VOLUME 2, NUMBER 3, MARCH 2002 © Copyright 2002 by the American Chemical Society
Monoclonal Antibody Recognition of Histidine-Rich Peptide Encapsulated Nanoclusters Joseph M. Slocik, Joshua T. Moore, and David W. Wright* Department of Chemistry, VU Station B 351822, Vanderbilt UniVersity, NashVille, Tennessee 37235-1822 Received December 18, 2001
ABSTRACT Biological systems offer insights into approaches to the problems encountered in the synthesis of nanomaterials. Histidine-rich proteins have been implicated in the biomineralization of heme, copper, and zinc. We have used the repeating consensus sequence AHHAHHAAD from the histidine-rich protein II of Plasmodium falciparum to mediate the aqueous self-assembly of several metal sulfide, metal oxide, and zerovalent metal clusters. Additionally, the surface encapsulating histidine-rich epitope is recognized by a monoclonal antibody raised against HRP II.
A major objective in achieving robust, functional nanodevices is the development of the requisite chemistry to assemble larger architectures through the control of the interfaces and distribution of nanocomponents. Biological systems offer important insights into possible approaches to the problems encountered in the synthesis of extended materials.1 The array of materials produced by biological systems include laminate composites and ceramics such as bone, teeth, and shells;2 magnetic materials such as the forms of magnetite found in magnetobacteria and the brains of migratory animals;3 novel silver or cadmium sulfide nanoclusters produced as a result of heavy metal detoxification mechanisms by bacteria;4,5 and arrays of precisely fabricated diffracting architectures resulting in the multitude of intense colors observed in insects and birds.6 Consequently, there has been increasing interest in the use of biomolecules to control not only the synthesis * Corresponding Author. Tel.: 1-615-322-2636. Fax: 1-615-343-1234. E-mail:
[email protected]. 10.1021/nl015706l CCC: $22.00 Published on Web 02/08/2002
© 2002 American Chemical Society
of nanoparticles, but also to direct the formation of extended supermolecular assemblies. Several approaches have been developed to form biological interfaces for nanoparticles. These include (1) the covalent linking of a suitably modified nanocluster surface to a protein or antibody which then recognizes a complement such as a receptor or antigen to form larger structures;7-9 (2) the use of oligonucleotide functionalized nanoparticles linked by complimentary DNA;10 (3) the use of combinatorial selection to identify peptides with binding specificities for nanocrystal assembly;11-13 and (4) the creation of bifunctional proteins capable of associating dissimilar materials to form mesoscale assemblies.14 We report, herein, the first use of a peptideencapsulated nanocluster surface to assemble a nanoclusterantibody complex through an epitope interface. Histidine-rich peptides and proteins have been discovered in human saliva,15 the digestive vacuole of the human malarial parasite, Plasmodium,16 human blood serum,17 the
Figure 1. Comparison of several repeating histidine-rich protein sequences.
vitellaria of the liver fluke Fasciola hepatica,18 and the fangs of marine polychaetes (Figure 1).19 In many of these organisms, these peptides and proteins have been implicated in the biomineralization of heme, copper, and/or zinc. It has been reported that the histidine-rich proteins (HRP II and III) of P. falciparum mediate the formation of a magnetic heme biomineral known as hemozoin.20 Similarly, the fangs of marine polychaetes containing histidine-rich proteins mineralize zinc in concentrations as high as 3% of the metal by dry weight.21 Given both the use of histidine-rich peptides in biomineralization processes and the physical properties of histidine itself (good nucleophilicity, side chain pKa, known metal binding properties, good hydrogen bonding), we hypothesized that a histidine-rich peptide would make a versatile ligand for the stabilization of a wide range of water soluble nanoclusters. Here, we have used a histidine-rich epitope AHHAHHAAD (HRE) from HRP II as a suitable peptide to mediate the aqueous synthesis of a variety of metal sulfide, metal oxide, and zerovalent metal clusters. Screening the resulting clusters against a monoclonal antibody for HRP II revealed molecular recognition of the ligand epitope along the nanocluster surface for some of the peptide encapsulated nanoclusters. The HRE was screened as a stabilizing ligand in the aqueous synthesis of ZnS (1), Au0 (2), Ag0 (3), TiO2 (4), and Ag2S (5) nanoclusters. The HRE was synthesized using an Advanced Chemtech peptide synthesizer and standard fmoc protocols, purified by reverse-phase HPLC, and lyophilized as previously described (Supporting Information).22 The synthesis of the HRE stabilized nanoclusters 1-5 was based on modifications of literature procedures.23 Briefly, the synthesis followed a two-step procedure. In the first step, metal-HRE complexes were formed by the 1:1 reaction of appropriate metal ion to peptide in a deaerated solution of Tris buffer (0.10 M, pH 8.6) with concomitant stirring. In the second step, the nucleation of the nanocrystallites was achieved by either the addition of stoichiometric amounts of inorganic sulfide (Na2S) for 1 and 5 or by the addition of an appropriate reductant (sodium citrate or sodium borohydride) for 2 and 3. The condensation synthesis of 4 proceeded by the 1:1 reaction of peptide and Ti((CH3)2CHO)4 in absolute ethanol. To this solution, 3 mL of Tris buffer (0.10 M, pH 8.6) was added. Reaction times were 4 h with stirring. 170
Figure 2. UV-vis absorption spectra of the HRE-stabilized nanoclusters 1-5.
The HRE encapsulated clusters 1-5 were purified by repeated precipitation with cold absolute ethanol until no excess ligand was detected in the supernatant by HPLC and/ or NMR. It is important to stress that these reaction conditions were designed to screen the suitability of the HRE as a ligand to stabilize nanocluster formation and should not be viewed as optimized. Nanoclusters 1-5 have been analyzed by a variety of physical methods. The absorption spectra of clusters 1-5 (≈1 × 10-4 M) are shown in Figure 2. The absorption maximum of 1 is at 288 nm with a broad blue-shifted shoulder. In their preparation of simple model histidineZnS clusters, Mehra and co-workers have ascribed a similar spectrum as arising from two distinct populations of clusters (R ) 2.7 and 3.6 nm) that were resolvable by size-exclusion chromatography.24 Clusters 2 and 3 show the expected plasmon resonance absorptions at 524 and 402 nm for Au0 and Ag0 nanoclusters, respectively.25 Cluster 4 represents an unusual example of a water solubilized TiO2 nanocluster with absorption maximum at 282 nm. The absorption spectrum of 5 is essentially featureless as previously reported for the glutathione encapsulated form of this nanocluster.23 Fluorescence spectroscopy showed that only 1 exhibited significant fluorescence (λmax 425 nm). Infrared analysis of the purified epitope-encapsulated nanoclusters clearly indicated the presence of surface bound peptide (Supporting Information) dominated by the vibrations associated with the histidine residues of the stabilizing epitope. Additionally, vibrations attributable to the Amide I and II vibrations of the peptide backbone were observed. Analysis of the clusters by XRD, TEM, and EDS revealed that clusters of ZnS, Au0, Ag0, and Ag2S (1-3, 5) were crystalline, while clusters of TiO2 (4) were considerable less so (Supporting Information).26 Due to their relatively small size, ZnS nanoclusters afforded broad XRD peaks. Scherrer’s analysis of the diffraction peaks estimated an average particle diameter for 1 of 1.5 nm. XRD studies of 2 and 3 showed highly crystalline nanoclusters of cubic structure with sizes estimated at 5 and 7.4 nm, respectively. XRD studies of 4 Nano Lett., Vol. 2, No. 3, 2002
Figure 3. Antigen capture assay. (A) Nanocluster (star symbol ) HRE-NC antigen, oval symbol ) nonspecific NC-antigen) exposed to monoclonal antibody for HRP II of P. falciparum. (B) HREstabilized nanocluster selectively recognized by monoclonal antibody. (C) Assay visualized by recognition of antigen/antibody by the developing agent, a polyclonal antibody against P. falciparum conjugated to a dye containing liposome (circlel symbol).
were featureless, due to either small size or poor crystallinity. Although XRD of 5 showed the expected peaks for monoclinic Ag2S, there was unusually high intensity for the peak at 2-theta of 46°. Under TEM examination (Supporting Information), the crystalline nanoclusters appear nearly spheroidal with monomodal particle size distributions and average particle size for 1-3 and 5 of 3.1, 9.5, 11.2, and 11.3 nm, respectively. When comparing experimentally determined average particle diameter, TEM estimates represent number averaged values, while particle diameter estimates from XRD peak width measurements are volume weighted. Consequently, the presence of a fraction of particles having diameters much smaller or larger than the number average values will give disproportionately smaller or larger particle sizes by XRD. The differences in particle size diameters for 1-3 and 5 determined by XRD and TEM highlights the presence of these cluster populations at the extremes of the particle size distributions noted in the particle size distribution analysis of surveyed areas by TEM. Selected-area EDS scans of clusters 1-3 and 5 confirm the stoichiometric presence of the relevant elements over a one micron area. Monoclonal antibody recognition of the peptide encapsulated nanocluster was assayed using the commercially available Parasight F test for P. falciparum malaria, which is based on the detection of the soluble HRP II antigen by a monoclonal antibody (Figure 3).27 Briefly, premeasured solutions of purified peptide-encapsulated nanoclusters (15) were placed in test wells. The test strip with immobilized monoclonal antibodies specific for P. falciparum HRP II was placed in the well and the solution adsorbed. The assay was developed by the addition of the detection agent, a solution containing P. falciparum polyclonal antibodies coupled with dye-loaded liposomes. A solid pink line and the pink control dash at the top of the strip indicated a positive test, while the presence of only the pink procedural control dash and a white/light pink background indicated a negative test. The results of the antigen capture assay are shown in Figure 4. Clearly, clusters 1-3 (test strips 4-6) produced positive results, while clusters 4 and 5 (strips 7 and 8) tested negative for antibody recognition. The different intensities and band positions on each of the test strips are the manifestation of a number of variables including initial cluster concentration, nanocluster solubility, concentrations of epitope available on Nano Lett., Vol. 2, No. 3, 2002
Figure 4. Antigen capture assay of HRE stabilized nanoclusters by monoclonal antibodies raised against the histidine-rich protein II of P. falciparum: (1) positive control using a multiantigenic peptide with 16 HRE repeats, (2) negative control using histidine stabilized ZnS nanocluster, (3) negative control using a glutathione stabilized ZnS nanocluster, (4) HRE-encapsulated ZnS nanocluster, (5) HRE-encapsulated Ag0 nanocluster, (6) HRE-encapsulated Au0 nanocluster, (7) HRE-encapsulated Ag2S nanocluster, (8) HREencapsulated TiO2 nanocluster.
the nanocluster surface, and cluster Rf values. Control 1 (strip 1) shows a positive result from a multiantigenic peptide containing 16 repeats of the HRE. Controls 2 and 3 (strips 2, 3) show negative controls for a simple histidine encapsulated and a glutathione (γ-ECG) encapsulated ZnS cluster, respectively. These controls indicate that neither the nanocluster core nor alternate peptide coats are recognized by the antibody. Positive results for 1-3 suggest that the conformation of the HRE peptide along the surface of the nanocluster is in a conformation that is readily recognized by the antigen binding site of the antibody. With the exception of limited reports of individual amino acid stabilized nanoclusters, the only peptide-encapsulated nanoclusters previously studied in any detail have been thiol-coordinating phytochelatinencapsulated CdS and ZnS nanoclusters. A theoretical model of phytochelatin chelation to a CdS surface has suggested that the coordination modes of the ligands keep them in a generally extended conformation uniformly distributed along the nanocluster surface with those side chains not involved in coordination directed toward the bulk solvent.28 Given the size of these CdS nanoclusters (2.2 nm), such a model results in a nanocluster encapsulated by 22 7-mer peptides. These predictions have recently been experimentally confirmed.26 From the experimentally determined size of 1 and the possible coordination modes for zinc-histidine, an analogous compositional model suggests that 22-27 HREs envelop the cluster’s surface. When compared to the 16 complete HREs or the 51 AHH repeats within HRP II, it is not surprising that, assuming the peptides are in an appropriate conformation, the surface of the peptide-encapsulated nanocluster elicited a strong monoclonal antibody response. Similar arguments may be extended to the HRE surface stabilization of the Au0 (2) and Ag0 (3) clusters. Although the majority of recent work has focused on colloidal metal clusters stabilized by n-alkanethiols,30-32 it has been shown 171
that the interactions of amines with zerovalent metals can yield stable colloids. Au0 clusters have been previously stabilized by cationic polyelectrolytes containing ammonium functional groups.33 Leff et al. demonstrated that soluble gold clusters could be stabilized by n-alkylamines.34 Recently, poly(amidoamine) (PAMAM) dendrimers, and in particular G-2 and G-4, have been used to surface passivate Au0 nanoclusters.35 In the latter two examples, an initial combination of electrostatics and/or coordination of the amine to the metal ion followed by reduction resulted in a cluster surface capped by the amine ligand. Similar surface chemistry for the HRE-stabilized clusters 2 and 3 would produce a surface with multiple epitopes for antibody recognition, thereby explaining the positive responses to the antigen capture assay. There are two possible interpretations for the negative antibody binding of clusters 4 and 5. The first possibility is that the surface coordination of the HRE along the surface of 4 and 5 is in a significantly different conformation so as not to be recognized by the antibody. Given the lack of crystallinity in 4, peptide stabilizing ligands likely reside in a variety of conformations stabilizing the cluster surface. Similarly, the monoclinic nature of Ag2S (5) provides a number of reasonable lattice spacings for ligand coordination, resulting in a nonrepeating surface of epitopes. The net effect is a non- or less-antigenic cluster surface. In contrast, the crystalline ZnS, Au0, and Ag0 clusters maintain structures that allow the peptide conformation to repeat itself, regularly along the surface.36 A second possibility is that the effective concentration of the less soluble 4 and 5 is below the detection limit of the assay. This could stem from either lower initial concentrations of resolubilized clusters or the aggregation and/or precipitation of the clusters on the test strip at a rate faster than they can be recognized by the antibody. Experiments are underway to evaluate these alternatives. The ability to exploit the recognition capabilities of biological systems is the inspiration for an increasing number of approaches to a wide variety of nanotechnology applications. Included among them are the assembly of extended mesoscale and multifunctional materials, biological imaging and diagnostic applications, and biomolecular electronics. Inspired by the use of histidine-rich proteins in biomineralization systems, we have employed an epitope from the HRP II of P. falciparum to stabilize a variety of nanoclusters. Using a peptide ligand, it is possible in a simple “one-pot” reaction to increase the surface functionality of a nanocluster without resorting to more standard ligand exchange approaches. While ligand exchange techniques have been previously used to functionalize nanoparticles, they require multiple steps yielding mixed, ill-defined surface compositions with diminished properties relative to unmodified clusters.37 Monoclonal antibodies raised against the AHHAHHAAD peptide sequence recognized the histidine-rich epitope on crystalline nanocluster surfaces. The implication of this observation is that by incorporating biologically active antigen epitopes along nanocluster surfaces, it is possible to synthesize immunoreactive nanoclusters. The recognition of antigen encapsulated nanoclusters by antibodies demonstrates 172
novel possibilities for highly selective antibody-based selfassembled materials and diagnostic agents. Acknowledgment. We wish to thank Dr. Sandra J. Rosenthal for helpful discussions and Becton Dickinson for the donation of the ParaSight F antigen capture test kits. This research was supported by NSF-CAREER (CHE0093829) and Vanderbilt University. Supporting Information Available: The complete amino acid sequence of HRP II from P. falciparum; HPLC trace and mass spectral data for the prepared peptide HRE; fluorescence spectra of cluster 1-5; composite IR of HREencapsulated nanoclusters 1-5; powder XRD scans of clusters 1-5 including patterns of the appropriate standard phases; TEM micrographs and histograms of metal particle sizes of clusters 1-3; EDS scans of crystalline samples 1-5. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Biomimetic Materials Chemistry; Mann, S., Ed.; VCH Publishers: New York, 1996. (2) Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Reike, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 1098-1105. (3) Magnetite Biomineralization and Magnetoreception in Organisms: A New Biomagnetism; Kirschvink, J. L., Jones, D. S., Macfadden, J. B., Eds.; Plenum: New York, 1985. (4) Klaus, T.; Joerger, R.; Olsson, E.; Granqvist, C.-G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 13611-13614. (5) Dameron, C. T.; Reese, R. N.; Mehra, R. K.; Kortan, A. R.; Carroll, P. J.; Steigerwald, M. L.; Brus, L. E.; Winge, D. R. Science 1989, 338, 596-541. (6) Srinivasarao, M. Chem. ReV. 1999, 99, 1935-1961. (7) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Maweni, G. J. Am. Chem. Soc. 2000, 122, 12142-12150. (8) Sondi, I.; Siiman, O.; Koester, S.; Matijevic, E. Langmuir 2000, 16, 3107-3118. (9) Mamedova, N. N.; Kotov, N. A.; Rogach, A. L.; Studer, J. Nano Lett. 2001, 1, 281-286. (10) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849-1862 and references therein. (11) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665-668. (12) Brown, S.; Sarikaya, M.; Johnson, E. J. Mol. Biol. 2000, 299, 725735. (13) Whitling, J. M.; Spreitzer, G.; Wright, D. W. AdV. Mater. 2000, 12, 1377-1380. (14) Brown, S. Nano Lett. 2001, 1, 391-394. (15) Brewer, D.; Lajoie Rapid. Commun. Mass Spec. 2000, 14, 17361745. (16) Wellems, T. E.; Howard, R. J. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 6065-6069. (17) Morgan, W. T. Biochemistry 1985, 24, 1496-1501. (18) Waite, J. H.; Rice-Ficht, A. C. Biochemistry 1989, 28, 6104-6110. (19) Voss-Foucart, M. F.; Fonze-Vignaux, M. T.; Jeuniaux, C. Biochem. Syst. 1973, 1, 119-122. (20) Sullivan, D. J., Jr.; Gluzman, I. Y.; Goldberg, D. E. Science 1996, 271, 219-222. (21) Bryan, G. W.; Gibbs, P. E. J. Mar. Biol. Assoc. U. K. 1979, 59, 969-973. (22) Ziegler, J.; Chang, R. T.; Wright, D. W. J. Am. Chem. Soc. 1999, 121, 2395-2400. (23) Note: All clusters were synthesized under anaerobic conditions in the dark. Synthesis of 1 was based on ref 24. A solution of ZnSO4 (2 µmol) in 0.010 M HCl was added to an aqueous solution of peptide (2 µmol) and diluted to a volume of 3.00 mL with Tris buffer (0.10 M, pH 8.6). After 15 min, an aqueous solution of Na2S (2 µmol) Nano Lett., Vol. 2, No. 3, 2002
was added dropwise and the reaction continued for 4 h. Cluster products were isolated and purified by repeated precipitation with cold absolute ethanol. Syntheses of 2 and 3 were based on the methods of Burst, M.; Fink, J.; Bethel, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656 and Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 1978, 790-799, respectively. Briefly, an aqueous solution of HAuCl4 (2 µmol) was slowly added to an aqueous solution of peptide (2 µmol) and the volume adjusted to 3 mL. After 15 min, the reductant was added (10 µmol sodium citrate for 2, 20 µmol sodium borohydride for 3), immediately turning the solution a deep purple/brown. The reaction was continued for 4 h. Cluster isolation and purification was as for 1. The condensation synthesis of 4 was based on Chemseddine, A. In Characterization of Nanophase Materials, Wang, Z. L., Ed., Wiley-VCH: New York, 2000; pp 315-352. An absolute ethanolic solution of Ti((CH3)2CHO)4 was added dropwise to an aqueous solution of peptide (2 µmol) and the volume adjusted to 3.0 mL with Tris buffer (0.10 M, pH 8.6). The solution became cloudy. The reaction was stirred for 4 h and products isolated as above. Cluster 5 was synthesized using modifications of the method of Brelle, M. C.; Zhang, J. Z.; Nguyen, L.; Mehra, R. K. J. Phys. Chem. A 1999, 103, 10194-10201. A solution of AgNO3 (2 µmol) in 0.010 M HCl was added to an aqueous solution of peptide (2 µmol) and diluted to a volume of 3.00 mL with Tris buffer (0.10 M, pH 8.6). After 15 min, an aqueous solution of Na2S (1 µmol) was added dropwise and the reaction continued for 4 h. Cluster isolation was as above. (24) Kho, R.; Nguyen, L.; Torres-Martinez, C. L.; Mehra, R. K. Biochem. Biophys. Res. Commun. 2000, 27, 29-35. (25) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. J. Chem. Phys. 2001, 115, 998-1008. (26) Materials characterization: Nanocomposite materials were characterized by powder X-ray diffraction (XRD) scans that were obtained using a Scintag X1 θ/θ automated powder diffractometer with a Cu target, a Peltier-cooled solid-state detector, and a zero-background, Si(510) sample support. For particle size determinations, each XRD scan was corrected for background scattering and was stripped of the KR2 portion of the diffracted intensity using the DMSNT software (version 1.30c) provided by Scintag. Observed peaks were fitted with a profile function to extract the full-width-at-half-maximum (fwhm) values. Average crystallite size, L, was calculated from Scherrer’s equation, L ) Kλ/βcosθB, assuming that peak broadening arises from size effects only (where β is the peak at fwhm measured in radians
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(27) (28) (29) (30)
(31) (32) (33) (34) (35) (36)
(37)
on the 2θ scale, λ is the wavelength of X-rays used, θB is the Bragg angle for the measured hkl peak, and K is a constant equal to 0.90 for L taken as the volume-averaged crystallite dimension perpendicular to the hkl diffraction plane) according to H. P. Klug and L. E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 5th ed.; Wiley: New York, 1974. Clusters were also examined using a Phillips CM20T transmission electron microscope (TEM) operating at 200 kV. Samples for TEM were prepared by putting one drop of a nanocomposite/CH2Cl2 suspension onto a 3 mm diameter copper grid covered with holey carbon film as a substrate and allowing the solvent to evaporate. Particle size distributions were obtained by manually measuring particle diameters from the bright-field micrographs obtained. Uguen C.; Rabodonirina M.; De Pina J. J.; Vigier J. P.; Martet G.; Maret M.; Peyron F. Bull. WHO 1995, 73, 643-649. Dameron, C. T.; Dance, I. G. In Biomimetic Materials Chemistry; Mann, S., Ed.; VCH Publishers: New York, 1996; pp 69-91. Spreitzer, G.; Whitling, J. M.; Madura, J. D.; Wright, D. W. J. Chem. Soc., Chem. Commun. 2000, 209-210. Terrill, R. H.; Postlethwaite, T. A.; Chen, C. H.; Poon, C.-D.; Hutchinson, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548. Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772. Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. F. Science 1996, 273, 1690-1693. Bonnemann, H.; Brijoux, W.; Brinkman, R.; Dinjus, E.; Joussen, T.; Korall, B. Angew. Chem., Int. Ed. Engl. 1991, 30, 1312-1314. Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 47234730. Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256-258. Crystallographic data from WWW-MINICRYST, an information calculating system on crystal structure data for minerals supported by the Russian Foundation of Basic Research (grants 96-07-89162, 96-07-89323, 01-07-90052). Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12886-12887.
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