8.1 7.7 8.8 9.0 6.8 7.6 6.8 6.8
'Reference 27. bConformation of dominant conformer in solution, a = axial; e 5: equatorial hydroxy group. 'Reference 50. "Reference 51. cCalculated with the method described in refs 34-36. /Errors in values are given in parentheses.
temperature. It was found that the hydration of a carbohydrate is mostly dependent on the relative position of the OH(4) in conjunction with its next nearest neighbor, the OH(2). It was concluded that carbohydrates can be divided into three groups with decreasing fit into the three-dimensional hydrogen-bonded structure of water:' (1) OH(4) is axial and OH(2) is axial; (2) OH(4) is equatorial and OH(2) is either axial or equatorial; (3) OH(4) is axial and OH(2) is equatorial. Additional kinetic experiments with hexoses, methylpyranosides, and disaccharides confirmed this trend.25 In an endeavor to further substantiate the results obtained through the kinetic experiments we measured partial molar volumes and isentropic partial molar compressibilities. Earlier studies have shown that the equilibrium composition of a carbohydrate in water is not strongly affected by pressure.26 Previously obtained experimental data on compres~ibilities~' support the conclusions that were drawn from the kinetic measurements. It was therefore incumbent on us'to extend the range of relevant volumetric and compressibility data in order to increase our knowledge of the stereochemical aspects of hydration. By measuring the limiting values one avoids problems with the definition of the standard s t a t e ~ . ~ * , ~ ~
Experimental Section Miteriais. All carbohydrates were of the quality described earlier.25 All solutions were made up by weight, and where necessary, corrections were made for the water content of the carbohydrate. The water used was doubly distilled. All measurements were carried out for at least six different concentrations of carbohydrate. The same solutions were used for density and compressibility measurements. Density Measurements. Density measurements were carried out at 25.00 f 0.05 OC by using an A Paar digital densitometer Model DMA 02C. The error in these density measurements was estimated to be f3 X lod g ~ m - ~ . Compressibility Measurements. Isentropic coefficients of compressibility were determined by the "sing-around" principle30 which involves the passing of repeated pulses of ultrasonic radiation through the solution and measurement of the frequency of this radiation. The frequencies were measured by a Hewlett-Packard 5326 A time counter, averaging over periods of 10 s. The ultrasonic cell was a gold-plated brass cylinder, which had a length of approximately 4 cm. It was calibrated with water as standard, using the data of Del Grosso and Mader." The cell was ther(25) Galema, S. A.; Blandamer, M. J.; Engberts, J. B. F. N. Submitted for publication. (26) O'Connor, C. J.; Odell, A. L.; Bailey, A. A. T. A u t . J. Chem. 1983, 36, 79. (27) Heiland, H.; Holvik, H. J. Solution Chem. 1978, 7 , 587. (28) Douheret, G.; Moreau, C.; Viallard, A. Fluid Phose Equilib. 1985,
22, 277, 289. (29) CesHro, A. In ThermodynamicData for Biochemistry and Biotechnology; Him, H.-J., Ed.; Springer: Berlin, 1986;Chapter 6. (30) Garnsey, R.; Boc, R. J.; Mahoney, R.; Litovitz, T. A. J. Chem. Phys. 1969. 50, 522.
mostated in a water bath to better than 10.05 OC. The error in the speed of sound was estimated to be fO.10 m s-l.
Results Partial molar volumes were obtained by measuring densities of carbohydratesolutions at different molalities of carbohydrate.'2 The apparent partial molar volume can be obtained by using eq 1, where d and doare the densities of a carbohydrate solution and V, = 1000(do- d)/(mddo)
+ M2/d
(1)
pure water, respectively, m is the molality of carbohydrate, and M 2 is the molar mass of the carbohydrate. The partial molar volume is obtained by plotting the apparent partial molar volume versus molality of carbohydrate and applying linear regression. At infinite dilution the partial molar volume equals the apparent partial molar volume. Isentropic compressibility coefficient^^^ were measured by monitoring the speed of sound through a solution in an adiabatic and reversible way, which ensures that the compressibility is isentropic. From this the isentropic compressibility coefficient 8, can be obtained:
Herein u is the speed of sound and d is the density of the solution. The 8, values can be used to calculate the apparent partial molar compressibility via the difference method
(3) where 8, and &, are the isentropic coefficients of compressibility of the solution and water, respectively, m is the molality of carbohydrate, d is the density of the solution, and Vt is the apparent molar volume at that concentration. By definition the limiting value of the isentropic apparent molar compressibilityK",(I#J)is equal to the isentropic partial molar compressibility: K2(,) = &(a)
+ m(dK+/dm)
(4)
Hydration numbers can be obtained from the ultrasound ex riments and are calculated by a method described earlier E 3 6
in which nh is the hydration number and n, and n, are the mole fractions of water and carbohydrate, respectively. The hydration numbers given in the tables are the limiting values obtained at (31) Del Grosso, V. A.; Mader. C. W.J. Acourt. Soc. Am. 1972,5, 1442. (32) Sce also Heiland, H. J. Solution Chem. 1980, 9, 857.
(33) Sec also: Hailand, H. In ThermodynamicDatafor Bimhemistry and Biotechnology;Hinz, H.-J., Ed.; Springer: Berlin, 1986; Chapter 2. (34) Shiio, H. J. Am. Chem. Soc. 1958,80, 70. (35) Moulik, S. P.;Gupta, S.Can. J . Chem. 1989, 67, 356. (36) Bockris, J. 0. M.; Reddy, A. K.N. Modern Electrochemistry; Plenum: New York, 1977; Vol. I, p 127.
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5323
Hydration of Carbohydrates in Aqueous Solutions
TABLE II: Partial Molar Volumes, Isentropic Partial Molar Compressibilities, and Hydration Numbers in Aqueous Solutions at 298 K Methylpynnosidesr
V*O, cm3mol-'
Vt(lit.), cm3mol-'
Me-a-glucopyranoside
132.6 (0.1)
Me-8-glucopyranoside
134.1 (0.2)
3-O-methylglucose Me-a-galactopyranoside Me-8-galactopyranoside Me-a-mannopyranoside Me-j3-xylopyranoside Me-8-arabinopyranoside
133.9 (0.1) 131.0 (0.1) 131.7 (0.1) 132.5 (0.1) 117.5 (0.1) 113.8 (0.1)
132.9b (0.3) 132.6d 133.6b (0.5) 133Sd 134.0" (0.5) 1 32.6d (0.4) 1 32.9d (0.2) 132.9d (0.4) 117.2'
cm3 mol-' bar-'
1o'KP(s), cm3 mol-' bar-'
nb'
-14.7 (0.1)
-13.e (3.0)
9.2
-11.1 (0.4)
-5.96 (3.0)
8.8
Io'K?(s),
-5.6 -17.0 -17.1 -12.3 -7.1 -15.3
8.1 9.4 9.4 8.9 7.4 8.2
(0.2) (0.2) (0.1) (0.2) (0.3) (0.6)
'Calculated with the method described in refs 34-36. 6Reference49 error estimated from the plot. 'Reference 53. dReference 51. 'Errors in values are given in parentheses. TABLE III: Partial Molar Volumes, Isentropic Partial Molar Compressibilities, and Hydration Numbers in Aqueous Solutions at 298 K Disaccharides*
comDsn6
VP, cm3mol-'
v#(lit.), cm3mol-'
212.0 (1.0)
210.1' (1.0) 208.8d 213.6d (1.0) 211.9, 206.9d (0.5)
maltose
gl-gl( 1 - 4 4
cellobiose
gl-gl(l-48)
trehalose gentiobiose sucrose
gl-gl( 1 1aa) gl-gl( 1-6a) gl-fr(1 -5a)
208.0 (0.1) 209.3 (1 .O)
turanose' palatinose lactose
gl-fr( 1 - 3 4 gl-fr(1 -6a) gI-ga( 1-48)
21 1.4 (0.2) 219.1 (0.2) 208.8 (0.2)
melibiose lactulose
gl-ga(1-6a) fr-ga(1-48)
210.7 (0.2) 208.5 (0.1)
-
104~t(s),
cm3 mol-' bar-'
21 1.6' (0.3) 210.2d 21 1.38 (0.2)
the
-23.7' (1 .O)
14.5
-25.0 (0.1)
14.8
-30.2 (0.3) -32.1 (0.4) -17.8' (0.5)
15.3 15.6 13.9
-30.4' (1 .O)
14.2 14.1 15.3
-20.2 (0.2) -16.1 (0.2) -31.1 (0.2)
209.1' (1.0) 207.6d 204.0" (0.7)
lo'KP(s)(lit.), cm3 mol-' bar-'
-31.2 (1.0) -29.5 (0.2)
15.5 15.2
'Reference 27. bCompsition of disaccharides: gl = glucose; ga = galactose; fr = fructose; between parentheses the type of linkage is given. 'Fructose moiety is in the pyranose form. dReference 51. 'Calculated with the method described in refs 34-36. /Reference 52. #Reference 53. Errors in values are given in parentheses. K2(s)
190
lO'.cm'
mol-' bar-'
'
-
-- 5
I
170
-
150
-
130
-
110
.
90
.
-- I150 I
-
--
-
I------
-
I
I
--
-
--
---35
'
0.0
0.1
0.2
0.3
0.4
m, mol kg"
infinite dilution. Equation 5 assumes that the compressibility of the hydration layer is zero. This affects the value of the hydration number. If the compressibility of the hydration layer is not zero, the hydration numbers will all be higher (maximal lo%), but leaving the trend in the numbers the same. We report partial molar volumes, isentropic partial molar compressibilities, and hydration numbers for a series of hexoses and pentoses (Table I), methylpyranosides (Table II), and disaccharides (Table 111). When available, literature data are given as well. There is generally good agreement with literature data, except for the isentropic partial molar compressibilities for the
Figure 2. Isentropic apparent molar compressibilitiesas a function of concentration: (t lyxose; ) (-0-) fructose; (-0-) trehalose; (-A-) 3-O-methylglucose; (-A-) cellobiose.
methylglucopyranosides. Figures 1 and 2 show repreaentative plots of the partial molar volumes and partial molar compressibilities, respectively. Discussion Partial Molar Volumes. The partial molar volume is the sum of the intrinsic volume of the solute and the volume contribution due to solute-solvent interactions
V, = Vim. + V s o ~ u t d v c n t
(6)
Galema and Hailand
5324 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 O
b
K2 I 10
Pentoses
Methylpyranosider
Hexoses
cm’ mol-‘ b a i l
-
Pentoses
1 -
Oisac c harides
Hexoses
-
0-3 0-Me-glucose .-Me -n-xylopy
8 -
- 12 - 16
.,Ribose \
*
- 20
-
-21
-
.-Me -tY-glucoPY 0-Me-u-mannopy
o-Talose
O-XY lose .
Lyxose .-Arabinose
.-Me -0-arabinopy
*-Mannose .-Glucose
-
.;Me u -galac topy M e 4 -gahctopy
ocGalactose *;Fructose Sorbose
-28 r
1 I
-32
.-Me -u-glucopy *-Palatinose .-Sucrose 0-turanose .-Maltose .-Cellobiose
0-Lactulose .-Trehalose .-Lactose *-Gentiobiose .-Melibiose
Figure 3. Partial molar compressibilities for pentoses, hexoses, methylaldopyranosides, and disaccharides: a schematic representation.
and is therefore informative for the character of solutesolvent interactions. Overall the carbohydrates have small partial molar volumes due to extensive solute-solvent interactions.” On average we observe an increment for an additional -CHOH of +16.6 cm3 mol-’, an increment of +21.2 cm3 mol-’ if a hydroxy group is replaced by a methoxy group, whereas disaccharides have slightly less than twice the partial molar volume of the monosaccharides. Information on the stereochemical aspects of hydration is quite sparse because the largest part of the partial molar volume is caused by the intrinsic volume of the carbohydrate. There are some significant differences in partial molar volumes within a set of isomeric carbohydrates (for instance, for the hexoses, talose has a significantly different V20from the other hexoses) but overall the differences are fairly small. isentropic Partial Mdar Compredbilitks.A pressure derivative of c” will directly reflect carbohydrate-water interactions, if it is assumed that the carbohydrate molecules are incompressible. Compressibilities can be measured at constant temperature or at constant entropy. In order to determine the isothermal apparent molar compressibilityone needs to know the expansibilities and heat capacities of the solution to calculate it from the isentropic compressibility. The isentropic and isothermal compressibilities are different numerically; however, when one is interested in a trend the use of either of them is r e d ~ n d a n t . ~ We u ~ have chosen to measure isentropic compressibilities. The limiting values of the compressibilitiesdirectly give insight into the compressibility of the hydration layer compared to that for pure water. Pure water has a molar compressibility of +8.17 X lo-‘ cm3 mol-’ bar-’. In the case of hydration of an apolar, hydrophobic solute, the water molecules in the hydration layer will form stronger hydrogen bonds to each other (hydrophobic hydration) and therefore the hydration layer will be less compressible than pure water. Consequently, a slightly negative limiting compressibility per methylene group is found. When ions are introduced into water they usually break the water structure by electrostriction. The water around ions is dense and less compressible than bulk water, leading to typically large negative partial molar compressibilities ((-30 to -50) X lo-‘ cm3 mol-’ bar-’). The partial molar compressibilities for the carbohydrates have intermediate values. This suggests that the hydrogen-bonded structure of water is only slightly disturbed by the presence of the carbohydrate. It seems that the more negative the partial ~
~~~
(37) Barone, 0.;Castronuovo, 0.;hucas. D.; Ella, V.; Mattia, C. A. J . Phys. Chem. 1983,87, 1931. (38) See also: Hailand, H. In ref 33. Chapter 4.
molar compressibility is, the more the hydration layer will be disturbed compared to pure water. We observe significant differences in the partial molar compressibilities of the carbohydrates (see Figure 3 for a schematic representation of the results). Starting with the hexoses, we observe that the compatibility of the hexoses into the three-dimensional hydrogen-bonded structure of water definitely depends on the stereochemistry of the hexose. The hydration layer of D-talose is least disturbed. The greatest disturbance is found for @galactose, whereas @mannose and @glucose show intermediate behavior. It turns out that hexoses can be divided into three groups that fit into the three-dimensional hydrogen-bonded network of water, depending on their stereochemistry. D-talose, with axial OH(2) and OH(4), fits into the water structure best while the hydration layer is least disturbed compared to bulk water. Hexoses with an equatorial OH(4) and an equatorial or axial OH(2) show a moderately disturbed hydration layer. Hexoses with an axial OH(4) and an equatorial OH(2) are the least compatible with the three-dimensional hydrogen-bonded structure of water. The ketohexoses D-fructose and L-sorbose exhibit the same partial molar compressibility,although their stereochemistry is different and they overall disturb the hydration layer more than, for instance, D-glucose. This is probably due to the fact that the exocyclic -CHOH moiety is situated at the anomeric center. Pentoses show the same trend as the hexoses: those pentoses with an equatorial OH(4) and either an equatorialor axial OH(2) only have a moderately disturbed hydration layer (D-xylose, D lyxose, and @ribose). Only @arabinose with an axial OH(4) and an equatorial OH(2) exhibits a larger disturbing effect on the three-dimensional hydrogen-bonded structure of water. The most interesting results are obtained for the methylhexopyranosides: if one studies the methoxy derivatives of the “parent hexoses”, we observe that the same trend occurs. Methylpyranosides with an equatorial OH(4) and either an equatorial or axial OH(2) have a moderately disturbed hydration layer (methy1-8-glucopyranoside.methyl-a-mannopyranoside), compared to methyl-8-galactopyranoside (axial OH(4) and an equatorial OH(2)) which shows a lesser fit into the three-dimensional hydrogen-bonded structure of water. The same trend is observed for the methyl-8-pentopyranosides. Methyl-B-xylopyranoside fits better into the water structure than methyl-8arabinopyranoside does. Further investigation on the compressibility data of these compounds show that, for the g1uCOse derivatives, the position of the methoxy group within the carbohydrate molecule is of direct importance for the compatibility with the water structure. Methyl-a-glucopyranosideis less compatible
Hydration of Carbohydrates in Aqueous Solutions with the three-dimensional hydrogen-bonded structure of water than methyl-8-glucopyranoside,while when the methoxy group is on the 3-position as in 3-O-methylglucose, the hydration layer seems hardly disturbed at all. The sensitivity to the relative position of the methoxy group depends also on the relative position of the other hydroxy groups in the carbohydrate molecule: the partial molar Compressibilityof methyl-a-galactopyranosideequals that of methyl-/3-galactopyranoside. Hence the relative position of the methoxy group within the carbohydrate molecule can be important in determining the overall compatibility of the carbohydrate with the three-dimensional hydrogen-bonded structure of water. However, this is dependent on the stereochemistry elsewhere in the carbohydrate molecule. The disaccharides can be roughly divided into three groups depending on the monosaccharide moieties present in the disaccharide. Disaccharides composed of a glucose and a fructose unit have the least disturbed hydration layer (sucrose, turanose, and palatinose). For some disaccharides which consist of two glucose units the hydration layer is moderately disturbed (maltose and cellobiose) while any disaccharide which contains a galactose subunit (galactose has an axial OH(4) and an equatorial OH(2) is least compatible with the water structure (lactose, melibiose, and lactulose). Comparing the results for the disaccharides which consist of two glucose units, we observe first that maltase and cellobiase have the same isentropic partial molar compressibility. This means that the hydration characteristics are similar, although the type of linkage is different. This immediately rules out the pr~position'~ that maltose can form an intramolecular hydrogen bond in water while cellobiose cannot. The other disaccharides, which contain two glucose subunits, have a more disturbed hydration layer compared to maltose and cellobiose, apparently in disagreement with the kinetic data' (trehalose and gentiobiose). Both trehalose and gentiobiose have the same fit into the three-dimensional hydrogen-bonded structure of water as lactose. We suspect that the type of linkage between the two glucose units has caused this difference with maltose and cellobiose. Unexpectedly, trehalose does not fit the water structure very well while it does have the ability to replace waterm and prevent irreversible dehydration. The disaccharides, which consist of a glucose and a fructose subunit, exhibit the least negative partial molar compressibilities. Only turanose shows different behavior from sucrose and palatinose. This can be caused by the fact that the fructose subunit is now present in the pyranose form instead of the furanose form (which suggests that for fructose the furanose form fits better into the water structure than the pyranose form) or it can be caused by the fact that there is a 1-3 type of linkage between the subunits. This has the same effect as the 1-6 and 1-1 type linkages between the glucose moieties (compare trehalose and gentiobiose). Disaccharides which contain a galactose subunit have a bad compatibility with the three-dimensional hydrogen-bonded structure of water, independent of the type of linkage between the two subunits or whether there is a glucopyranose or fructofuranose ring present as the other moiety (lactose, melibiose, and lactulose). This provides further support for the notion that the relative position of the OH(4) is of crucial importance for the compatibility of the carbohydrate with water structure. The importance of the relative position of OH(4) in disaccharideshas been stressed earlier in the literat~re:~'when sucrose is changed into lactosucrose the sweetness completely disappears. Hydratian Numbers. Hydration numbers for carbohydrates can be obtained by applying NMR,'24 near-infrared spectroscopy," (39) Neal, J. L.; Goring, D. A. I. Can. J . Chem. 1910, 18, 3745. (40) (a) Weisburd, S. Sci. News 1988, 133, 107. (b) Crowe, L. M.; Mouradian, R.; Crowe, J. H.; Jackson, S. A.; Womersley, C. Biochim. B b phys. A 1984, 769, 141. (41) Birch, G. G.; Shamil, S. J . Chem. Soc., Faraday Trans I 1988,84, 2635. (42) Udaira, H.; Ikura, M.; Uedaira, H. BUN. Chem. Soc. Jpn. 1989,62, 1. (43) Birch, G. G.; Grigor. J.; Derbyshire, N. J . Solution Chem. 1!%9,18, 795. (44) Harvey, J. M.; Symons, M. C. R. J . Solurion Chem. 1978, 7, 571.
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5325 and HPLC.4 Also, measurements on frozen aqueous solutions of carbohydrates" and ultrasound measurements34a have bcen used to obtain hydration numbers. However, it is emphasized that the hydration number depends on the method used.35 Here we use eq 5 to calculate hydration numbers, which are indicative of the number of water molecules that are disturbed by the presence of the carbohydrate. Equation 5 is based on the assumption that the hydration layer is incompressible. If the compressibility of the hydration layer is not zero, then all hydration numbers will change by a few percent. The actual numbers cannot be directly compared to other hydration numbers, since they are mostly obtained through other methods. For the hexoses the hydration numbers illustrate exactly how small the differences in the hydration characteristics are. The overall trend is that the better the compatibility of the carbohydrate with the three-dimensional hydrogen-bonded structure of water, the smaller the number of water molecules, which is disturbed by the presence of the carbohydrate (e.g., D-talose, = 7.7; D-galactose, nh = 8.7). The small difference in hydration numbers (for the hexoses it ranges from 7.7 to 9.0) illustrates why partial molar volume measurements are not sensitive enough to differences in hydration. The pentoses have a slightly smaller hydration number due to the lack of the exocyclic -CHOH group. The difference for the pentose with an axial OH(4) and an equatorial OH(2) (D-arabinose, nh = 7.6) is more pronounced for the pentoses. All other pentoses, which have an equatorial OH(4) and either an axial or an equatorial OH(2), disturb a smaller number of water molecules (D-xylose, Dlyxose, and D-ribose: nh = 6.8). The methylhexopyranosides disturb on average more water molecules than the hexoses due to the presence of an additional methoxy group. Again the number of water molecules, which have a different compressibility, is governed by the stereochemistryof the carbohydrate. 3-O-Methylglucose, the structure of which is most compatible with water structure, disturbs the smallest number of water molecules (nh = Kl), whereas the hydration number of the methylgalactopyranosides (nh = 9.4) is indicative of a more disturbed hydration layer. Again, the hydration numbers are found in a small range (nh = 8.1-9.4). The hydration numbers of the methylpentopyranosidessupport that the carbohydrate with an axial OH(4) and an equatorial OH(2) (methyl-6-arabinopyranoside, nh = 8.2) disturbs more water molecules than an isomer for which OH(4) is equatorial and OH(2) is either equatorial or axial (methyl-8-xylopyranoside, nh = 7.4). Overall, the disaccharides have a hydration number that is twice the hydration number of the monosaccharides. Again, the range of hydration numbers for the disaccharides is fairly small (nh = 13.9-1 5.6) but stresses the importance of the relative position of OH(4). Conclusion
We submit that the hydration of carbohydrates is governed by their stereochemistry. Most important for the hydration characteristics is the relative position of the next-nearest-neighbor hydroxy groups within the carbohydrate molecule. The extent of hydration is largely determined by the position of the OH(4) in conjunction with the relative position of OH(2). The molecular picture is that the oxygen distances between the next-ntarestneighbor hydroxy group within the carbohydrate molecule should be compatible with the oxygen distances in water. The carbohydrates can be divided into three groups each with a different (45) Hollenberg, J. L.; Hall, D. 0. J . Phys. Chem. 1983,87, 695. (46) Silveston, R.; Kronberg, B. J . Phys. Chem. 1989, 93,6241. (47) Daoukaki-Diamanti,D.; Pissis, P.; Boudouris, G. Chem. Phys. 1984, 91, 315. (48) Uedaira, H.; Ishimura, M. Bull. Chem. Soc. Jpn. 1989, 62, 574. (49) Franks, F.; Ravenhill, J. R.; Reid, D. S. J . Solurion Chem. 1972, I , 3. (50) Bernal. P. J.; Van Hook, W. A. 1.Chem. Thermodyn. 1986.18.955. (51) Shahidi, F.; Farrell, P. G.; Edward, J. T. J. Solution Chem. 1916. 5, 807.
(52) Ihnat, M.; Goring, D. A. 1. Can. J . Chcm. 1967, 15, 2353. (53) J a m , R. V.; Ahluwalia, J. C. Can. J . Chcm. 1984, 16, 583.
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fit into the water structure. This is dependent on whether there will be a fit of the hydroxy groups of the carbohydrate with the nearest-neighbor oxygens (Dtalose) and next-nearest-neighbor oxygens (~-glucuse,Dmannare) or no @-galactose) fit with oxygen neighbors in water structure. The precise molecular details are still under investigation. The worst compatibility in the threedimensional hydrogen-bonded structure of water is found when a carbohydrate has an axial OH(4) and an equatorial OH(2). Interestingly, if this is so, there is no influence by the relative position of the methoxy group on the anomeric center or the type of linkage between the two monosaccharide moieties. Only when OH(4) is axial and OH(2) is also axial is the best fit with water structure found. For a carbohydrate with an equatorial OH(4) and either an equatorial or an axial OH(2) there is a moderate compatibility with the water structure. The relative position of a methoxy group on the anomeric center and the type of linkage between two moieties is important in this case in determining the extent of compatibility. Further studies are underway to model the carbohydratewater interactions.
Acknowledgment. The investigations were supported by the Netherlands Foundation for Chemical Research (SON) with
financial aid from the Netherlands Foundation for Scientific Research (NWO). Also part of this work was supported by the Netherlands program for Innovation Oriented Carbohydrate Research (IOP-k) with financial aid of the Ministry of Economic Affairs and the Ministry of Agriculture, Nature Management and Fisheries. S.A.G. thanks the University of Bergen for their hospitality. Also we thank Prof. J. B. F. N. Engberts and Dr. M. J. Blandamer for their interest in this work and discussion. Registry No. D-Talose, 2595-98-4; D-gahCtose, 59-23-4; D-mannose, 3458-28-4; D-ghCOSe, 50-99-7; D-fructose, 57-48-7; L-sorbose, 87-79-6; D-XylOSe, 58-86-6; D-lyxose, 11 14-34-7; D-ribose, 50-69-1; D-arabinose, 10323-20-3; methyl-j3-glucopyranoside, 709-50-2; methyl-a-mannopyranoside, 6 17-04-9; methyl-j3-galactopyranoside,1824-94-8; mcthylj3-xylopyranoside, 612-05-5; methyl-j3-arabinopyranoside, 5328-63-2; methyl-a-glucopyranoside,97-30-3; methyl-a-galactopyranoside, 182494-8; sucrose, 57-50-1; turanose, 547-25-1; palatinose, 13718-94-0; maltose, 69-79-4; cellobiose, 528-50-7; lactose, 63-42-3; melibiose, 58599-9; lactulose, 4618-18-2; trehalose, 99-20-7; gentiobiose, 554-91-6; 3-O-methylglucose, 146-72-5.
Supplementary Material Available: Tables of densities, compressibility coefficients, apparent molar volumes, apparent molar compressibilities, and hydration numbers a t each concentration of carbohydrate at 298 K (4 pages). Ordering information is given on any current masthead page.
A Theoretical Study of Interstitial Transition-Metal Impurities in Group II - V I Compounds K. K. Stavrev,* S. I. Ivanov, Department of Chemistry, University of Sofa, 1 Anton Ivanov Avenue, S o f a 1 1 26, Bulgaria
and G. St. Nikolov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria (Received: December 26, 1990)
The problem of first transition metal ions (Sc2+to Cu2+)located as impurities at interstitial sites in group 11-VI compounds (cubic ZnS and ZnSe) is studied by semiempirical quantum chemical methods. The relative stability of the two sites and the barriers to transitions between the interstitial sites were evaluated within the cluster approach by considering channels of 22 atoms. The relative ionic diffusion rates have been estimated on the basis of the channel barrier heights calculated for different tracers moving in the channels through the lattices. The location of Mn2+ions at interstitial sites cannot account for the 1.3-eV emission band observed experimentallyfor Zn1.,Mn$ and Znl.,Mn$e and the reason for this emission should be searched elsewhere.
Introduction The problem of interstitial atoms in crystals has, for a long time, been the subject of extensive studies.'.* A number of theoretical models are available for neutral atoms or ions at interstitial sites in a great variety of crystal^.^*^ At the same time, the experimental data on interstitial transition-metal (TM) ions in group 11-VI compounds are scarce. It is of interest to study in detail the electronic structure of such centers: one could expect that they will differ drastically from the substitutional centers as to their magnetooptical behaviores Nowick, A. S.,Burton, J. J., Eds. Dl//uctonin Solids; Academic Ress: New York, 1975. Murch, G. E., Nowick, A. S.,Eds. Di//urlon in Crystalline Solids; Academic Ress: Orlando, FL, 1984. (2) Hartmann, H.; Selle, B.; Mach, R. Current Topics in Material Science; Kaldis, E., Ed.; North-Holland: Amsterdam, 1982. (3) Fujita, S.Phys. Status Solidi B 1987, 143, 443. (4) Fujita, S.;Neugebauer, J. J. Phys. Chem. Solids 1988, 49, 561. (1)
0022-3654/91/2095-5326$02.50/0
For example, interstitial MnZ+ions could be responsible for the infrared (IR) 1.3-eV emission observed at high Mn concentrations with all semimagnetic II,.,Mn,VI compounds.6.' This subject has already been analyzed by ab initio calculations for small clusters comprising explicitly only the first neighbors of the interstitial MnZ+ions in the cubic ZnSa This analysis rejects the possibility that interstitial MnZ+ions could be responsible for the infrared emission band of Zn,.,Mn>. The cluster considered is, however, a poor replica of the solid-state problem; expanding the coordination cluster drastic may cause changes to occur in the ( 5 ) Schulz,
H.-J. J. Cryst. Growth 1982, 59, 65. H.; Benecke, C.; Busse, W.; Gumlich. H.-E.; Krast, A.
( 6 ) Waldmann,
Semicond. Sci. Technol. 1989, 4, 7 1. (7) Benecke, C.; Busse, W.; Gumlich, H.-E.; Hoffmann, H.; Hoffmann, S.;Krost, A.; Waldmann, H. Phys. Status Solidi B 1989, 153, 391. (8) Janssen, G. J. M.Ph.D. Thesis, University of Groningen, 1986. For detailed analysis of substitutional Mn2+ in ZnS, see: Richardson, J. W.; Janssen, G. J. M. Phys. Reo. B 1989, 39, 4958.
0 1991 American Chemical Society
