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The role of bivalent metals in hydroxyapatite structures as revealed by molecular modeling with the HyperChem software Izabela Gutowska,1 Zygmunt Machoy,1 Bogusław Machaliński2 Department of Biochemistry and Chemistry, Pomeranian Medical University, Szczecin, Poland. 2 Department of General Pathology, Pomeranian Medical University, Szczecin, Poland 1 Received 22 March 2005; accepted 20 April 2005 Published online 1 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.30511 Abstract: Hydroxyapatite, the major component of bone, demonstrates significant reactivity with metals. Knowledge of spatial structure and energy data of the molecule helps understand the binding of metals by hydroxyapatite and elucidate the chemical and physical properties of such complexes. We used HyperChem software (Hypercube Inc.) to analyze the structure of hydroxyapatite when the central calcium atom is replaced by one of the metal ions (Mg, Cu, Zn, Fe, Cr, Mn) marked by us in bone. Our results show that hetero-ionic exchange affects composition and leads to deformation of hydroxyapatite crystals. Replacement was ac- companied by changes in bond lengths between oxygen and calcium atoms in the hydroxyapatite molecule and by displacement of groups of atoms surrounding the central calcium atom. The use of molecular modeling as a computational tool enabled a preliminary and theoretical understanding of chemical structure without the need for laboratory tests. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res 75A: 788 –793, 2005 INTRODUCTION Ca10(PO4)6(OH)2 or 3Ca3(PO4)3 䡠 Ca(OH)2. Spatially, hydroxyapatite is a triangular structure made up of a central Ca(OH)2 and three surrounding Ca3(PO4)3 groups.2 Hydroxyapatite crystals are encased in an aqueous envelope that sticks to the crystals thanks to polarized and hydratized calcium and phosphate ions. This envelope plays an important role during exchange of ions between the hydroxyapatite crystal and extracellular fluid. Ion exchange depends on ion type, size, and site in bone structure. Potassium and chlorine ions of the aqueous envelope undergo rapid and total displacement by ions present in the extracellular space. Exchange is slower for magnesium, sodium, and carbonate and is slowest in the case of calcium and phosphate.3 In comparison to ions located in the crystal core, those in external layers are much more predisposed to substitution.3,4 Much less is known about substitution of the central Ca2⫹ atom. Okazaki5 is of the opinion that calcium may be substituted by magnesium and sodium, phosphate by carbonate and citrate and hydroxyl groups by fluoride and chloride. As molecular structure and function can nowadays be studied through model building and computation known as molecular modeling,6,7 we devised a theoretical study to elucidate replacement of the calcium atom by another bivalent metal. An important advan- Pollution of the natural environment with heavy metals is among the most concern-raising problems in contemporary ecology. Metal toxicity in human and animal organisms develops during accumulation, which principally targets hard tissues. On the molecular level, incorporation of metals into bones depends on interactions with hydroxyapatite, their chief structural component. Bone hydroxyapatite is a natural compound exhibiting significant reactivity to metals. This property is important in view of the role of bone in living organisms. Hexagonal cells located in outermost layers of hydroxyapatite and capable of binding other ions are called the “valency surplus” of bone. Additional valency is used not only to bind ions in the hydroxyapatite lattice but also for uptake of exogenous metals.1 The immense surface of hydroxyapatite predisposes its chemical components to displacement by other atoms with a similar atomic radius. The chemical formula of hydroxyapatite has been approximated in the middle of the 20th century as Correspondence to: Zygmunt Machoy; e-mail: IzaGut@ poczta.onet.pl © 2005 Wiley Periodicals, Inc. Key words: hydroxyapatite; structure spatial; bivalent metals substitution; molecular modeling HYDROXYAPATITE STRUCTURE MOLECULAR MODELING 789 Figure 1. Spatial model of the hydroxyapatite molecule with calcium as the central atom (Ca, big white; O, small black; P, small white; H, smallest black). tage of molecular modeling is that it respects the basic physical and chemical laws of the micro-world of molecules. Molecular modeling is the science and art of studying molecular structure and function through model building and computation.6 configuration. The next step is to minimize the chair structure by performing a molecular mechanics optimization. The optimization method is selected from a palette of ab initio and eight semi-empirical calculations.10 We chose ETH method developed by Hückl12 as a special case of the molecular orbital method with pi-electron approximation.10 Now we compare the structural properties of the minimized system with those of the model-built structure. MATERIALS AND METHODS Samples (10 mg) of powdered deer mandibular bone were dissolved in 65% nitric acid and the content of Mg, Ca, Cu, Zn, and Fe were determined with atomic absorption spectroscopy, whereas Cr and Mn were determined with inductively coupled plasma–atomic emission spectrometry. We used the HyperChem software8 –10 to determine the properties of molecules by analyzing energy parameters at various energies of the molecule and estimating differences between energy states of the system. This program can perform specific actions on selected atoms, residues, and molecules. HyperChem also enables conformational studies of the molecule through computations of the energy surface in relation to two spatial angles. For this purpose, HyperChem performs the so-called limited optimization to render conformations of the molecule, depending on the distribution of electron and spin densities, electrostatic potentials, electrostatic maps, and energy diagrams.11 An important feature of HyperChem is the ability to manipulate and compute properties of macromolecules, such as proteins and nucleic acids. With HyperChem, you can mix classical and quantum mechanical calculations in the same molecular system.8,9 The first step involved drawing of the molecule’s structural formula. Before we build the structure and perform a molecular mechanics optimization, we should choose a molecular mechanics force field provided with HyperChem. A force field contains atom types and parameters that must be assigned to the molecule before molecular mechanics calculation. We measure the structural properties of the model-built structure and compare this with geometry measurements from the optimized structure and perform the calculation to obtain the total energy of the unoptimized RESULTS Figures 1–3 display theoretical models of the bone hydroxyapatite molecule obtained by substituting the central calcium atom with other bivalent metals. Such substitution between calcium and magnesium in bone and tooth take place in living organisms indeed.13 Regardless of the substituting metal, however, Ca3(PO4)2 groups of natural hydroxyapatite remain intact. Symmetric distribution of calcium atoms around the central one was found by us only in the outermost layer of the hydroxyapatite molecule (Fig. 1), in accordance with the hydroxyapatite molecule model of Aurich.2 Substitution of the central calcium atom with another metal (Figs. 2 and 3) resulted in asymmetry, with hydroxyl groups positioned on one side of the central atom and the six external calcium atoms displaced contralaterally. Table I presents computed bond lengths between the central atom (Ca, Mg, Cu, Zn, Fe, Cr, Mn) and surrounding oxygens. Bond length was greatest in the case of natural hydroxyapatite (Fig. 1). Replacement of the central calcium atom by any of the six metals previously detected by us in bone hydroxyapatite produced shortening of the bond and spatial shrinking of the model. Information on chemical stability can be obtained 790 GUTOWSKA, MACHOY, AND MACHALIŃSKI Figure 2. Model of the hydroxyapatite molecule. Central Ca atom replaced by (a) Mg; )b) Cu; (c) Zn. by looking at molecular energy data. The lower is the total energy of the molecule (greater negative energy), the more stable is the hydroxyapatite complex. Table II shows that natural hydroxyapatite with calcium as the central atom is least stable among the hydroxyapatite complexes studied by us. This finding appears to be relevant to bone metabolism. One may expect that increasing chemical stability of hydroxyapatite will impair bone turnover and reduce the availability of calcium from its stores in the bone. Such an increase accompanies displacement of the central calcium atom by any of the six bivalent metals studied by us, stability being greatest in the case of Cu-hydroxyapatite. Theoretically, metals are incorporated into natural hy- droxyapatite by substitution of the central calcium atom. We assessed molecular stability by determining bond energy of the metal in the hydroxyapatite structure. Bond energies are shown in Table III. The greatest negative bond energy (⫺1447.4 kcal/mol) was obtained for zinc, whereas the positive bond energy of 1585.2 kcal/ mol calculated for manganese prohibits bond formation by this metal with the hydroxyapatite molecule. DISCUSSION There is a spate of biologically active compounds that contain metals. Often, metals form complexes and HYDROXYAPATITE STRUCTURE MOLECULAR MODELING Figure 3. 791 Model of the hydroxyapatite molecule. Central Ca atom replaced by (a) Cr; (b) Fe; (c) Mn. are positioned centrally, as in the case of heme with iron or chlorophyll with magnesium.14,15 Seven bivalent metals (Ca, Mg, Cu, Zn, Fe, Cr, Mn) were marked by us in mandibular bone of deer using mentioned analytical methods, and only these metals were included to the further investigation. HyperChem has already been applied to experimental studies of enamel crystals with the electron microscope,16,17 in research on synthetic hydroxyapatite with reduced calcium content,18 and in theoretical investigations on changes in properties of heme and chlorophyll in- duced by replacement of the central atom with another metal.14,15 We have now decided to use HyperChem for modeling of hydroxyapatite molecules containing bivalent metals previously detected by us among mineral components of bone. Calcium atoms of hydroxyapatite may be substituted by magnesium, lead, strontium, sodium,3 iron, copper, or manganese,19 whereas phosphate may be replaced by carbonate or hydroxyl anions. This property makes bone an ideal ion exchanger and a reservoir for many ions.3,4,20 792 GUTOWSKA, MACHOY, AND MACHALIŃSKI TABLE I Comparison of Bond Length between the Central Atom and Surrounding Oxygen Atoms in the Hydroxyapatite Molecule Bond length [Å] No. of atoms MgCuZnFeCrMnHydroxyapatite hydroxyapatite hydroxyapatite hydroxyapatite hydroxyapatite hydroxyapatite hydroxyapatite *(1)–O(2) *(1)–O(3) *(1)–O(4) *(1)–O(5) *(1)–O(6) *(1)–O(7) *(1)–O(8) from OH *(1)–O(9) from OH 2.4242 2.4259 2.4198 2.4244 2.4315 2.4297 2.0832 2.0788 2.1049 2.0845 2.1152 2.0990 1.9297 1.9154 2.0077 1.9419 2.0382 1.9702 1.9897 1.9758 2.0273 2.0028 2.0370 2.0181 1.9282 1.9136 2.0103 1.9413 2.0364 1.9705 1.9351 1.9213 2.0099 1.9481 2.0399 1.9746 1.8642 1.8956 1.9139 1.9468 1.9238 1.9378 2.4085 2.0362 1.8648 1.9437 1.8653 1.8744 1.8957 2.4111 2.0426 1.8789 1.9897 1.8781 1.8862 1.9276 *Central atom. Hetero-ionic exchange results in composition changes and may lead to spatial deformation of the hydroxyapatite crystal.1,3,21 Replacement of calcium by another metal (ion) produces deformation and alteration of bond lengths between calcium and oxygen atoms in the hydroxyapatite molecule (Table I)4,19 or displacement of atom groups surrounding the central atom (Figs. 1–3). Perhaps this property explains why hydroxyapatite of bones and teeth is susceptible to structural and crystallographic changes described in the literature.1 The symmetrical hexagonal structure of bone hydroxyapatite crystals may be subjected to stretching vibrations and it is presumed that the surface of expansion exerts a destructive effect on the symmetry and structure of growing crystals.22 Expansion takes place near the surface of the polarized layer containing deformed channels with OH⫺ groups and deformed tetrahedral molecules of phosphate.22 Chemical stability of compounds may be judged according to molecular energy data. Hydroxyapatites with lower total energy, i.e., those with greater negative energy, are more stable. Data in Table II show that replacement of calcium with any of the six metal ions studied by us increases stability, with copper forming the most stable complex. Substitution by magnesium is accompanied by a minimal change in the molecule’s energy. This finding confirms the key role of magnesium in stabilizing amorphous hydroxyapatite23 and preventing transition to the crystalline form. It also explains why calcium ions reduce the inhibitory effect of magnesium ions on growth of enamel crystals.24 These interactions were observed in human teeth.13,25 Admassu and Breese26 investigated the possibility of applying fish apatite to remove heavy metals from aqueous solutions. Binding to the hydroxyapatite matrix decreased in the following order: Pb, Zn, Cu, Cd, Ni, Mg. Out of the six metals in the present study, the zinc bond was found to possess the greatest negative energy. By displacing calcium, zinc appears to modify the rate of crystal growth.18 Calcium, copper, chromium, iron, and magnesium follow zinc in bond strength. Unlike previous reports,3,4,20 our calculations show that manganium does not form chemical bonds with the hydroxyapatite molecule. We speculate that manganium is attached in a different manner to the crystal surface and is not incorporated into its structure. Displacement of calcium by another ion (Mg, Zn, Fe, Cu, Mn, Cr) produces changes in bond lengths, deforms the crystalline structure, and affects chemical TABLE II Total Energy of the Complex (Complex Stability)a TABLE III Bond Energy of the Metal in the Complexa Complex Total Energy of the Complex [kcal/mol] Complex Bond Energy of the Metal in the Complex [kcal/mol] Hydroxyapatite Mg-hydroxyapatite Cu-hydroxyapatite Zn-hydroxyapatite Fe-hydroxyapatite Cr-hydroxyapatite Mn-hydroxyapatite ⴚ95159 ⫺95175 ⫺98219 ⫺96759 ⫺97266 ⫺96616 ⫺96761 Hydroxyapatite Mg-hydroxyapatite Cu-hydroxyapatite Zn-hydroxyapatite Fe-hydroxyapatite Cr-hydroxyapatite Mn-hydroxyapatite ⫺366.6 ⫺59.1 ⫺260.3 ⫺1447.4 ⫺59.4 ⫺178.4 1585.2 a The greater is the negative value of total energy, the more stable is the complex. a The greater is the negative value of total energy, the stronger is the bond energy of the metal in the complex. HYDROXYAPATITE STRUCTURE MOLECULAR MODELING stability of hydroxyapatite. Application of molecular modeling as a computation tool is useful for preliminary theoretical analysis of chemical compounds without resorting to laboratory testing. 12. 13. 14. The authors thank Department of Physical Chemistry, Technical University in Szczecin, for facilities of HyperChem molecular modeling program. 15. 16. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Ichijo T, Yamashita Y, Terashima T. Observations on structural features and charakteristics of biological apatite crystals. Vol. 7. Observation on lattice imperfection of human tooth and bone crystals. Bull Tokyo Med Dent Univ 1993;40:193–205. Aurich H. The laboratory of life. Warszawa: PWN; 1974. p 222–226. Kokot F. Water-electrolyte and acid-base balance in physiology and pathology. Warszawa: PZWL; 1998. Leroy N, Bres E. Structure and substitutions in fluoroapatite. Eur Cells Bul Materials 2001;2:36 – 48. Okazaki M. Physicochemical properties of functionally graded fluoridated hydroxyapatite. Otsu, Japan: XXIVth World Conference of ISFR, September 4 –7; 2001. 25 p. Schlick T. Molecular modeling and simulation. An interdisciplinary guide. New York: Springer-Verlag; 2002. p 3. Höltje HD, Sippl W, Rognan D, Folkers G. Molecular modeling. Düsseldorf-Zürich: New York: John Wiley-VCH GmbH and Co.; 2003. 228 p. HyperChem. Computational Chemistry; Hypercube Inc.; 1996. HyperChem—Getting started, molecular visualization, and simulation. Hypercube Inc.; 1994. Straszko J, Paprota S. Molecular modeling. Szczecin: Technical University, Department of Chemical Engineering and Physical Chemistry; 1998. Kotfica M. Computer-assisted spectral analysis of some chemical compounds [thesis]. Szczecin: Technical University; 1998. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 793 Kasprzak M. Structure modeling of inorganic molecules [thesis]. Szczecin: Technical University; 2000. Boshey AL, Posner AS. Kinetics of amorphous calcium phosphate to microcristaline hydroxyapatite. J Phys Chem 1973;77: 2313–2315. Czaja J, Nowacka N, Straszko J. Impact of industrial pollution on the properties of chlorophyll. Zakopane: Conference 25–28 Juny 2003. p 33– 41. Olszak-Humienik M, Kotfica M, Możejko J. Comparison of binding energies of selected environmental xenobiotics by porphyrin complexes. Bull Environ Contam Toxico 2002;69:41– 48. Ichijo T, Yamashita YT. Observations on the structural features and characteristics of biological apatite crystals. Vol. 2. Observation on the ultrastructure of human enamel crystals. Bull Tokyo Med Dent Univ 1992;39:71– 80. Kodaka T, Debari K, Abe M. Hexahedrally based crystals in human tooth enamel. Caries Res 1992;26:69 –76. Ingram GS, Horay CP, Stead WJ. Interaction of zinc with dental mineral. Caries Res 1992;26:248 –253. Sutter B, Ming DW, Clearfield A, Hossner LR. Mineralogical and chemical characterization of iron-, manganese-, and copper-containing synthetic hydroxyapatites. Soil Sci Soc Am J 2003;67:1935–1942. Michel V, Ildefonse P, Morin G. Chemical and structural changes in Cervus elaphus tooth enamels during fossilization (Lazaret cave): a combined IR and XRD Rietveld analysis. Appl Geochem 1995;10:145–159. Hing KA, Best SM, Bonfield W. Characterization of porous hydroxyapatite. J Mater Sci Mater Med 1999;10:135–145. Lee WT, Dove MT, Salje EKH. Surface relaxations in hydroksyapatite. J Phys Condens Matter 2000;12:9829 –9841. Mathai M, Shozo T. Structures of biological minerals in dental research. J Res Natl Inst Stand Technol 2001;106:1035–1044. Sendur A. Magnesium content in the hard dental tissues. Czas Stomat 1998;51:785–790. Pawlus G, Gutowska I, Machoy Z, Machaliński B. Magnesium to calcium quantitative ratios in human tooth germs and teeth. J Elementol 2003;8(2):75– 81. Admassu W, Breese T. Feasibility of using natural fishbone apatite as a substitute for hydroksyapatite in remediating agueous heavy metals. J Hazard Mat 1999;69:187–196.