Anal. Chem. 2003, 75, 671-677
Complexation of Copper by Zwitterionic Aminosulfonic (Good) Buffers Heath E. Mash and Yu-Ping Chin*
Environmental Science Program, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, Ohio 43210 Laura Sigg, Renata Hari, and Hanbin Xue
Swiss Federal Institute of Environmental Science and Technology (EAWAG), Ueberlandstrasse 133 Duebendorf, Switzerland CH8600
Copper binding properties were investigated for several popular zwitterionic buffers. The two buffers 4-morpholinoethanesulfonic acid (MES) and 3-N-morpholinopropanesulfonic acid (MOPS) did not bind copper and would be good choices for metal speciation studies within their operational pH range. Conversely, 3-(N-morpholino)-2hydroxypropanesulfonic acid (MOPSO) was observed to weakly bind copper directly (log Kc 2.02). Moreover, strong copper binding was observed for 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 4-(2hydroxyethyl)piperazine-1-propanesulfonic acid (HEPPS), and N-(2-hydroxyethyl)piperazine-N′-(2-hydroxypropanesulfonic acid) (HEPPSO). Log Kc values range from 7.04 to 7.68 and are indicative of strong copper binding ligands. The latter buffer also exhibited weak binding characteristics with a log Kc of 2.05. The strong Cu binding ligands were present in HEPES, HEPPS, and HEPPSO at much lower concentrations than the total buffer concentration. MES, HEPES, MOPSO, and HEPPSO were analyzed by electrospray-ionization quadrapole time-of-flight mass spectroscopy. The most prominent feature of the spectra for each buffer analyzed was the presence of multiple oligomers, indicating a propensity of interaction between buffer molecules. In addition, the presence of several contaminants was identified in the mass spectrum of the HEPES matrix, including a prominent contaminant (at m/z 131) present in levels similar to those obtained from the modeling of the copper titration data. Other contaminants were found in the other matrixes but were not identified as possible copper binding agents. The activity of the hydrogen ion (pH) plays an important role in the speciation of metals in aqueous environments.1-4 The accurate determination of metal-ligand complexes requires measurement at a stable solution pH. While many different * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 614-292-6953. Fax: 614-292-7688. (1) Stumm, W.; Morgan, J. J. Aquatic Chemistry, Chemistry Equilibria and Rates in Natural Waters, 3rd ed.; John Wiley & Sons: New York, 1996. (2) Roy, R. N.; Bice, J.; Greer J.; Carlsten, J. A.; Smithson, J.; Good, W. S.; Moore, C. P.; Roy, L. N.; Kuhler, K. M. J. Chem. Eng. Data 1997, 42, 41-44. 10.1021/ac0261101 CCC: $25.00 Published on Web 12/21/2002
© 2003 American Chemical Society
buffering agents are available, systems involving trace metals require buffers that do not chelate them. Many inorganic and organic buffers used in environmental studies involving trace metal-ligand speciation may complex a number of trace metals.3-9 Thus, the degree to which they bind must be determined for their use in speciation studies. Two major disadvantages of this approach include the following: (1) the uncertainty associated with the stability constant between the trace metal and the buffer and (2) side reactions that cannot be accounted for in a given system. Zwitterionic N-substituted aminosulfonic acid buffers first introduced by Good and co-workers10-12 are commonly used in metal speciation studies because of their purported weak-tononexistent complexation properties. Buffers were created that cover a wide range of pH by the substitution of increasingly larger alkyl chains between the tertiary amine group and the sulfonic group, the addition of hydroxyl groups on the molecule, or other modifications to the backbone N-substituted structure (Figure 1). For example, 4-morpholinoethanesulfonic acid (MES) and 3-Nmorpholinopropanesulfonic acid (MOPS) reportedly had very little or negligible metal binding characteristics. Similarly, 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 4-(2hydroxyethyl)piperazine-1-propanesulfonic acid (HEPPS), and N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) were reported to not complex Cu2+, Mn2+, Mg2+, or Ca2+.12 As a result Good buffers have been extensively used in studying the (3) Sunda, W. G.; Hanson, P. J. In Chemical Modeling in Aqueous Systems: Speciation, Sorption, Solubility, and Kinetics; Jenne, E. A., Ed.; American Chemical Society: Washington, 1979; pp 151-180. (4) Temminghoff, E. J. M.; van der Zee, S. E. A. T. M.; de Haan, F. A. M. Environ. Sci. Technol. 1997, 31, 1109-1115. (5) Du, Q.; Sun, Z.; Forsling, W.; Tang, H. Water Res. 1999, 33, 693-706. (6) Wiese, C.; Schwedt, G. Fresenius J. Anal. Chem. 1997, 358, 718-722. (7) Abu Zuhri, A. Z.; Voelter, W. Fresenius J. Anal. Chem. 1998, 360, 1-9. (8) Harris, W. R.; Cafferty, A. M.; Abdollahi, S.; Trankler, K. Biochim. Biophys. Acta 1998, 1383, 197-210. (9) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1974. (10) Good, N. E.; Winget, G. D.; Winter, W.; Connolly, T. N.; Izawa, S.; Singh, R. M. M. Biochemistry-US 1966, 467, 467-477. (11) Good, N. E.; Izawa, S. In Methods in Enzymology; San Pietro, A., Ed.; Academic Press: New York, 1972; pp 53-68. (12) Ferguson, W. J.; Braunschweiger, K. I.; Smith, J. R.; McCormick, J. J.; Wasmann, C. C.; Jarvis, N. P.; Bell, D. H.; Good, N. E. Anal. Biochem. 1980, 104, 300-310.
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Table 1. Previously Reported Stability Constants for Several Zwitterionic Buffers buffer
log K
ref
MOPSO MOPS
3.81 4.00 none 4.71 5.02a 4.74 4.1a 4.3a “incipient complexation” none none
24 24 25 24 21 24 21 21 20 21 25
DIPSO TAPSO POPSO HEPPSO HEPES PIPES
a Only CuL stability constants are shown here for comparative 1 purposes.
Figure 1. Chemical structure of several zwitterionic buffers that have been used for pH control in environmental research.
speciation of metals in natural waters, including HEPES,13 MES,14 piperazine-1,4-bis(2-ethanesulfonic acid) (PIPES),15 and HEPPS.16 Each of these buffers operates within environmentally relevant pH regions. Recent evidence has shown that some of the Good buffers are capable of weakly binding a variety of metals (Table 1). Hegetschweiler and Saltman reported a small, but measurable interaction between Cu2+ and HEPES.17,18 Their conclusion was based on indirect evidence gained from the increased reduction rates of Cu2+ to Cu+ in a series of alcohols and heme proteins in the presence of HEPES. They hypothesized that the lone pair of electrons from the nitrogen participated in the reaction thereby increasing reaction rates. Vasconcelos and co-workers19-23 published a series of papers demonstrating the interaction of several zwitterionic buffers with trace metals. In their system, the toxicity of copper to Amphidinium carterae was dramatically decreased in the presence of 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO).19 This was attributed to a strong interaction between DIPSO and Cu2+ that was effectively removing bioavailable copper. Conversely, HEPES increased the toxic effects of Cu2+ even though a weak interaction between Cu2+ and HEPES20 was observed when HEPES was present in excess of (13) Xue, H.; Sigg, L. Aquat. Geochem. 1999, 5, 313-335. (14) Rozan, T. F.; Benoit, G.; Mash, H.; Chin, Y.-P. Environ. Sci. Technol. 1999, 33, 1766-1770. (15) Rue, E. L.; Bruland, K. W. Mar. Chem. 1995, 50, 117-138. (16) van den Berg, C. M. G. Mar. Chem. 1985, 16, 121-130. (17) Saltman, P.; Eguchi, L.; Hegetschweiler, K. In Frontiers in Bioinorganic Chemistry; Xavier, A. V., Ed.; VCH: Weinheim, Federal Republic of Germany, 1986; p 241. (18) Hegetschweiler, K.; Saltman, P. Inorg. Chem. 1986, 25, 107-109. (19) Lage, O. M.; Vasconcelos, M. T. S. D.; Soares, H. M. V. M.; Osswald, J. M.; Sansonetty, F.; Parente, A. M.; Salema, R. Bull. Environ. Contam. Toxicol. 1996, 31, 199-205. (20) Vasconcelos, M. T. S. D.; Azenha, M. A. G. O.; Lage O. M. Anal. Biochem. 1996, 241, 248-253. (21) Vasconcelos, M. T. S. D.; Azenha, M. A. G. O.; Almeida, C. M. R. Anal. Biochem. 1998, 265, 193-201. (22) Vasconcelos, M. T. S. D.; Almeida, C. M. R. Anal. Chim. Acta 1998, 369, 115-122. (23) Soares, H. M. V. M.; Conde, P. C. F. L.; Almeida, A. A. N.; Vasconcelos, M. T. S. D. Anal. Chim. Acta 1999, 394, 325-335.
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total copper by 103 times. Subsequent work by these investigators revealed significant interaction between Cu2+ and the Good buffers N-(2-hydroxyethyl)piperazine-N ′-(2-hydroxypropanesulfonic acid) (HEPPSO) and piperazine-N,N′-bis(2-hydroxypropanesulfonic acid) (POPSO) (Table 1).21,22 Vasconcelos et al.21 also observed a decrease in the value of the stability constant with increased buffer concentration, attributing this to aggregation between buffer molecules. Yu and co-workers25 proposed a complexation mechanism whereby substituted hydroxyl groups on N-substituted aminosulfonic acids initiated metal binding. Complexation did not occur for structurally similar compounds that lacked the hydroxyl groups. In all cases, binding of metals to these buffers was believed to be weak. They present a series of noncomplexing zwitterionic buffers, on the basis of this assumption, that extend over the pH range from 3 to 11. To date, much of the work involving Good buffers’ complexation to Cu2+ and other metals resulted from observed anomalies in biological and toxicological studies. The objective of this work was to determine whether several commonly used zwitterionic aminosulfonic acid pH buffers can potentially interfere in copper speciation studies in natural waters. Specifically, we wish to (1) determine the extent of Cu binding (if any) to MOPS, MES, MOPSO, HEPES, HEPPSO, and HEPPS and (2) measure the conditional binding constants for those buffers that exhibit complexation behavior. EXPERIMENTAL SECTION All chemicals used in this research were obtained commercially (Sigma Chemicals) in their highest purity form (>99.5%). Stock solutions (0.1 M) of HEPES, HEPPS, HEPPSO, MES, MOPS, and 3-(N-morpholino)-2-hydroxypropanesulfonic acid (MOPSO) were prepared without further purification. A stock copper solution (1 × 10-3 M) was prepared weekly from CuNO3‚2.5H2O, diluted in Milli-Q water and acidified to pH 2 with concentrated trace metal reagent-grade HNO3. This solution was standardized against EDTA. A dilute copper solution (6.25 × 10-5 M) was prepared every other day and acidified to pH 2. Experimental pH conditions (24) Anwar, Z. M.; Azab, H. A. J. Chem. Eng. Data 1999, 44, 1151-1157. (25) Yu, Q.; Kandegedara, A.; Xu, Y.; Rorabacher, D. B. Anal. Biochem. 1997, 253, 50-56. (26) Mash, H.; Chin Y.-P.; Sunda, W. Manuscript in preparation.
were fixed and maintained with 1.0 and 0.1 M solutions of HNO3 and KOH to circumvent any drift. Potentiometric titrations were performed using a modified rotating Orion model 96-29 cupric ion-selective electrode (CuISE) and an Orion model 900200 Ag/AgCl double-junction reference electrode in conjunction with a Mettler DL70-ES titrator. pH was monitored using a Mettler InLab-417 combination pH electrode. The CuISE was spun at a rate of 2300 rpm to decrease the equilibrium time between subsequent copper additions. Employing a spinning electrode cut response time to approximately 10 min at low Cu concentrations (
