Structural study of the copper(II)–enrofloxacin metallo-antibiotic

Inorganica Chimica Acta 382 (2012) 186–190

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Structural study of the copper(II)–enrofloxacin metallo-antibiotic Hussein Ftouni, Stéphanie Sayen, Stéphanie Boudesocque, Isabelle Dechamps-Olivier, Emmanuel Guillon ⇑ Institut de Chimie Moléculaire de Reims (ICMR), UMR CNRS 6229, Université de Reims Champagne-Ardenne, Faculté des Sciences, B.P. 1039, F-51687 Reims Cedex 2, France

a r t i c l e

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Article history: Received 26 September 2011 Received in revised form 5 December 2011 Accepted 8 December 2011 Available online 14 December 2011 Keywords: Copper Antibiotic Enrofloxacin Complex EXAFS XANES

a b s t r a c t The structure of [Cu(erx)2(H2O)2] complex was assessed using copper K-edge X-ray absorption spectroscopy (XAS) through X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) combined with EPR spectroscopy. A Jahn–Teller distorted octahedra was evidenced for Cu2+ geometry. Four equatorial oxygen atoms from two enrofloxacin ligands were found with Cu– Oeq distances of 1.93 Å (±0.02). The coordination was completed by two water molecules in axial position at an average distance of 2.11 Å. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Quinolones are a family of broad-spectrum synthetic antibiotics. The quinolone(s) term refers to potent synthetic chemotherapeutic antibacterials [1,2]. Due to the emergence of microbial resistance that results from the bacterial adaptations, a derivative class was introduced, named fluoroquinolone [3]. Thus, the majority of quinolones in clinical use belongs to the subset of fluoroquinolones, which have a fluorine atom attached to the central ring system, typically at the C-6- or C-7 position. Enrofloxacin (erx, Fig. 1) is a second generation fluoroquinolone used in veterinary medicine for treatment of individual pets and domestic animals. Its mechanism of action is not thoroughly understood, but it is suspected to act by inhibiting bacterial DNA gyrase (a type-II topoisomerase), thereby preventing DNA supercoiling and synthesis. In September 2005, the FDA withdrew approval of enrofloxacin for use in water to treat flocks of poultry, as this practice was noted to promote the evolution of fluoroquinolone-resistant strains of the bacterium Campylobacter, a human pathogen [4]. Thus, an alternative is necessary. In that way, it was shown that metallo-antibiotics are very stable at physiological pH and they seem to be a good approach for the development of drugs with similar activity against bacteria

⇑ Corresponding author. E-mail address: [email protected] (E. Guillon). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.12.012

but with the possibility of lowering their level of resistance [5]. Quinolones can bind several metallic cations, including Mg2+, Ca2+, Mn2+, Fe2+/3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Al3+ [6,7], which may result in change in their activity. Mg2+ and Al3+ complexes were found to decrease the drugs activity [8], whereas Fe3+ and Zn2+ ones were found to exhibit higher activities [9]. The crystal structures of the Ni2+ and Cu2+ cinoxacin and ciprofloxacin complexes have been solved, in which the metal is found to be bound to the a-carboxylketo moiety to form 1:2 metal:drug complexes [7]. The complexes have a pseudo-axial symmetry with the two drug ligands bound symmetrically in equatorial positions. More recently, the formation of Cu2+(erx)(1,10-phenantroline) ternary complexes was underlined [10]. A recent theoretical study suggested that metal binding to these drugs is associated with the action of these drugs. Fluorescence quenching measurements indicate the presence of a p–p stacking which has been suggested to be associated with the DNA intercalation capacities of the drugs and their Cu2+ complexes [11]. Recently, we have reported the zinc and copper complexes of the flumequine antibiotic which is a first generation antibacterial fluoroquinolone [12]. The coordination occurs through the carbonyl and carboxylate oxygen atoms from two flumequine molecules in the equatorial plane. The coordination sphere is completed by two water molecules in axial position. To our knowledge, no structural information of the Cu2+(erx)2 complex (without other organic ligand) was reported in the literature, in spite of numerous Zn and Ni with enrofloxacin as well as other Cu-quinolonato complexes [13,10,14,15]. Efthimiadou et al. [10]

H. Ftouni et al. / Inorganica Chimica Acta 382 (2012) 186–190 O

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2.2. Structural characterization

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The simulation of the EPR spectra was performed using X-sophe software version 1.1.4 for Mandriva 2006 x86-64 developed by the centre for Magnetic Resonance and the Department of Mathematics of the University of Queensland, Brisbane, Australia, for Bruker Biospin GmbH [16]. The software uses a linewidth model with an angular dependence of g and a simplex optimization method with the copper element in an natural abundance.

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Fig. 1. (a) Enrofloxacin, 1-cyclopropyl-7-(4-ethyl-piperazin-1-yl)-6-fluoro-4-oxo1,4-dihydro-quinoline-3-carboxylic acid: erx, (b) proposed Cu(erx)2(H2O)2 structure.

reported that the lack of such information prevents a detailed description of its reactivity. Indeed, they reported the possible formation of aggregates. Nevertheless, the antimicrobial activity of the Cu2+(erx)2 complex, which was tested on three different microorganisms, provided the best inhibition against Escherichia coli and Pseudomonas aeruginosa. Thus, the aim of this paper is to gain new insight into the structural information of the Cu2+(erx)2 complex by combining EPR spectroscopy and X-ray absorption spectroscopy (XAS). We report here the synthesis and characterization of the [Cu(erx)2(H2O)2] complex.

2. Experimental 2.1. Synthesis of [Cu(erx)2(H2O)2] An ethanol–water solution (% v/v = 40/60, 5 mL) of Cu(NO3)2, (H2O)3 (13 mg, 0.055 mmol) was slowly added dropwise at room temperature to a magnetically stirred aqueous solution (10 mL) of ligand erx (40 mg, 0.111 mmol) in presence of KOH (0.111 mmol). After 2 h shaking, a pale green solution was formed, then filtered and washed with an ethanol–water solution to remove any pure solid as microcrystalline powder. Copper percentage was determined using ICP-AES (Varian Liberty Series II) by dissolving around 10 mg in 25 mL distilled water. Calibration was performed using a copper standard solution at 100 ppm. Anal. Calc. for C38H46F2N6O8Cu: C, 55.86; H, 5.63; N, 10.29. Found: C, 55.68; H, 5.62; N, 10.36%. ICP-AES, calc.: 7.78%, found: 7.49%.

2.2.1. XAS data collection XANES and EXAFS spectra were collected at the ELETTRA Synchrotrone (Trieste, Italy) on the Beamline 11.1 of the storage ring operating at 2/2.4 GeV with an optimal storage beam current around 300/130 mA. The measurements were carried out at the copper K-edge (8979 eV) and the incident beam was monochromatized with a Si(1 1 1) double crystal. The energy was calibrated by measuring the absorption edge of a copper metallic foil (fixed at 8979 eV). The detectors were low-pressure (0.2 atm) air-filled ionization chambers and the measurements were performed at room temperature in the transmission mode. The XANES spectrum was the sum of three recordings in the 8920–9080 eV range. The EXAFS spectrum was the sum of three recordings in the 8830– 9830 eV range. The spectra were recorded using sampling steps of 0.2 eV (XANES) and 2 eV (EXAFS), and an integration time of 2.0 s per point was used. The spectrum of the 0.5 lm metallic foil was recorded just before the unknown sample XANES spectrum to check the energy calibration. Spectra were measured on the wellpowdered sample mixed with boron nitride to give a sample thickness of ca. 1 mm, placed in an aluminium sample holder and held in place by Kapton tape. 2.2.2. XAS data analysis EXAFS data analysis was performed using the ‘‘Multi-Plateform Applications for X-ray Absorption’’ package, including Cherokee and Roundmidnight programs [17]. The v(k) function was extracted from the data with a linear pre-edge background, a polynomial atomic-absorption background, and was normalized using the Lengeler–Eisenberg method [18]. The extraction procedure was conducted as previously described [19]. The energy threshold, E0, was taken at the middle of the absorption edge, and was corrected in the fitting procedure. The k3 weighted v(k) function was Fourier transformed in the 2–14 Å1 k range, by means of a Kaiser–Bessel window with a smoothness parameter s equal to 2.5 (k is the photoelectron wave number). The peaks corresponding to the two first coordination shells were then isolated and back-Fourier transformed into k space to determine the mean coordination number, N, the bond length, R, and the Debye–Waller factor, r2, by a fitting procedure realized in the framework of single scattering. The estimated errors for distances and coordination numbers are ±0.02 Å and ±20%, respectively. The S20 value was taken equal to 1 and kept constant. The backscattering phase, Ui(k, Ri), and amplitude, Ai(k, Ri), functions were obtained using the ab initio FEFF 7 code [20], with input data based on the X-ray crystal structure of CuO [21] for the Cu–O wave and of Cu(cyclam)(ClO4)2 [22] for the Cu–C wave. 3. Results and discussion The reaction of Cu(NO3)2,(H2O)3 with enrofloxacin in 1:2 molar ratio in the presence of KOH in ethanol–water, at room temperature, for 2 h yielded to a pale green microcrystalline solid. Unfortunately, all attempts to obtain well-formed crystals suitable for X-ray determination have failed, thus the complex was structurally characterized by using EPR spectroscopy combined with

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B (G) Fig. 2. Experimental and simulated EPR spectra of Cu(erx)2(H2O)2. Recording conditions: 1 mmol L1, T = 150 K, modulation frequency: 100 kHz, modulation amplitude: 6.684 G, time constant: 81.92 ms, and sweep time: 167.7 s.

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Fig. 3. Normalized XANES spectrum of Cu(erx)2(H2O)2 and its first derivative.

X-ray absorption spectroscopy (XAS). No hyperfine splitting was resolved by EPR in the powder sample due to line broadening caused by the anisotropy of the g factors and hyperfine interactions and by the interaction of the Cu2+ centres with neighbouring spins. Thus, the EPR spectrum was recorded in a 50/50 DMSO/CHCl3 solution at 150 K (Fig. 2). The frozen solution spectrum displayed a typical anisotropic Cu2+ signal with four lines in parallel region arising from the hyperfine coupling of the S = 1/2 electron spin of Cu2+ with its I = 3/2 nuclear spin. The spectrum was consistent with an axial Cu2+ centre (gk > g\ > 2.0). The gk value and Ak constant could be experimentally determined, whereas the perpendicular parameters could not be determined because of poor resolution in this region. Thus, the parameters were determined by simulation of the EPR spectrum using the XSOPHE software version 1.1.4 [16]. The obtained values gk = 2.358, Ak = 144.3  104 cm1, g\ = 2.073, A\ = 10.7  104 cm1, and gk/Ak = 163 cm, consistent with a dx2–y2 ground state for the Cu2+ ion, were in the range expected for mononuclear Cu2+ carboxylate complexes

that contain the CuO4  O2 chromophore in a tetragonally distorted octahedral geometry [23,24]. Thus, we can propose (confirmed by XAS) in the complex, two carboxylate and two pyridone oxygen atoms from two enrofloxacin molecules coordinated in Cu2+ equatorial plane, whereas two water molecules weakly interact in the axial positions to form the CuO4  O2 chromophore. As the [Cu(erx)2(H2O)2] could not be obtained as well-formed crystals, X-ray absorption spectroscopy was used as an alternative direct method to probe the electronical and structural nature of the metal site. Firstly, X-ray absorption near edge structure (XANES) spectrum recording was used to probe the copper geometry. Fig. 3 shows the Cu K-edge XANES spectrum of [Cu(erx)2(H2O)2] and its first derivative. The spectrum exhibited a very weak pre-edge feature at 8977.4 eV, which corresponds to the 1s ? 3d transition. The very weak intensity of this pre-edge was consistent with an octahedral symmetry. Indeed, for this point group, a very weak pre-edge feature is observed although theoretically

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forbidden by the symmetry centre. This is because the point group represents a static model derived by time-averaging the asymmetric vibrations within the molecule, whereas XAS reacts to the individual and constantly changing structures of the local environment as intramolecular vibrations, which momentarily eliminate the symmetry centre. In addition to the pre-edge peak, there was a shoulder in the low energy side of the edge (8984.0 eV) due to 1s ? 4pz and shakedown electron transitions, which revealed a tetragonal distortion [25]. The first derivative of the XANES spectrum (Fig. 3, right part) revealed detailed information which was not directly obvious in the XANES spectrum. A splitting of the main peak into 1s ? 4pz transition (a, 8984.0 eV) and 1s ? 4px,py transitions (b, 8988.7 and 8992.1 eV) is observed. The XANES region is sensitive to changes in the ligand environment and provides an independent measure for estimating how electron transitions and multiple scattering are influenced by the complexation, particularly with regard to the relative intensity of the two a and b peaks. The decrease in a peak intensity, compared to the one of Cu(H2O)62+ [19], shows that the Cu2+ coordination environment depends on the ligand field. Actually, a decrease in the a peak intensity is expected because the water molecules field undergoes ligand exchanges with a different organic moieties field (carboxylic acid and pyridone moiety) in the equatorial plane. Moreover, the XANES region also contains information about electron transitions. It was found that the a peak intensity is influenced by the degree of axial distortion [26] and by the covalence of the equatorial ligands bound to the Cu2+ atom [27]. The source of the a electron transition has been attributed to a shakedown effect, where the final electron

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-2 Table 1 Structural data of the two first coordination shells of Cu(erx)2(H2O)2 obtained from fitting of the EXAFS spectruma; N number of neighbours, R absorber-neighbour distance, r2 Debye–Waller factor; uncertainties are estimated in coordination numbers to ±20%, in R to ±0.02 Å, and in r2 to ±0.001 Å2.

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Cu–O Cu–O Cu–C Cu–C Cu–O Cu–C

3.60 1.66 4.43 4.38 2.45 3.69

1.93 2.11 2.85 3.24 3.68 3.97

0.0024 0.01b 0.0068 0.0071 0.01b 0.01b

5.67 3.60 4.77 0.30 5.55 1.83

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R (Å) Fig. 4. Raw k3-weighted EXAFS spectrum and its corresponding Fourier transform (uncorrected for phase shift) of Cu(erx)2(H2O)2.

state is lower in energy than the direct 1s ? 4p transition [27]. The path to the final state involves a 1s ? 4pz excitation combined with a ligand-to-metal charge transfer [28]. As demonstrated by the Cu(H2O)62+ spectrum [19], water molecules coordinated in the equatorial plane produce a strong a peak intensity. When comparing the XANES spectra derivatives of [Cu(erx)2(H2O)2] and Cu(H2O)62+, the diminished a peak for the former suggested that enrofloxacin was more sterically hindered due to its three-dimensional structure. Indeed, Cu2+ cannot bond to enrofloxacin in the equatorial plane with the same degree of angular overlap as water. Such steric hindrance would affect the ability of the ligand to

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transfer charge to the metal when core electrons are removed in 1s ? 4pz transition. Fig. 4 shows the raw k3-weighted EXAFS spectrum and the corresponding Fourier transform of the [Cu(erx)2(H2O)2] complex in solid state. The EXAFS exhibited the declining amplitude envelope and simple sinusoid of single shells of low-Z scatterers. In its turn the nonphase-corrected Fourier transform spectrum was dominated by a single first shell peak at 1.53 Å. Two small features appearing between about 2.0 and 4.0 Å principally reflected firstshell multiple scattering and/or second-shell single scattering. Nevertheless, FEFF calculations underlined that multiple scattering was negligible in this part. The experimental and least-squares fitted EXAFS spectra corresponding to the filtered two first shells are shown in Fig. 5. They are plotted together with their corresponding Fourier transform. The peak positions in the radial structure function (RSF) correspond to relative distances (uncorrected for phase shift) between the metallic cation and atoms in local coordination shells. Table 1 displays the EXAFS linear least-squares fitting results obtained from FEFF calculations within the framework of the single scattering approach. The strongest peak, which appears around 1.53 Å in Fig. 5 on the Fourier transform, corresponds to first-shell O atoms. This first shell was fitted with four Cu–O contributions coming from the carbonyl and carboxylic moieties of two enrofloxacin ligands with an average distance of 1.93 ± 0.02 Å (Table 1). Two other Cu–O contributions with a distance of 2.11 ± 0.02 Å completed the first coordination shell of the metallic cation and confirmed the tetragonally distorted octahedral geometry evidenced by XANES and EPR spectroscopy. This hexacoordination was achieved by two water molecules in accordance with elemental analysis. The other peaks at higher R values were attributed to carbon and oxygen atoms from enrofloxacin molecules. The results of the least-squares fitting, reported in Table 1, revealed a first mean metal–C distance (four carbon atoms) of 2.85 Å, a second average metal–C distance (four carbon atoms) of 3.24 Å, a mean metal–O bond length (two oxygen atoms) of 3.68 Å, and a last metal–C contribution (four carbon atoms) at a mean distance of 3.97 Å. These parameters allowed us to propose a tetragonally distorted octahedral structure for the [Cu(erx)2(H2O)2] complex, in which two enrofloxacin molecules are bidentates to the Cu2+ cation in the equatorial plane through carboxylate and carbonyl oxygen atoms. The axial positions are taken up by two water molecules. This geometry disagrees with the one proposed by Efthimadiou and co-workers [10] who proposed, on the basis of UV–vis spectroscopy, that the copper atom was fivecoordinated and could be described as having a slightly distorted square pyramidal geometry. However, such octahedral structures have been reported for other similar antibiotic–copper complexes based on ciprofloxacin [25] or flumequine [12]. In both cases, the copper complex has a distorted octahedral geometry with Cu–Oeq bond distances around 1.92 Å and a higher axial distance (around 2.25 Å). 4. Conclusion In summary, a Cu2+–enrofloxacin complex has been synthesized and fully characterized by combining XAS and EPR studies. The complex is held in a tetragonally distorted octahedral geometry, wherein the copper centre is bridged by two carboxylic and pyridone moieties from two enrofloxacin molecules in equatorial position. The coordination sphere is completed by two water molecules

in axial position. This new result could allow to gain new insight into the very high enrofloxacin antimicrobial activity against E. coli and P. aeruginosa [10]. Acknowledgements The ELETTRA Synchrotrone is acknowledged for providing beamtime at the XAS Beamline. The authors thank Drs. Luca Olivi and Andrea Cognigni (Beamline Staff) for their help in XAS data recording and helpful discussions. References [1] J.M. Nelson, T.M. Chiller, J.H. Powers, F.J. Angulo, Clin. Infect. Dis. 44 (2007) 977. [2] D.V. Ivanov, S.V. Budanov, Antibiot. Khimioter. 51 (2006) 29. [3] M. Martinez, P. McDermott, R. Walker, Vet. J. 172 (2006) 10. [4] FDA statement on withdrawal of Baytril for use in poultry. Available from: [5] P. Gameiro, C. Rodrigues, T. Baptista, I. Sousa, B. de Castro, Int. J. Pharm. 334 (2007) 129. [6] (a) K. Yukinori, M. Kyuichi, H. Hideo, Chem. Pharm. Bull. 44 (1996) 1425; (b) G. Mendoza-Diaz, R. Perez-Alonso, R. Moreno-Esparza, J. Inorg. Biochem. 64 (1996) 207; (c) P.B. Issopoulos, Analyst 114 (1989) 627; (d) D.S. Lee, H.J. Han, K. Kim, W.B. Park, J.K. Cho, J.H. Kim, Pharm. Biomed. Anal. 12 (1994) 157; (e) I. Turel, N. Bukovec, Polyhedron 15 (1996) 269; (f) I. Turel, Coord. Chem. Rev. 232 (2002) 27. [7] (a) I. Turel, I. Leban, N. Bukovec, J. Inorg. Biochem. 56 (1994) 273; (b) M. Ruiz, R. Ortiz, L. Perello, J. Latorre, J. Server-Carrio, J. Inorg. Biochem. 65 (1997) 87; (c) B. Macias, M.V. Villa, I. Rubio, A. Castineiras, J. Borras, J. Inorg. Biochem. 84 (2001) 163. [8] (a) G. Höffken, K. Borner, P.D. Glatzel, P. Koeppe, H. Lode, Eur. J. Clin. Microbiol. Infect. Dis. 4 (1985) 345; (b) I. Turel, A. Sonc, M. Zupancic, K. Sepcic, T. Turk, Met. Based Drugs 7 (2000) 101. [9] F. Gao, P. Yang, J. Xie, H. Wang, J. Inorg. Biochem. 60 (1995) 61. [10] E.K. Efthimiadou, Y. Sanakis, M. Katsarou, C.P. Raptopoulou, A. Karaliota, N. Katsaros, G. Psomas, J. Inorg. Biochem. 100 (2006) 1378. [11] J. Robles, J. Martin-Polo, L. Alvarez-Valtierra, L. Hinojosa, G. Mendoza-Diaz, Met. Based Drugs 7 (2000) 301. [12] D. Perez-Guaita, S. Boudesocque, S. Sayen, E. Guillon, Polyhedron 30 (2011) 438. [13] R. Saraiva, S. Lopes, M. Ferreira, F. Novais, E. Pereira, M.J. Feio, P. Gameiro, J. Inorg. Biochem. 104 (2010) 843. [14] J. Recillas-Mota, M. Flores-Alamo, R. Moreno-Esparza, J. Gracia-Mora, Acta Crystallogr., Sect. E 63 (2007) M3030. [15] E.K. Efthimiadou, Y. Sanakis, M. Katsarou, C.P. Raptopoulou, A. Karaliota, N. Katsaros, G. Psomas, Polyhedron 27 (2008) 1729. [16] (a) M. Griffin, A. Muys, C. Noble, D. Wang, C. Eldershaw, K.E. Gates, K. Burrage, G.R. Hanson, Mol. Phys. Rep. 26 (1999) 60; (b) G.R. Hanson, K.E. Gates, C.J. Noble, M. Griffin, A. Mitchell, S. Benson, J. Inorg. Biochem. 98 (2004) 903. [17] A. Michalowicz, J. Moscovici, D. Muller-Bouvet, K. Provost, J. Phys. Conf. Ser. 190 (2009) 012034. [18] B. Lengeler, P. Eisenberg, Phys. Rev. B 21 (1980) 4507. [19] E. Guillon, P. Merdy, M. Aplincourt, Chem. Eur. J. 9 (2003) 4479. [20] S.I. Zabinsky, J.J. Rehr, A.L. Ankoudinov, R.C. Albers, M.J. Eller, Phys. Rev. B 52 (1995) 2995. [21] G. Tunell, E. Posnjak, C.J. Ksanda, J. Wash. Acad. Sci. 23 (1933) 195. [22] P.A. Tasker, L. Sklar, J. Mol. Struct. 5 (1975) 329. [23] R.G. Bhirud, T.S. Srivastava, Inorg. Chim. Acta 173 (1990) 121. [24] M. Devereux, D.O. Shea, M.O. Conner, H. Grehan, G. Conner, M. McCann, G. Rosair, F. Lyng, A. Kellett, M. Walsh, D. Egan, B. Thati, Polyhedron 26 (2007) 4073. [25] I. Turel, L. Golic, P. Bukovec, M. Gubina, J. Inorg. Biochem. 71 (1998) 53. [26] J. Garcia, M. Benfatto, C.R. Natoli, A. Bianconi, A. Fontaine, H. Tolentino, Chem. Phys. 132 (1989) 295. [27] L.S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahn, K.O. Hodgson, E.I. Solomon, J. Am. Chem. Soc. (1987) 6433. [28] R.A. Blair, W.A. Goddard, Phys. Rev. B 22 (1980) 2767.