NATIONAL BUREAU OF STANDARDS REPORT

9820

Progress Report

THE REACTIVITY OF THE COMPONENTS OF TOOTH STRUCTURE

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

311.05- NBS PROJECT NBS REPORT

11-3110560 311.05-

11-3110561

December 29, 1967 9820

Progress Report

on THE REACTIVITY OF THE COMPONENTS OF TOOTH STRUCTURE

by

Eugene F. Huget* and Gerhard M. Brauer**

* Guest Worker, National Bureau of Standards, Washington,

D. C., from U. S. Army Institute of Dental Research,

Walter Reed Army Medical Center, Washington, D. C.

** Chemist, Dental Research Section, National Bureau of

Standards, Washington, D. C.

This investigation is part of the dental research program

conducted by the National Bureau of Standards, in cooperation with the Council on Dental Research of the American

Dental Association; the National Institute for Dental Research; the Army Dental Corps; the Aerospace Medical Division, USAF School of Aerospace Medicine; and the Veterans

Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, th< whole or in part, is not author Bureau of Standards, Washingtc

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in

! Office of the Director, National

y the Government agency for which

spies for its own use. <NB|>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

- 1 -

The Reactivity of the Components of Tooth Structure

Eugene F. Huget and Gerhard M. Brauer

A restorative material capable of adhering

to tooth structure is greatly needed. Bonding

is complicated by the complex nature of tooth

structure as well as the mechanical, toxicologic

and bacteriologic conditions which occur in the

oral cavity. Heats of immersion (-AH ) measurements for dentin, enamel, and anorganic whole tooth

tissue of known specific surface have been used to

study the reactivity and degree of surface modification produced by various ions and functional

groups in an aqueous environment.

A decrease in the -AH value of dentin in

w

2

water from 10.3 cal/g (4100 ergs /cm ) to 4.02

2 cal/g (1590 ergs /cm ) in 2^ - 4o# aqueous solutions

of organic solvents suggests the strengthening of

electrostatic bonds between polar groups of the

collagenous matrix of dentin. In absolute ethanol

and hexane, dentin absorbs heat indicating the

weakening of hydrophobic bonds. -AIT values for

anorganic whole tooth in aqueous solutions of organic solvents are not appreciably different from those
- 2- o

obtained in water (109 ergs /cm ). The presence

of N0^, NOj* CNS~ , So£, and I ”, at pH 5 depressed

the -AHW

of dentin to 4.9ical/g (1910 ergs /cm2

).

Other anions (F~ and Cl“ ) had no effect. At pH 8,

however, anions did not alter the -AH- of dentin.

The addition of aldehyde groups to the liquid adsorbate at pH 8, resulted in the lowering of the

apparent -AR^ value of dentin. Significant changes

in -AHW

values for anorganic whole tooth were not

observed in the presence of anions or aldehyde

groups. The surface of dentin may be modified by

treatment with non-polar solvents. Ionic bonding

to exposed polar groups can be achieved at the

optimal pH.

1. INTRODUCTION

The most common cause of failure of a dental n filling”

or restoration Is the Inadequate seal between the prepared

surfaces of the hard tooth tissue and the restorative material. Faulty marginal seals facilitate the ingress and

diffusion of salivary ions and bacteria which lead to

further decay and often the eventual death of the vital

pulp organ. The development and use of a restorative material capable of bonding to one or more of the components

of tooth tissue would enhance the quality of the marginal

seal and thus prolong the period of useful service of a

finished restoration.
- 3- Bonding to tooth structure Is complicated not only by

the mechanical, toxicologic, and bacteriologic conditions

which occur in the oral cavity, but also by the complex

nature of tooth structure itself. Many investigators believe that bonding to tooth structure may best be accomplished by Interaction of the restorative material with

the mineral rather than the organic phase. Indeed, the

only evidence obtained for the mechanism of a potentially

successful dental adhesive suggests that interaction occurred mainly with the mineral component . 1

Little information is available regarding the c hemic al

and physical characteristics of the surfaces of the components of tooth structure. Such information would contribute

significantly to the eventual solution of the many problems

still to be resolved in obtaining a truly adhesive restorative material.

In previous studies, we have determined the apparent

heats of wetting of well-characterized powders of the cora- 2 3 ponents of tooth structure, ' as well as their heats of

reaction In the presence of dilute acids, chelating agents

and stannous fluoride. In this investigation, calorimetric

measurements of the heats of immersion ( -AE^} of human

dentin, enamel, and anorganic whole tooth tissue of known

specific surface have been used to determine quantitatively

the nature and degree of surface modification produced by
-4-

various ions and functional groups in an aqueous environment . 2 r EXPERIMENTAL METHODS AND MATERIALS

2.1. Methods

A thermistor calorimeter similar to the instrument

..li designed by Sunner and Wadso was used in this investigation. The techniques employed In calibration of the calorimeter, heats of immersion determinations, and specific surface measurements of finely ground human dentin, enamel, and

2

*5 anorganic whole tooth tissue have been described previously. * All heats of wetting measurements were made at least in

duplicate.

2.2. Materials

Freshly extracted human teeth were cleaned by brushing and rinsing with water. The crown portions were removed

with carborundum disks and the necrotic tissue from the pulp

chambers and canals removed with round steel burs. No. 4

and No. 3 steel burs were then used to grind out the dentin

which was subsequently sieved for 16 hours on a mechanical

shaker using No. 100, No. 200, and No. 3^5 mesh sieves. The

powder passing through the No. 3^5 sieve was placed on a No.

400 mesh sieve and shaken for six hours. The powder which

passed through the No. 400 sieve was examined petrographic ally for impurities. It was then stored in a deep freeze unit

and later placed In a desiccator over anhydrous calcium
-5-

sulfate for at least one week before use. Some samples

were further dried under vacuum (10~ to 10“^ ram Hg at

room temperature). Other samples were saturated with

water vapor prior to their use.

The enamel hulls which remained after the removal of

the dentin were then ground to a fine powder with No. 558

carbide cross-cut fissure burs. The finely ground product

was analyzed petrographically, sieved and stored in the

manner described for the sieving and storage of powdered

dentin. Finely ground whole tooth tissue (enamel, dentin,

and cementum) was rendered anorganic by ethylene diamine

extraction of the organic constituents.

5 . RESULTS

Since all determinations were made at least in duplicate, it v;as possible to obtain a measure of the reproducibility of the measuring process among replicates. The

standard deviation of reproducibility did not appreciably

depend on the magnitude of the measured value but was somewhat less for anorganic whole tooth than for dentin. The

pooled estimates obtained from all Individual sets of duplicates or triplicates are; for dentin: A * 0.095; for anorganic whole tooth: A = 0.059 where A is the estimate^

standard deviation for individual measurements.

All but a few of the values reported in Tables 1

through 4 are averages cf duplicates. Their standard error
- 6 - is therefore: for dentin: 0.067; for anorganic whole tooth: 0.

023. For convenience, all values are reported to the

nearest 0.01 cal/g.

2 1 After conversion to ergs/cm , (1 cal * 4. 1840 £0 ergs),

the rounding was made at the nearest 10 unit for dentin, and

the nearest whole unit for anorganic whole tooth.

The apparent -AHW

values obtained for the three powdered

constituents of human tooth structure are presented in Table

1 . The observed -AH values were highly dependent upon

the residual water content of the dentin sample powders

prior to immersion. The careful drying of dentin powders

under vacuum resulted in a significant increase in the apparent values. With the exception of anorganic whole

tooth powder which had been equilibrated with water vapor

prior to immersion, the method of sample powder storage and

relative degree of dryness of anorganic whole tooth and

enamel did not appreciably effect the apparent -AHW

values.

3.1. In Organic Solvents

In 2 to 40 percent (by volume) aqueous solutions of

organic solvents (ethanol, acetone, butanol, ethylene glycol),

a decrease in the apparent -AH^ value of dentin from that

o

obtained in water (4100 ergs /cm ) was observed (Table 2) # When immersed in either absolute ethanol or hexane, dentin

absorbed rather than evolved heat. On the other hand, -AH^
-7-

values for anorganic whole tooth In organic solvents (~ 84

2

ergs /cm ) were not appreciably different from those obp tained In distilled water (100-117 ergs /cm ).

3.2. In Salt Solutions

The Immersion of dentin powders in water neutralized

with sodium hydroxide and made basic so that the resulting

solution had a pH of approximately 8 yielded slightly lower

-AH values than those observed upon the immersion of this

w

substance in water (Table 3). No significant differences

in -AH values accompanying the immersion of anorganic whole

W

tooth tissue in dilute Sodium hydroxide and water were found.

Using two percent solution of the respective salts, it

can be seen from Table 3 that the presence of NO^, NOg, SO*,

CNS" and 1“ at pH 5 depressed the - AHWvalue of dentin to as

low as 4.8l cal/g, (1910 ergs /cm ). Presence of fluoride or

chloride, however, did not change the -AJ^ values. Adjustment of the solution to pH 8 by addition of aqueous NaOH

gave -AH

w

values for dentin and anorganic whole tooth that

differed little from those obtained in water adjusted to pH 8.

3.3. In Dilute Aldehyde Solutions

The addition of aldehyde groups to the aqueous phase

at pH 5, (glutaraldehyde or dialdehyde starch), had only a

slight effect upon the observed -A}^ of dentin and anorganic

whole tooth powders (Table 4). At pH 8, however, a decrease

in the -AHW

of dentin to approximately 4.8 cal/g (1900 ergs/cm )
- 8 - was found. The -AH^ values of anorganic whole tooth powders

in aldehyde solutions (pH 8), however, were essentially the

same as those obtained upon the immersion of these powders

in water.

4. DISCUSSION

The cutting of tooth structure, whether in vivo or in

vitro, will result In significant structural changes in the

organic matter near the surface. It is hoped that the technique employing rather mild conditions used In the preparation of the odontic powders produced surface changes approxi

mating those encountered under clinical conditions. Freshly

cut dentin presents surfaces that are especially complex by

virtue of the deposition of non- homogenous collagenous films

However, despite the difficulties in preparing reproducible

surfaces, the surfaces of the dentin powders used in this

study should be net unlike those occurring at the dentinal

floors and walls of actual cavity preparations. The treatment of these^dentinal surfaces with the reagents used in

this study should yield modifications similar to those obtained with powdered dentin. Comparison of the reactivities

of the surfaces of dentin and anorganic whole tooth powders

strongly suggests that the organic matrix of dentin presents

reactive sites at which bonding may occur. Modification of

the surface of dentin with the appropriate reagent may also

result in the formation of easily accessible reactive groups
-9-

that could act as potential sites for bonding.

The apparent -AHW

values obtained for human dentin

upon immersion in water greatly exceeded the -AH

w

values

for inert solids (29 to 628 ergs /cm ) previously reported

g by other investigators for other substances. Therefore,

the collagenous component of dentin binds water through

hydrophilic groups and may further absorb, dissolve or react upon contact with water. The observed -AHW

values for

anorganic whole tooth, however, were found to be within this

range. Zettlemoyer' has suggested that an inert substance

must present a reasonably large surface area (10-20 m /g)

to yield a measurable evolution of heat upon immersion.

The specific surfaces of the human enamel powders used in this

study were apparently much too small to promote the evolution

of sufficient amounts of heat to allow measurement by means

of the calorimeter employed. The small, but reproducible,

endothermic values obtained for enamel cannot be readily

explained.

3 Analysis of human dentin by Hess has revealed that

this substance contains 10-19 percent proteinacious material

and that the most abundant dentinal protein is collagen,

Kanagy^ has shown that bovine corium collagen has a greater

capacity for the adsorption of water than other fibrous

proteins, i.e., wool, nylon, and silk. Differences in capacity

for water adsorption are related to both physical and chemical
- 10- structure. A material capable of adsorbing vast quantities

of water must not only contain water- attracting groups, but

must also possess physical characteristics that permit

water to come into contact with potentially available hydrophilic groups. The amine, guanidyl and carboxyl groups, as

well as the hydroxyl groups of serine and hydroxyproline, are

important in the adsorption of water by collagen. Spreading

of the peptide chains and the accompanying access to polar

groups is facilitated by the presence of large concentrations

of proline and hydroxyproline and the long side chains of

arginine and lysine that * probably inhibit the formation of

extensive crystalline areas. Although subtle variations in

sequential arrangements may exist, the characterization of

dentinal and corium collagens by Veis and Schlueter1 ^ did

not reveal the presence of striking differences in regard

to amino acid moieties. Heats of wetting values of +37 cal/g

11 have been reported by Kanagy for purified bovine corium

collagen. Since the -Af^ values for anorganic whole tooth

are low (- 2.06 cal/g), it is likely that the unexpectedly

large -AH values obtained for dentin were caused by the

presence of collagenous material at the surface of the

powder particles. Hence, the immersion of dentin powders

resulted in the binding of water by the accessible polar

groups of the collagenous matrix as well as the adsorption

of water by the inorganic phase.
- 11 - The -AH values obtained upon the immersion of powdered

dentin in aqueous solutions of organic solvents also reflected the presence of a coating of collagenous material

at the surfaces of the powder particles. The substitution

of mixed solvents having lower dielectric constants for

water as adsorbates yielded significant reductions in the

magnitudes of the observed -AH^ values obtained for powdered

dentin.

A precise explanation for the decrease in the -AH

values of dentin as the solvent medium became more organic

in nature cannot be offered. This behavior may be attributed

to the strengthening of electrostatic linkages of the dentinal

collagen which in turn could result in (l) the reduction of

the number of sites accessible to water or (2) a slight decrease in the surface area of the specimen through tightening of the structure of collagen. An endothermic interaction

of the substrate with compounds such as butanol or ethylene

glycol could also be a possible cause. The apparent absorption of heat which was observed when dentin was immersed in

absolute ethanol and hexane could also be the result of the

endothermicity of the transfer of the non- polar side chains

of the collagenous matrix of dentin from the aqueous environment provided by its residual wa$*r content to an overwhelm12 ing hydrophobic environment.

Because of the absence of water reacting groups at the
- 12- surface, reductions of the dielectric constants of the adsorbate mixtures induced by the addition of organic solvents

did not profoundly alter the -AH^ values observed for anorganic whole tooth tissue. The magnitude of the observed

heat of immersion value of a polar solid is dependent not

only upon available surface area, but also upon the ability

1*5

of the polar solid to orient the dipoles of the adsorbate. ^ Even In a less polar environment, as long as small quantities

of water are present, the heat of immersion of this polar

solid is essentially the same as that in water Itself. The

striking influence of the presence of trace amounts of water

has been shown by Boyd and Harkins for the immersion of

anatase In benzene.

The ability of a protein to interact with ions or functional groups to which it may be exposed is dependent, among

other considerations, upon the pH of the ambient environment.

The isoionic point of native collagen has been located in

the pH range of 7 to 8. The exact location of this value

is highly dependent upon the previous history of the tissue

IS in question.

It can be assumed that the isoionic point of the major

protein constituent of human dentin, was approximately 7.

The depressed values observed for powdered dentin

upon immersion in dilute aqueous solutions containing certain

anions (l“, NO^, NOg, SO*, CNS“) at pH 5 suggest the direct
-13-

endothermic interaction of certain anions with R-NH^+

groups.

An alternative mechanism may involve the preferential binding of these anions to positively charged sites on the dentinal collagen at the expense of water. Chloride and fluoride

ions apparently possess a low affinity for dentinal collagen.

The affinity of the chloride ion for other proteins has also

been shown to be less than that of other anions.

Although a slight decrease in the apparent -A^ values

for dentin upon immersion in dilute base was observed, ionic

bonding to charged groups of the dentinal collagen was not

assumed to have taken place. It is more likely that the exposure of dentin to this medium resulted in the removal of

protons from positively charged amine groups, thereby limiting the ability of these groups to bind water. The binding

of anions to dentinal collagen did not occur at pH 3 as

evidenced by -AH values approximately the same as those obtained for the immersion of dentin in a slightly basic

medium (pH 8)

.

Although it has been shown by Gustavson 1 that aldehyde groups react with amino groups in the acid region, causing the discharge of hydrogen ions from the NH^+

groups

which exist under acid conditions, the adsorption and binding

of aldehydes by dentinal collagen did not appear to occur at

pH 5. Interaction between amino groups and the aldehyde

groups supplied by the adsorbate mixture did appear to occur
- 14. at pH 8, however. This observation Is In agreement with

the older belief that aldehyde groups can react with amino

, i Q

groups only in the uncharged state. However, since -AH*

determinations are concerned with nearly instantaneous

measurements of changes In heat content. It is possible that

the rate of interaction between charged amino groups and

aldehyde groups proceeded at too slow a rate to allow detection by this means.

5 . CONCLUSIONS

The magnitudes of the observed values of human

dentin in water indicate that factors other than the wetting

of a crystalline inorganic solid are involved. The collagenous material present at the surface of the powder particles

readily adsorbs water.

Under the proper environmental conditions, certain ions

and functional groups interact preferentially with the collagenous matrix. The reactions which occur at the surface

of dentin are similar to those exhibited by purified collagen.

The collagenous material at the cut dentinal surfaces of

a tooth which has been mechanically prepared to receive a

restoration offers potential sites to which bonding of a

restorative material may be achieved.
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references

1* Bowen, R. Lv J. Dent , Res .# 44, 1369 (1965),

2m Brauar, G. M. , in R. H, Austin,. H. G. F. Wilsdorf,

and R. W. Phillips, eds., Adhesive Restorative Materials -

XI, pp. 203-211, Public Health Service Publication No#

1494, (1966)

.

3# Huget, E. F., Brauer, G. M. , and Loebenstein, W. V.,

J. Dent , Res ,, 47 , March-April 1968.

4. Sunner, S., and Wadso, I., Acta Chem. Seand .^ 13, 97 (1959),

5# Peckham, S* C. , Losee, F, L., and Ettleman, X,, J# Dent . Res., 35, 1947 (1956).

6. Greg, S. J. , The Surface Chemistry of Solids , 2d ed#

,

Reinhold, New York 1961, p. 228.

7. Zettlemoyer, A. C., Chem. Revs . , 59 , 987 (1959).

8. Hess, W. C. , in R. W. Phillips and G. Ryge, eds.. Adhesive

Restorative Dental Materials pp. 10-14, National Institutes of Health and U. S* Public Health Service of H.E.W.

1961
9. - 20- Kanagy, J. R., J Soc Leather Trades Chem, 49 , 646 (1954)

•

10. Veis, A., and Schlueter, R. J., Biochem , , 1650, (1964)

•

11. Kanaqy , R. J . , J _Research NBS 44, 31 (1950).

- i —

12. Kauzmann, W, , Advan Protein Chem , 14, 1 (1959)

.

13. Zettlemoyer, A. C., JCn Chemistry and Physics of Interfaces

pp. 139-148, American Chemical Society Publications,

Washington, D # C. , 1965.

14. Boyd, Go E . , and Harkins, W. D a , J Am Chem Soc , 64 , 1190 (1942)

.

15. Gustavson, K. H. , The Chemistry and Reactivity of

Collagen , Academic Press, New York, 1956 pp. 94-95.

16. Ibid., pp. 128-129.

17. Gustavson, K. H. , Svensk Kern Tidskr , 52 , 261 (1940)

.

18. Mellon, E. F., in 0* Flaherty, F., Roddy, W. T. and

Lollar, R. Mo , eds.. The Chemistry and Technology of

Leather . Vol. II 0 Reinhold Publishing Corp., New York

1958, p. 75.
SKI HO Nl 30NV1S1S3H

Fig.

l.

Typical

Heat

of

Imaerslon

Curve

Obtained

for

Dry

Powdered

Dentin.