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THE RELATIONSHIP BETWEEN CRYSTAL STRUCTURE AND CHEMICAL COMPOSITION OF ENAMEL AND DENTIN BY J. W. GRUNER, DUNCAi\ McCONh’ELL, AND W. D. ARMSTRONG (Prom the Department of Geology and Mineralogy and the Laboratory of Physiological Chemistry, University of Minnesota, Minneapolis) (Received for publication, July 19, 1937) De Jong (7) presented the first direct evidence of the crystalline phosphate nature of the mineral phases of calcified tissues. He demonstrated, by x-ray methods, the similarity of the structure of bone to the apatite group of phosphate minerals. Taylor and Sheard (14) extended this investigation and showed that the x-ray diffraction lines of enamel and dentin closely matched those of fluorapatite. They also found the optical constants of enamel and dentin to be in close agreement with those of dahllite, a carbonate-apatite with the empirical formula CaC03[Ca3(P0&],. Roseberry, Hastings, and Morse (13) demonstrated, by x-ray methods, the absence in calcified tissues of such crystalline substances as CaC03 and CaHPO*. These investigators con- cluded that the mineral phases of enamel, dentin, and bone are carbonate-apatites. It has been shown that the chemical composition of enamel differs from that of dentin and that the compositions of various specimens of the same material are not exactly identical (1). There is a similar, and often greater, variation in composition among the minerals of the apatite group. The following minerals form an isomorphous seriescapable of solid solution in one another: (a) CaF2[Ca3(P0&],, fluorapatite; (b) CaCb[Ca3(P0& chlor- apatite; (c) Ca(OH)z[Cas(PO& hydroxyapatite; (d) CaC03- [CWP04& carbonate-apatite; (e) CaO[Ca3(P0& oxyapa- tite, where z is 3 or very nearly 3 in all formulas except (d), in which case it lies between 2 and 3, and is sometimes closer to 2 than to 3. The mineral francolite is an isomorphous mixture of (a) and (d), and dahllite is a solid solution of (c) and (d). Oxy- apat,ite (e) occurs in rather limited amounts under special con- 771 by guest on May 24, 2018 http://www.jbc.org/ Downloaded from

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Page 1: AND CHEMICAL COMPOSITION OF ENAMEL AND · PDF filethe relationship between crystal structure and chemical composition of enamel and dentin by j. w. gruner, duncai\ mcconh’ell, and

THE RELATIONSHIP BETWEEN CRYSTAL STRUCTURE AND CHEMICAL COMPOSITION OF ENAMEL

AND DENTIN

BY J. W. GRUNER, DUNCAi\ McCONh’ELL, AND W. D. ARMSTRONG

(Prom the Department of Geology and Mineralogy and the Laboratory of

Physiological Chemistry, University of Minnesota, Minneapolis)

(Received for publication, July 19, 1937)

De Jong (7) presented the first direct evidence of the crystalline phosphate nature of the mineral phases of calcified tissues. He demonstrated, by x-ray methods, the similarity of the structure of bone to the apatite group of phosphate minerals. Taylor and Sheard (14) extended this investigation and showed that the x-ray diffraction lines of enamel and dentin closely matched those of fluorapatite. They also found the optical constants of enamel and dentin to be in close agreement with those of dahllite, a carbonate-apatite with the empirical formula CaC03[Ca3(P0&],. Roseberry, Hastings, and Morse (13) demonstrated, by x-ray methods, the absence in calcified tissues of such crystalline substances as CaC03 and CaHPO*. These investigators con- cluded that the mineral phases of enamel, dentin, and bone are carbonate-apatites.

It has been shown that the chemical composition of enamel differs from that of dentin and that the compositions of various specimens of the same material are not exactly identical (1). There is a similar, and often greater, variation in composition among the minerals of the apatite group. The following minerals form an isomorphous series capable of solid solution in one another: (a) CaF2[Ca3(P0&],, fluorapatite; (b) CaCb[Ca3(P0& chlor- apatite; (c) Ca(OH)z[Cas(PO& hydroxyapatite; (d) CaC03-

[CWP04& carbonate-apatite; (e) CaO[Ca3(P0& oxyapa- tite, where z is 3 or very nearly 3 in all formulas except (d), in which case it lies between 2 and 3, and is sometimes closer to 2 than to 3. The mineral francolite is an isomorphous mixture of (a) and (d), and dahllite is a solid solution of (c) and (d). Oxy- apat,ite (e) occurs in rather limited amounts under special con-

771

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772 Structure of Enamel and Dentin

ditions. It may exist in solid solution with (c) and (d), according to Bredig, Franck, and Fiildner (3), and it has been described as voelckerite (12). Gruner and McConnell (6, 9) have recently investigated the minerals francolite and ellestadite in detail and have found some remarkable stereochemical and structural rela- tionships. From this work certain new conceptions have been derived with regard to the position of some of the ions in the apa- tite crystal lattice. These new principles pertaining to the crystal structure of apatite can be applied directly to enamel and dentin, whereby the apparent discrepancies between composition and structure of these substances can be satisfactorily explained.

Structures of Carbonate-Hydroxyapatites

Mehmel (10) and Nkay-Szabb (11) determined the crystal structure of fluorapatite in 1930. This structure is shown as a projection upon the basal plane of the hexagonal space lattice in Fig. 1. For a detailed description of this structure the literature (6, 9-11) should be consulted. Essentially it is hexagonal and possesses two sets (or kinds) of S-fold axes of symmetry, which are normal to the plane of projection. One set of axes is called “external axes,” but this is not a term of structural crystal- lography and is used here purely for convenience. On these axes are located the F ions which may be partly or entirely replaced by Cl ions or OH groups yielding the corresponding members of the apatite group. According to previous investigators, including Hendricks, Jefferson, and Mosley (8), these F positions can also be occupied by the much larger CO3 groups. To preserve the symmetry of the structure such a group must be on a 3-fold axis.

The second set of S-fold axes is called “internal axes” for con- venience; it is the loci of two-fifths of the Ca ions in the struc- ture. As will be shown, our investigations indicate that some C ions are located on these axes, in which case 1 C ion substitutes statistically for enough Ca ions to maintain the electrostatic equilibrium of the structure.

The P ions have a covalency of 4 in the structure, and each of these ions is surrounded by a regular tetrahedral arrangement of 0 ions, which together form the PO, groups. These groups, of which there are six in the unit cell, form the foundation of the structure. The unit cell is the smallest structural unit with all the properties of the substance and is, therefore, the structural

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Gruner, McConnell, and Armstrong 773

molecule. Each unit cell possesses the following types of ionic positions: 24 0, a few of which may be occupied by OH or F ions, if an excess of either should be present; all of the 0 ions are bonded to P ions; 2 F, which may be entirely, or in part, occupied by OH, Cl, or 0 ions; 6 P, a small number of these positions, probably not exceeding 10 per cent, may be occupied by C, V, or As; Si and S may also occupy these positions (9) ; 10 Ca, a small amount of Ca may be replaced by C, in which case all of the ten positions

OXYGEN

FLUORINE

CALCIUM

l PlKuFwmJS

FIG. 1. Projection of the simple hexagonal unit cell of apatite on the basal plane (0001). The heights of the various ions above the plane of projection are indicated as fractions of CO. The small diagram at the upper left shows the derivation of the simple hexagonal unit cell from the hexagonal prism. The a-fold axes are indicated by triangles, but the other symmetry elements of the space group C& are not shown.

are not necessarily filled, because C can replace more than one Ca, depending upon the number of charges required to produce electrostatic equilibrium.

x-Ray Diffraction Data

Powder diffraction photographs were made with unfiltered Cu and Fe radiation in precision cameras with an effective radius of 57.3 mm. The samples were introduced as rods of powder about

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774 Structure of Enamel and Dentin

TABLE I

Comparisons of Di$raction Patterns of rlpatite, Dahllite, Enamel, and Dentin

Unfiltered Fe radiation; radius = 57.3 mm.; d, interplanar distance; I,

intensity.

Line NO. Indices

1 002 2 120, 210 3 3008 4 121, 211

5 112

6 300

7 202 8 301 9 130/3

10 122, 212 11 130, 310 12 131, 311

13 2220 14 113 15 1230

16 203 17 222 18 132, 312

19 123, 213 20 231, 321

21 140, 410 22 402

23 004 24 232, 322

25 133, 313 26 142, 412 27 240, 420

28 331

29 124, 214 30 502

31 304

32 233, 323

33 151, 511

Fluorapatite -I- Dahllite

d

3.432 3.060 2.975 2.798

2.769 2.702

2.616 2.517 2.477

2.289 2.248 2.135

2.057 2.026 2.001

1.937 1.883

1.838 1.795 1.769

1.745 1.720 1.637

1.605

1.533

1.521

1.498 1.468 1.452

1.445 1.424

I

2

3

s 210

4

6

3

3

: 2 1

1 1

3 5 1

6 3

3 3 3

i

d

3.437 3.077 2.990 2.811

2.778 2.712 2.628

2.524 2.493

2.256 2.145

2.031 1.993 1.941

1.888

1.840 1.805 1.778

1.753 1.721 1.645

1.538

1.502

1.473 1.455

1.434

2 3 1

>lO 3 6

i 3

3.428

3.082 2.995 2.809 2.771

2.712 2.624

2.530 2.488 2.294

2.260

2.142

2.061

2.027 1.989

1.941 1.892

1.837 1.807 1.778

1.752 1.716 1.645

1.608 ‘1.581 ,1.539

Dentin

d

3.418 3.066

2.791

2.714

2.624

2.259 (2.135

\

1.937

1.837

1.719

1.445

. .

-

I -

2

3

>lO

2 1

2 1

4

4

2

2

-

0.6 mm. thick. Photographs taken with iron radiation are par- ticularly suitable for measuring the exact sizes of the unit cells of various specimens. Table I contains the observed interplanar

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Gruner, McConnell, and Armstrong 775

distances (d) and the estimated intensities (I) of the lines pro- duced by fluorapatite, dahllite, enamel, and dentin.

The Miller indices of the reflecting planes are given wherever possible. When the value of d is smaller than 1.4 A., more than one plane usually reflects for a measured value of d, and for this reason the indices are not given for reflections beyond this value. The differences in the estimated intensities of the diffraction lines of the four substances compared in Table I are not of great conse- quence.

The intensities depend chiefly upon the chemical composition of the crystal lattice but also to a considerable extent on the size

TABLE II

Dimensions of Unit Cells and Densities of Various Apatite-Like Substances

Fluorspatite. ................... Dahllite. ....................... Enamel, Specimen 1.. ........... Dentin, “ 3. ............

a0 co

-d. H.

9.36* 6.88 9.41 6.88 9.41 6.87 9.40 6.87

--Y-x------ 0.7350 ‘i22 3.187 3.18t 0.7311 528 2.93 0.7301 527 3.055 2.967 0.7309 526 3.024 2.824

* All values for a0 and CO are kO.01 A. t Determined by Mr. A. S. Dadson who supplied the analyzed specimen

of fluorapatite. See Contrib. Canad. Mineral., No. 56, Univ. Toronto Studies (1933).

of the individual crystals. The deproteinized enamel gave sharp diffraction lines but those of deproteinized dentin were diiused and indistinct, indicating a much smaller grain size, with a prob- able upper limit of 0.1 P. For the same reason diffusion causes the weaker lines produced by enamel to be absent in the spectro- grams of dentin. Nevertheless, the lines, whose spacings can be measured, are easily correlated with those of enamel and dahllite.

The dimensions of the unit cells as calculated from the data in Table I are shown in Table II.

The theoretical densities of the mineral phases of enamel and dentin were calculated from the dimensions of the unit cells and the molecular weights. The latter were calculated from the analy-

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776 Structure of Enamel and Dentin

TABLE III Analyses of Enamel and Dentin and Distribution of Ions in Their Structures

Sl Decimen 1. Demoteinked enamel L

ca. .................. Mg ..................

P .................... co*. ................. Ignition loss .........

OH. ................. 0. ...................

9.087 9.238 18.476 370.17

0.099 0.101 0.202 2.46 5.604 5.697 28.485 176.78 0.698 0.710 2.840 8.52

per cent

36.41 0.24

17.39 3.07 5.4

~- 50.003

15.746

2.000 24.000

34.02 384.00

975.95

15.488

2.000 48.000

50.000 -

Specimen 2. Unaltered el

35.64 8.894 9.085 0.44 0.181 0.185

17.50 5.640 5.761 2.86 0.650 0.664

la me1

18.170 / / 364.04 Ca ............. Mg ... ........ P .............. cot. ...........

0.370 4.50 28.805 178.76

2.656 7.97

15.365 15.695

2.000 24.000

OH. ..........

0. ............ 2.000 34.02

48.000 384.00

50.000973.29 50.001

mcimen 3. Deproteinized

33.69 j 8.408 1 8.888

9 L pe .- :ntin

17.776 0.774

26.875 4.576

Ca ............. Mg ............ P .............. co*. ...........

356.14 9.41

166.77 13.73

34.02

384.00

964.07

0.89 15.78

4.76

0.366 5.085 1.082

15.798 2.000

24.000 OH. ........... 0 ..............

Ltin Specimt

j 26.10 0.80

12.48 3.44

L 4. Unaltered dc

6.514 8.821 0.329 0.446

4.022 5.446 0.782 1.059

-iixi?-15.772 2.000

24.000

17.642 353.46 0.892 10.85

27.230 168.99

4.236 12.71

50.000

2.000 34.02 48.000 384.00

50.000 964.03

Ca. ...........

Mg ........... P ............. co,. ..........

OH ........... 0 .............

......

......

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Gruner, McConnell, and Armstrong 777

ses shown in Table III. The observed densities were obtained by centrifuging the deproteinized materials in Thoulet’s solution1

In order to prevent coagulation, the powders were shaken for an hour in Thoulet’s solution with an approximate density of 3 be- fore the density determinations were made. This procedure has been found very successful for clay minerals. The determined density of enamel is slightly higher than 2.95, the value usually given for the protein-containing material. Dentin apparently has a texture which produces results by all available methods which are considerably lower than the theoretical value.

Chemical Composition

Chemical analyses of enamel and dentin possess at least one serious inherent defect. It is not possible to distinguish between the substances constituting the ignition loss. Although one can be practically certain, in the case of the deproteinized materials, that all of the organic matter has been removed, there remains an uncertainty as to the state of the water in these substances. It may be present as water filling the interstices or in the form of OH groups actually forming part of the structure. Because the specimens of deproteinized enamel and dentin lost very little weight on being heated to 300”, the assumption is warranted that most of the water is present in combination in these materials, as OH groups.

The analysis for monovalent alkalies has been commonly neg- lected. Those reported by Gassmann (5) 30 years ago cannot be used for our purposes because he did not separate the enamel and dentin. Fortunately, the small amounts of sodium and potas- sium present in enamel and dentin do not affect the structure materially. The sodium ion, being nearly the size of the calcium ion, can easily substitute for the latter. The potassium ion, how- ever, is somewhat larger than the calcium ion. As potassium is present in amounts which probably rarely exceed 0.1 per cent (2), all of it can enter the Ca positions.

Column 1 of Table III contains a number of analyses which one of us has previously reported (1).

Specimens 1 and 3 are the purified mineral phases of enamel and dentin, respectively. They were prepared from the separated

1 This is a concentrated water solution of potassium and mercuric iodides.

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778 Structure of Enamel and Dentin

enamel and dentin obtained from four sound teeth. The protein, and presumably all organic material, was removed from the speci- mens by the method of Crowell, Hodge, and Line (4). Specimens 2 and 4 are analyses selected at random from those previously reported for protein-containing enamel and dentin. Column 2 of Table III contains the results of the multiplication of the weight per cent by 10 and the division of this result by the atomic

TABLE IV

Number of Ions Relegated to the Ca and P Posiiions of Mineral Phases of Enamel and Dentin

Specimen No.

1. Deproteinieed enamel ........................

2. Unaltered enamel ............................

3. Deproteixhed dentin ........................

4. Unaltered dentin ............................

P positions Ca positions

5.697 P 9.238 Ca 0.303 c 0.101 Mg

0.407 c

6.000 9.746

5.761 P 9.085 Ca

0.239 c 0.185 Mg 0.425 C

6.000 9.695

5.375 P 8.888 Ca 0.625 C 0.387 Mg

0.519 c

6.000 9.794

5.446 P 8.821 Ca 0.554 c 0.446 Mg

0.505 c

6.000 9.772

weight (the molecular weight in the case of carbon dioxide). These values happen to be similar to the number of ions required for the unit cell. The assumption is made that each unit cell contains 24 0 ions and two OH groups. The 50 negative charges of these ions must be balanced by a corresponding number of positive charges. It is possible to find a factor by which the num- bers in Column 2 may be multiplied to give the requisite number of positive ions, and these so called adjusted ionic ratios are given

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Gruner, McConnell, and Armstrong 779

in Column 3. Multiplication of the atomic ratios by their respec- tive atomic weights yields the. results shown in Column 6, the sum of which equals the molecular weight of the unit cell. This weight was used to calculate the theoretical densities shown in Table II.

Enamel and dentin apparently contain sufficient OH groups to be classed as hydroxyapatit.es. The ignition loss of deprotein- ized enamel (Specimen 1) was 5.4 per cent, and the carbon dioxide content of this material was 3.07 per cent. Even if all carbon dioxide were lost on ignition, there remains to be accounted for an additional ignition loss of 2.33 per cent, which is unquestionably water. Because the material lost only a very little weight on prolonged heating at 300°, it seems probable that the water exists in combined form in this material; that is, as OH groups. Essen- tially similar results were obtained with the other materials whose analyses are shown in Table III. As the theoretical water con- tent of hydroxyapatite is 1.79 per cent, the amount of water in both enamel and dentin is entirely sufficient for this struct,ure.

A statistical distribution of the positive ions in each specimen, whose analysis is shown in Table III, is given in Table IV. These distributions are based on the principles described above and which may be restated as follows: (a) The P positions are the most important and must be completely filled. If the amount of phosphorus is insufficient, carbon will substitute, making up the deficiency, to produce a total of 6 ions. (b) Any remaining carbon will occupy positions on the internal 3-fold axes. (c) The number of calcium ions will depend upon the number necessary to produce electrostatic equilibrium, and, consequently, the sum of the ions, which occupy the Ca positions, is usually slightly less than 10.

SUMMARY AND CONCLUSIONS

As a result of the previous investigations by Gruner and McCon- nell of the minerals francolite (6) (a carbonate-fluorapatite) and ellestadite (9) (a sulfate-silicate-apatite), it becomes possible to explain completely the phosphate structures of the mineral phases of enamel and dentin, notwithstanding their variations in Ca : COZ and P:COz ratios. It is shown that the formulas previously assigned to enamel and dentin are incorrect, because they are based on the assumption that all of the carbon dioxide is present in the structure as carbonate groups. In the older formulas the

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780 Structure of Enamel and Dentin

carbonate groups were supposed to substitute for the F ions and OH groups in the apatite structure.

For reasons discussed in detail elsewhere (6), the following con- clusions as to the crystal structure of apatites are justifiable. (a) The CO, group is too large to fit into the F and OH positions. There is no x-ray evidence of the expansion of the crystal lattice which would be required by such a substitution. (b) If the CO, groups occupied these positions, a relatively large increase in density would result, but this has not been found to occur. (c) These positions are already occupied by OH ions in dahllite and F ions in francolite and are, therefore, barred to COs groups.

Furthermore, on the basis of the older assumptions it was im- possible to reconcile the chemical composition with the structure (1). The true structure is one of the most interesting solid solu- tions (or isomorphous mixtures) now recognized. In it carbon substitutes for phosphorus to the extent of 10 atomic per cent or less. Carbon also substitutes for calcium by occupying a position between 3 0 ions to form a CO, group, which is slightly larger than the CO3 group in simple carbonates. This type of substitu- tion is also limited and probablydoes not exceed that shown in the case of deproteinized dentin (Specimen 3) in Table IV.

The structural formula of enamel and dentin may be written as Ca(OH)z(Ca, C),-, [(P, C)O&. It is well known that mag- nesium and sodium may replace calcium in this structure, but for the sake of simplicity these constituents have been omitted from the formula. The formula may also be written, (OH),- Cad@, C>OddCa, CL This formula indicates that all Ca ions are united to the PO4 groups, and that only the Ca ions on the internal a-fold axes can be replaced by carbon. It shows fur- ther that the OH groups are in contact with 6 Ca ions only, and are separated from the PO, groups. The essential difference in the structures of the mineral phases of enamel and dentin, which is reflected in their different compositions, is the fact that in the mineral matter of dentin more carbon substitutes for phosphorus and calcium than in the mineral matter of enamel. According to the mean results obtained by Armstrong and Brekhus (1) for enamel and dentin the exact formulas arc

Enamel, (0H)&ad(P~. ,C,.l)O,,l(Ca,.1Mgo.lCo.r) Dentin, (OH)~Ca~[~P~.~Co.~)O,41(CBil.?MgO.LC0.~)

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Gruner, McConnell, and Armstrong

BIBLIOGRAPHY

1. Armstrong, W. D., and Brekhus, P. J., J. Biol. Chem., 120, 677 (1937). 2. Bowes, J. H., and Murray, M. M., Biochem. J., 29,272l (1935). 3. Bredig, M. A., Franck, H. H., and Ftildner, H., Z. Elektrochem., 39,

959 (1933). 4. Crowell, C. D., Hodge, H. C., and Line, W. R., J. Dent. Research, 16,

251 (1934). 5. Gassmann, T., Z. physiol. Chem., 66, 455 (1908). 6. Gruner, J. W., and McConnell, D., Z. Krist., 97, 208 (1937). 7. de Jong, W. F., Rec. trav. chim. Pays-Bas, 46,445 (1926). 8. Hendricks, S. B., Jefferson, M. E., and Mosley, V. M., Z. Krist., 81,

352 (1932). 9. McConnell, D., Am. Mineral., 22, 977 (1937).

10. Mehmel, M., Z. physik. Chem., Abt. [B], 16,223 (1931). 11. NQray-Sxab6, S., Z. Krist., 76,387 (1930). 12. Rogers, A. F., Am.J. SC., 33,475 (1912). 13. Roseberry, H. H., Hastings, A. B., and Morse, J. K., J. Biol. C&m.,

90, 395 (1931). 14. Taylor, N. W., and Sheard, C., J. Biol. Chem., 81,479 (1929).

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ArmstrongJ. W. Gruner, Duncan McConnell and W. D.

ENAMEL AND DENTINCHEMICAL COMPOSITION OF

CRYSTAL STRUCTURE AND THE RELATIONSHIP BETWEEN

1937, 121:771-781.J. Biol. Chem. 

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