Complexation of calcium and copper with carbohydrates

Marine Chemistry, 19 (1986) 29~304

299

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

COMPLEXATION OF CALCIUM AND COPPER WITH CARBOHYDRATES Implications for seawater speciation JOHN W. HAAS, Jr.

Department of Chemistry, Gordon College, Wenham, MA 01984 (U.S.A.) (Received September 17, 1985; revision accepted March 6, 1986)

ABSTRACT Haas, J.W., Jr., 1986. Complexation of calcium and copper with carbohydrates. Implications for seawater speciation. Mar. Chem., 19: 299--304. The extent of complexation of calcium(If) and copper(If) with 33 carbohydrates found in seawater has'been determined potentiometrically using calcium and copper ionselective electrodes. The measurements were performed in an essentially neutral aqueous m e d i u m at 25°C in 0.70 M K N O 3. Neutral sugars form very weak I :I complexes with Ca(If) and Cu(II) ions. Sugars with carboxylate groups from much stronger complexes. K'cu(ii)~ K'ca(ii) for a given sugar. The extent of complexation is dependent on the conformation of the sugar ligand.

INTRODUCTION

The chemical speciation of metals in aquatic environments is important in determining metal bio-availability, geochemical cycling and toxicity. In turn, metal complexation may dramatically influence reaction rates and product distribution for reactions involving the associated ligands. Complexation studies in natural environments have followed two general lines. One approach seeks to determine the overall complexation of a metal at a specific site where the composition and structure of the organic matter may not be known. Other studies involve metal complexes with specific ligands of known structure, the approach followed in this report. A wide variety of carbohydrates and polyols are found in marine environments (Mopper et al., 1980; Yamaoka, 1983). Various studies have demonstrated sugar distribution patterns and microbiological processes involved in the cycling of dissolved carbohydrate (Haug and Larsen, 1969; Mopper et al., 1980; Goche et al., 1981). Sugars have long been known to form labile complexes with metal ions in neutral aqueous solution (Rendleman, 1966; Angyal, 1973; Dheu-Andries and Perez, 1983). Sugar--metal complexes have been implicated in sugar glycosidation (Angyal, 1975; Lonnberg and Vesala, 1980), enolization and isomerization (Haug and Larsen, 1969; Harris and Feather, 1975). Evaluation of the significance of sugar--metal complexes in marine processes requires a knowledge of the extent of complexation for specific metal ions and sugars in an environment approximating seawater. Most studies of 0304-4203/86/$03.50

© 1986 Elsevier Science Publishers B.V.

3O0 sugar--metal complexation have dealt with the structure of the complexes with only scattered reports providing numerical data for formation constants. We have employed a potentiometric m e t h o d using electrodes sensitive for calcium(II) and copper(II) ions to investigate the extent that these ions complex with a wide variety of sugars found in seawater. Ion selective electrodes (ISE) provide ease o f measurement, sensitivity, selectivity and, most importantly, the ability to measure ion activity in a d i r e c t fashion by p o t e n t i o m e t r y (Rechnitz, 1970; Buffle et al., 1980). Ca(II) was chosen for study because it is a major ion in natural water and has been implicated in biological processes as calcium storage (Farber et al., 1957), isomerization (Schray and Benikovic, 1978) and stereospecific carbohydrate chain linkage (DeLucas et al., 1975). Cu(II), although found in only trace amounts in aquatic systems, is one of the most strongly complexing cations and has been established as toxic to marine organisms (Mills and Quinn, 1981; Lazar et al., 1981). EXPERIMENTAL

Reagents The sugars and inorganic reagents were obtained from the best sources available. Sugars were routinely purified by recrystallization from aqueous alcohol after treatment with activated charcoal. The potassium salts of sugar acids were prepared as solutions b y adding KOH to the parent sugar acid.

Instrumentation Cupric ion measurements were made with the Orion Model 94-29 cupric ion electrode. Calcium ion was measured with the Orion Model 93-20 electrode. The Orion Model 90-02 Hg/HgC1/sat. KC1//0.7MKNO3 double junction electrode was used as the reference electrode, pH was measured with the Orion Model 81-02 combination electrode. Potential measurements were made with a Coming Model 112 digital pH/ion meter connected to a H o u s t o n Instruments Series 5000 recorder. Potentials were measured to + 0.1 mV.

Method To a thermostatted beaker containing the ISE and reference electrode was added 100 ml of 0.70 M KNO 3 solution. The solution was kept under nitrogen and stirred at a constant rate with a mechanical glass stirrer under uniform lighting conditions. 3 to 5 aliquots o f standard Ca(II) or Cu(II) solution in 0.70 M KNO 3 were added from a calibrated pipet to establish the electrode response over the range o f metal concentration desired. 5 to 15 aliquots of known amounts of equilibrated sugar solution in 0.70 M KNO 3 were added

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b y pipet in each run. Potential measurements were recorded when the meter readings became stabilized, normally within 1 min (Uemasu and Umezawa, 1982). Electrode response calibration data for each run were treated b y a least squares linear regression computer program which provided a 'best fit' potential/concentration curve. In cases where anionic ligands were studied the pH was maintained at a constant value by addition of 0.01 M KOH by micropipet. Solution pH values were designed to provide o p t i m u m performance for the ISEs, to avoid hydrolysis of the metal cation and to ensure that ligands derived from sugar acids were in the anionic form. ISE potentials decrease when complexing ligands are added. Glycerine was used as a 'non-complexing' carbohydrate to correct for the small increase in potential observed when organic molecules affect the environment at the electrode surface (Llenado, 1975). The range of sugar and metal ion concentrations employed ranged from 10 -3 to 10 -1 M and 10 -6 to 10 -3 M, respectively. Values for specific sugars were chosen to maximize the precision of measurement.

Calculation of formation constants Conditional formation constants (Dickson et al., 1981) were calculated from eq. 1 where [M], [S] and [MS] are the molar concentrations for uncomplexed metal cation, sugar and metal--sugar complex, respectively. K ' = [MS]/[M] [S]

(1)

A computer program (COMPLEX) was employed to convert electrode response data to [M] values via the electrode calibration algorithm. Formation constant values were calculated from the initial and equilibrium concentrations of [M], [S] and [MS]. K ' values over the range of ligand concentration were compared to establish any deviation from 1 : 1 stoichiometry. The results reported represent values obtained from at least 15 different concentrations obtained from 3 or more different runs. The ionic strength of the solution was n o t significantly affected b y the addition of small concentrations of metal cations and anionic ligands. Thermodynamic stability constants cannot be calculated since activity coefficients for the ionic species are unknown. RESULTS AND DISCUSSION

Tables I and II present conditional formation constant values for a series of carbohydrates at 25°C in 0.70 M KNO 3. All the sugars reported exhibited 1 : 1 metal--sugar complexes in the concentration range investigated. As the potentiometric approach measures 'free' metal ion concentration it cannot distinguish the complexing ability of individual sugar species present in situations where various ring forms are present. Rather, the formation constant represents a mixed value for all the sugar forms at equilibrium. An analysis

302 TABLE I Stability constants for the 1:1 complexes of calcium(II) and copper(II) and neutral carbohydrates in 0.70 M KNO3 at 25°C

Compound

K'Ca (din 3 tool -1 ) at pH 8.1

K ' c u (din 3 mol -i ) at pH 6.1

Arabitol Ribitol Xylitol Galactitol Glucitol Mannitol Myo-inosotol Arabinose Lyxose Ribose Xylose Galactose Glucose Mannose Sorbose Fructose Lactose Trehalose

0.22 0.16 0.29 0.31 0.30 0.24 0.13 0.15 0.16 0.46 (0.1 ~0.1 (0.1 ~0.1 ~0.1 ~0.1 ~0.1 ~0.1

1.24 0.59 0.61 0.83 0.77 0.70 0.29 1.5 0.51 1.67 0.51 0.24 0.15 0.54 0.28 0.26 0.16 0.41

TABLE

II

Stability constants for the 1 : 1 complexes of calcium(II) and copper(II)with carbohydrate acid anions in 0.70 M KNO 3 at 25°C

Compound

K'Ca (dm 3 mol -I ) at pH 8.1

K ' c u (dm a mo1-1 ) at pH 6.1

Glycolate Arabonate Lyxonate Ribonate Xylonate Galactonate Gluconate Mannonate Galacturonate Glucuronate Mannuronate CeUobionate Lactobionate Maltobionate Melibionate

12.9 23.3 16.6 18.1 17.3 6.1 20.4 23.0 20.5 10.8 11.1 58 46 93 98

209 287 241 279 259 228 260 281 241 77 84 610 453 654 718

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of inherent measurement error and potential values over wide variations in metal/sugar concentration suggests that the precision of the formation constants is +8--12% for K' values ~ 2 and 3--5% for K ' > 5. The data are similar to values f o u n d in other studies under comparable conditions (Evans and Frampton, 1977; Jaques et al., 1979; Aruga, 1981). g 'cu(n) > K'ca(iD for a specific sugar. The neutral carbohydrates are uniformly seen to provide only weak complexes with calcium(II) and copper(II). Sugars containing carboxylate groups form significantly stronger complexes whose strength depends on the structure of the ligand and metal ion involved. Relative ligand binding strength is related to the number of oxygen groups which can bind the cation. Sugar pyranose ring forms which contain an axial--equatorial--axial sequence of oxygen atoms favor complexation. An assessment of the significance of complexation for specific metals and sugars is complicated b y the presence of other competing metals and ligands in seawater. However, an estimate of the maximum extent of complexation can be obtained from K ' values in Tables I and II and by assuming that only one metal and ligand are present. Assuming initial Cu(II) (Florence, 1982) and sugar concentrations (Mopper et al., 1980) of 1 x 10 -9 and 1 x 10 -7 M, respectively, and applying eq. 1 to calculate the equilibrium concentrations of M, S, and MS we find a vanishingly small concentration of copper--sugar complex even for sugars with relatively large K ' values. Calculation of the extent of calcium complexation using K ' values from Tables I and II and assuming initial values for [M] (Florence, 1982) and [S] (Mopper et al., 1980) of 1 x 10 -2 and 1 x 10 -7 M indicates that a significant percentage of the sugar may be b o u n d to the metal. The percentage of sugar in complex form ranges from 0.5% complex where K ' -- 0.5 to 50% complex for K ' = 100. The significance of calcium over copper in complex formation arises from the 10 ÷7 greater concentration in seawater even though copper complexes are stronger b y a factor of 10 +2 . The data suggest that complexation involving calcium or copper ions and neutral carbohydrates does not appear to be of great biological significance. However, complexes with soluble sugar acids from soluble monosaccharides and ionic polysaccharides such as the alginates would appear to be potentially important under typical marine conditions. ACKNOWLEDGMENTS

The author thanks Dwight Tsudy for development of the computer program and John Haas, III for synthesis of the uronic acids. REFERENCES Angyal, S.J., 1973. Complex formation between sugars and metal ions. Pure Appl. Chem., 35: 131- 146. Angyal, S.J., 1975. Complexes of carbohydrates with metal cations. V. Synthesis of

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