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The Study of Flouridated Hydroxyapatite by P- F Rotational-Echo, Double-Resonance NMR Yong Pan
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Miami Valley Laboratories, The Proctor & Gamble Company, P.O. Box 538707, Cincinnati, OH 45253-8707
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This work demonstrates the use of P- F Rotational-Echo, DoubleResonance (REDOR) NMR to investigatefluoridein apatitic matrices. For fluorapatite, F-observed P- F REDOR NMR of fluorapatite revealed the P- F connectivities. P-observed P- F REDOR NMR determined the nearest P- F distance. For F -treated hydroxyapatite and 25%F-substitutedfluorohydroxyapatite, the ratios of REDOR difference to full-echo intensity with various dephasing times were measured and then compared to the simulated values for models with variousfluoridationdepths and orientations. The simulation involved calculating the dephasing of multiple P's by multiple F's and then summing over all the P-F spin clusters, weighted by the occurring probabilities. The data showed that the OH ions in the first unit cell on the (001) surface were replaced by F ions. This result suggested thefluoridationwas via the ion-exchange process rather than by the diffusion or the dissolution and remineralization pathway. 19 31
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Hydroxyapatite (HAP), Caio(OH)2(P04>6, is one of the major inorganic constituents of dental enamel and cortical bone. Incorporation of fluoride into HAP has been proven effective in preventing dental caries (/). The effects offluorideon the mechanical properties of bone remain extremely controversial as exemplified by the contradictions in the literature (2, 3). The interaction between F" and HAP is complicated by its dependence onfluorideconcentration, pH, reaction time, and the type of HAP. The proposed pathways include ion exchange (specific adsorption) of F"(aq> with OH"(iattice); dissolution of enamel crystallites followed by reprecipitation of fluorohydroxyapatite (FHAP); and the diffusion of F" into an apatite lattice (4, 5). Several analytical techniques have been used to address the amount of F~ 168
©1998 American Chemical Society In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
169 incorporated into HAP and the reaction products offluoridewith HAP, Le., FHAP, CaF , or nonspecifically adsorbedfluoride(6, 7). However, none of these techniques can provide information on the spatial distribution of F" in HAP. The question is, then, are F ions uniformly distributed throughout apatite crystals or are they concentrated on the surface? On which crystal plane is the fluoridation preferred? Here we report our recent development of the use of P-F REDOR NMR to investigate the spatial distribution of F~ in apatite, which can provide detailed information on the history and mechanism offluorideand HAP interactions. The crystal structure of hexagonal (P6 /m) apatite has been determined and refined by X-ray diffraction (8, 9). Fig. 1 shows the arrangement of F and Ρ nuclei influorapatite(FAP). The F ions lie on [001] hexads separated by 9.37 À, whereas the nearest neighbor distance for F" along the six-fold axis is only 3.44 À. Associated with each F ion are three PO4 - groups located in the plane perpendicular to the F" axis. The PO4 - groups of each layer are rotated by 60° with respect to the layer below. The nearest P-F distance is 3.62 Â. Depending on the mechanism, the incorporation of F" in apatite could be different in both depth and orientation. Several models of F" incorporation will be discussed in section 3.2. Rotational-Echo, Double-Resonance (REDOR) NMR, a technique for retrieving information on weak heteronuclear dipolar coupling interactions, has been applied to observe the interface in polymer blends (10) and to measure heteronuclear distances and connectivities in various biological systems (11-14). For a pair of isolated spins, the REDOR difference signal can be easily measured and translated into internuclear distances. For systems where an observed nucleus is dephased by more than one heteronucleus, the calculation requires the knowledge of the relative geometry of the nuclei (15). The calculated values for various dephasing cycles can be compared with experimental data. P-F REDOR NMR does not require isotopic labeling. Forfluoridatedhydroxyapatite, a proton dilute system, high-power proton decoupling is not required. Therefore, a two-channel NMR spectrometer is adequate to perform the REDOR experiment (16). 2
Downloaded by COLUMBIA UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch004
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3ip.i9 REDOR NMR. The pulse sequence for P-observed ip-i F REDOR is shown in Fig. 2. The P magnetization is created by a single 90° pulse. A P 180° pulse in the center of the REDOR dephasing period refocuses all isotropic chemical shifts at the start of data acquisition and produces a rotational spin echo. The F 180° pulses inserted at each half-rotor period prevent the rotational spin echo from reaching full intensity. For a single spin pair, the reduced signal intensity, s, can be calculated according to the equation: F
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s = $οθθ8[ΔΦ(α, β, %d)]
(2.1) 19
where s i$ the signal intensity without the application of F 180° pulses; α is the azimuthal angle and β is the polar angle defined by the internuclear vector in a coordinate system with ζ axis parallel to the rotor axis; is the % is the product of the number of rotor cycles (N ), the dipolar coupling constant (D), and the rotor period a
D
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In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by COLUMBIA UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch004
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Figure 2. Puke sequence for P-observed P - F REDOR NMR. Two equally spaced l^F 180° pulses per rotor period result in the dephasing of transverse 31p magnetization produced by a 31p 90° pulse. Phosphorusfluorine dipolar coupling determines the extent of dephasing. A 31p 180° pulse in the middle of the dephasing period refocuses the isotropic 31p chemical shift difference at the beginning of data acquisition.
In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
171 (Τ ); and the amount of dephasing, ΔΦ, is a function of Euler angles α and β as well as λ^. For a powder sample, cos[A(a, β, Xq)] can be averaged over all orientations of spin pairs. The averaged (s -s)/s , known as AS/S , is calculated and plotted as a function of X as shown in Fig. 3 (17). The curve is universal. For an isolated spin pair, an experimental determination of AS/S can be readily translated into a dipolar coupling constant D and, therefore, an internuclear distance r since D=Wi7h/2m where γ]ρ and yp are P and F gyromagnetic ratios and h is Planck's constant. When an observed nucleus is dephased by more than one heteronucleus, the dephasing is normally dominated by the nearest heteronucleus since the dipolar coupling is inversely proportional to the cube of the distance. However, when the neighboring heteronuclear distances are comparable, the dephasing from multiple heteronuclei will be accumulated (18): Γ
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Downloaded by COLUMBIA UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 18, 1999 | doi: 10.1021/bk-1999-0717.ch004
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s = SoCOS[XAO/(a, β, λο)] (2.2) j For spin-1/2 interactions, such as between F and P, the dephasing with plus and minus signs occurs with equal probability; therefore, the spin average can be obtained by averaging the two possible dephasing values, ΔΦ|(α, β, λ^) = ± ΙΔΦ|(α, β, Arj))!, as shown here: 19
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