ENDOR study of irradiated tooth enamel
γ- and x-irradiated tooth enamel has been studied by EPR and ENDOR. Radiation-induced EPR spectrum of tooth enamel was found to be a superposition of signals with dominant contribution determined by CO₂- radicals. Two types of these radicals were observed: ordered and disordered centers. EPR spectra...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
1999
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| Цитувати: | ENDOR study of irradiated tooth enamel / S. Ishchenko, I. Vorona, S. Okulov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 84-92. — Бібліогр.: 29 назв. — англ. |
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Ishchenko, S. Vorona, I. Okulov, S. 2017-05-27T16:03:18Z 2017-05-27T16:03:18Z 1999 ENDOR study of irradiated tooth enamel / S. Ishchenko, I. Vorona, S. Okulov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 84-92. — Бібліогр.: 29 назв. — англ. 1560-8034 PACS 61.72; 76.30; 76.70 https://nasplib.isofts.kiev.ua/handle/123456789/117926 γ- and x-irradiated tooth enamel has been studied by EPR and ENDOR. Radiation-induced EPR spectrum of tooth enamel was found to be a superposition of signals with dominant contribution determined by CO₂- radicals. Two types of these radicals were observed: ordered and disordered centers. EPR spectra of both CO₂- centers are described by axial g-tensor with g = 1.9975 and g ⊥= 2.0021 with g || c. The ENDOR spectrum of unannealed enamel powder consists of a singlet at Larmor frequency of ³¹P nuclei and doublet at Larmor frequency of ¹H nuclei. Samples annealing at T = 200-250 ⁰C resulted in the destruction of disordered centers and appearence of superhyperfine structure of ENDOR spectra. Its analysis with advanced the powder ENDOR theory allows to determine the superhyperfine constants and to find for the first time that the ordered CO₂- radical is located in B sites (phosphorous position) of bioapatite lattice. This substitution is accompanied by the shift of the nuclei of the first ³¹P shell towards the defects by 0.04 nm and the formation of the OH vacancy in the nearest radical surroundings. The authors wish to thanks Prof. A.B. Roitsin for consultations on the theory of powder ENDOR and Dr. S.V. Virko for assistance in computer calculations. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics ENDOR study of irradiated tooth enamel Article published earlier |
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ENDOR study of irradiated tooth enamel |
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ENDOR study of irradiated tooth enamel Ishchenko, S. Vorona, I. Okulov, S. |
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ENDOR study of irradiated tooth enamel |
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ENDOR study of irradiated tooth enamel |
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ENDOR study of irradiated tooth enamel |
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ENDOR study of irradiated tooth enamel |
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endor study of irradiated tooth enamel |
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Ishchenko, S. Vorona, I. Okulov, S. |
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Ishchenko, S. Vorona, I. Okulov, S. |
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1999 |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Article |
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γ- and x-irradiated tooth enamel has been studied by EPR and ENDOR. Radiation-induced EPR spectrum of tooth enamel was found to be a superposition of signals with dominant contribution determined by CO₂- radicals. Two types of these radicals were observed: ordered and disordered centers. EPR spectra of both CO₂- centers are described by axial g-tensor with g = 1.9975 and g ⊥= 2.0021 with g || c. The ENDOR spectrum of unannealed enamel powder consists of a singlet at Larmor frequency of ³¹P nuclei and doublet at Larmor frequency of ¹H nuclei. Samples annealing at T = 200-250 ⁰C resulted in the destruction of disordered centers and appearence of superhyperfine structure of ENDOR spectra. Its analysis with advanced the powder ENDOR theory allows to determine the superhyperfine constants and to find for the first time that the ordered CO₂- radical is located in B sites (phosphorous position) of bioapatite lattice. This substitution is accompanied by the shift of the nuclei of the first ³¹P shell towards the defects by 0.04 nm and the formation of the OH vacancy in the nearest radical surroundings.
|
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1560-8034 |
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https://nasplib.isofts.kiev.ua/handle/123456789/117926 |
| citation_txt |
ENDOR study of irradiated tooth enamel / S. Ishchenko, I. Vorona, S. Okulov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 84-92. — Бібліогр.: 29 назв. — англ. |
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2025-11-25T23:07:37Z |
| last_indexed |
2025-11-25T23:07:37Z |
| _version_ |
1850581028734763008 |
| fulltext |
8 4 © 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 1. P. 84-92.
PACS 61.72; 76.30; 76.70
ENDOR study of irradiated tooth enamel
S. Ishchenko, I. Vorona, S. Okulov
Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kyiv, 252028, Ukraine
Abstract. γ- and x-irradiated tooth enamel has been studied by EPR and ENDOR. Radiation-
induced EPR spectrum of tooth enamel was found to be a superposition of signals with dominant
contribution determined by CO
2
- radicals. Two types of these radicals were observed: ordered and
disordered centers. EPR spectra of both CO
2
- centers are described by axial g -tensor with
||g = 1.9975 and g ⊥= 2.0021 with cg |||| . The ENDOR spectrum of unannealed enamel powder
consists of a singlet at Larmor frequency of 31P nuclei and doublet at Larmor frequency of 1H
nuclei. Samples annealing at T = 200-250 0C resulted in the destruction of disordered centers and
appearence of superhyperfine structure of ENDOR spectra. Its analysis with advanced the pow-
der ENDOR theory allows to determine the superhyperfine constants and to find for the first time
that the ordered CO
2
- radical is located in B sites (phosphorous position) of bioapatite lattice. This
substitution is accompanied by the shift of the nuclei of the first 31P shell towards the defects by
0.04 nm and the formation of the OH vacancy in the nearest radical surroundings.
Keywords: tooth enamel, electron-nuclear double resonance, radiation-induced electron-paramag-
netic resonance.
Paper received 11.12.98; revised manuscript received 01.04.99; accepted for publication 19.04.99.
1. Introduction
The investigation of bioapatites has been started a few
decades ago. Late in the 60s apatites have been consid-
ered as a perspective laser material and studied by dif-
ferent techniques. Radiospectroscopic investigations
have been directed to clarification of mechanisms of in-
corporation of impurity atoms such as carbon, silicon,
sulphur and different cations in apatites lattice. The com-
plex study of naturae, synthetic and bioapatites has re-
sulted in building up of laser operating in IR region [ 1].
Besides, calcium apatites had attracted considerable at-
tention because they form a base of the important bio-
logical tissues such as bone and dental enamel. In these
tissues crystallites of hydroxyapatite are organized into
small prisms plunged into organic substance. Both prisms
and crystallites reveal certain ordering. The composition
and orientation of crystallites are believed to be closely
related to the total state of organism�s health. Therefore,
the information about their changes can be used in med-
icine for early diagnostics of some diseases [2]. Besides,
ionizing irradiation of bioapatites induces paramagnet-
ic centers, the number of which is proportional to the
irradiation dose. This phenomenon is used in retrospec-
tive dosimetry to determine radiation dose received by
people and animals [3-5] and in EPR dating of archeo-
logical findings [6, 7].
Tooth enamel is the most interesting calcified bio-
logical tissue. It contains a mineral phase representing
about 94-98 % of the total weight, the rest is formed by
water and organic matter. The EPR signal near g = 2.0
appeared after irradiation by γ- and x-rays is well known
[8-14]. However, the nature of this radiation-induced sig-
nal is not clarified completely in spite of many publica-
tions devoted to its study.
EPR spectrum in the irradiated tooth enamel has been
found to be a composite signal [13]. It leads to variation
of the line shape depending on experimental conditions
and sample history. This is the reason of discrepancies
among the data reported in different papers and leading
to uncertainty and mistakes in EPR dosimetry.
The CO
2
-, CO
3
3-, CO
3
-, CO- radicals [13], O- center [14]
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
85SQO, 2(1), 1999
and the so-called �background signal� with g = 2.0045,
∆H
pp
= 0.64 mT give the main contribution to the
mentioned radiation-induced EPR spectrum. The back-
ground signal is observed in the enamel samples before
irradiation and dominates at irradiation doses up to 1
Gy. Note that radicals can be located in different sites of
hydroxyapatite lattice resulting in the increase of the
number of the EPR components. Besides, the irradiation
of tooth enamel induces short-half-life radicals. Some
centers are destroyed by temperature annealing whereas
some new defects can be formed in accordance with
annealing processing.
However, the contributions of the mentioned centers
are not equivalent. A dominant contribution is caused
by the centers of one type. Some authors have assigned
this more intensive signal to CO
3
3- radical [8-10, 15] while
others have classified it as CO
2
- radical. The most
convincing evidence of CO
2
- interpretation is the pres-
ence of g-tensor component equal to 1.997. This value is
characteristic for CO
2
- and varies slightly depending on
radical surroundings. Besides, the recent study of CO
3
3-
in synthetic hydroxyapatite [16] allows to determine the
following g values of the radical: g
x
= 2.0045, g
y
=
=2.0034, g
z
= 2.0014. These parameters do not describe
the dominant EPR signal in tooth enamel.
Callens et al. [17] have carried out the EPR study of
intact and powdered human tooth enamel both unheated
and dried at 400 0C. Existence of two main EPR
components with similar parameters has been found. One
of them does not exhibit anisotropy and determined by
disordered centers. This component has been attributed
to CO
2
- radical in organic matrix of enamel and/or sur-
face CO
2
- radical. Another EPR component has been
assigned to a bulk CO
2
- radical. It has been noted that
the annealing results in the destruction of the disordered
centers. However, the position in the lattice has not been
determined for both radicals.
ENDOR is the most useful technique to define the
location of a paramagnetic center in crystalline lattice.
A few ENDOR measurements were carried out on tooth
enamel [8, 10, 18, 19]. The dominant ENDOR signals
observed in these experiments were the structureless sin-
glet line at Larmor frequency of 31P nuclei and singlet or
doublet line at Larmor frequency of 1H nuclei. Such
ENDOR spectra do not allow to obtain information
about the location of CO
2
- radical in hydroxyapatite lat-
tice. More informative ENDOR results have been ob-
tained on the synthetic hydroxyapatite. The superhyper-
fine structure of the 31P and 1H ENDOR spectra per-
mits to determine the location of CO
3
3-[16] and O-[14]
radicals in this material.
This article is devoted to the detailed ENDOR study
of irradiated tooth enamel to define the location of CO
2
-
in hydroxyapatite crystallites. The structure of 1H and
31P ENDOR lines was observed on enamel samples dried
at 250 0C. Its analysis allowed to obtain the information
about CO
2
- surroundings and to determine the radical
location in bioapatite lattice.
2. Materials and methods
The powder samples were prepared according to the tra-
ditional procedure of retrospective EPR dosimetry [3].
The paramagnetic centers were created by irradiation
with x- or γ- rays. The absorption dose was estimated to
be approximately 10 kGy. The heating of the samples
was carried out on air at temperatures 250-300 0C dur-
ing 30-60 minutes. Such annealing conditions were opti-
mum to observe the superhyperfine structure of ENDOR
spectra.
ENDOR spectra were recorded using EYa-1301 su-
perheterodyne spectrometer operating in the 3-cm range
of wavelengths. Measurements were performed at the
temperatures 77, 4.2 and 1.5 K. A sample was placed
along the axis of a TE
011
cylindrical microwave cavity. A
radiofrequency (rf) field inducing nuclear transitions was
generated by four rods driven across the cavity parallel
to its axis and connected in accordance with Helmholtz
scheme. The amplitudes of rf field were 0.01-0.05 mT.
The ENDOR signals were registrated by pulse modula-
tion at 1.5 kHz of the applied rf power and phase-sensi-
tive detection. The ENDOR spectra were recorded for
powder samples of the human and swine tooth enamel.
On heated samples the structure of ENDOR lines was
obtained with microwave power of approximately 5 µW
at liquid helium temperature. The dependence of
ENDOR spectra on the type of irradiation (x- or γ- rays)
and type of enamel (human or swine) was not observed.
3. The theory of powder endor
The orientation dependencies of ENDOR spectra can
be studied on powders if g -tensor of paramagnetic cen-
ter is anisotropic. During ENDOR measurements the
magnetic field H is fixed in certain position of a powder
EPR spectrum. This induces the resonance only for those
centers which have definite orientation relative to the
direction of the magnetic field. The changing of H value
permits to observe the centers with different orientations
separately, i.e., to registrate the angular variation of
ENDOR lines. The obtained dependencies are similar to
that of single crystals if superhyperfine A-tensor and elec-
tron g -tensor have axial symmetry and coinciding di-
rections. In general case, ENDOR spectrum is partly
averaged. However, the lines of such spectrum alter their
spectral positions while changing the value of fixed mag-
netic field. This permits to obtain the information about
the parameters of superhyperfine interaction. The above
technique has been named �orientation ENDOR selec-
tion� [20, 21].
The general theory of powder ENDOR is very
complicated. Its particular cases have been described
before [21-24]. It is appropriate to present the simplified
expressions which were used in this work. The expres-
sions have been obtained in the first order of the pertur-
bation theory and given the satisfactory description of
our experimental results. Note that these expressions do
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
8 6 SQO, 2(1), 1999
not consider the effect of alternating fields, relaxation
processes and transition probabilities. They contain also
some limitations which will be pointed out below.
The spectrum of powder ENDOR caused by
unpaired electron and nucleus with spins S = I = 1/2 can
be described in g -tensor axes frame (see Fig. 1) by the
following expression:
∫ ∫ ∑ ×−=
2/
0
2
0
0 )),((sin),(
π π
ϕθθυ
m
mHHLcHI
∑ −×
M
M ddHN ϕθϕθυυ )),,(( 0 , (1)
where L(H-H
0
) and N(n-n
0
) are the form-factors of EPR
and ENDOR line shapes, respectively, H
0
and ν
0
are the
resonant field of EPR and resonant frequency of EN-
DOR, M and m are the projections of electron and nu-
clear spins on H direction, C is a constant. In (1) H is a
parameter determining the point where EPR spectrum
is saturated and, thus, the group of paramagnetic cen-
ters with the same definite orientation.
H
0
and ν
0
can be obtained from the spin-Hamilto-
nian accounting for electron Zeeman, nuclear Zeeman
and superhyperfine interactions [21, 25].
),(
),(
),(0 ϕθβ
ϕθυ
ϕθ
g
mAh
H mwm −
= , (2)
=),,(0 ϕθυ HM
2/12
)(
),(
−
= ∑ ∑
−
i
ni
j
ijjj HhAhg
hg
M υ
ϕθ , (3)
where
2/1
2)(),(
= ∑
i
ii hgg ϕθ , (4)
),(/),(
2/12
ϕθϕθ ghgAA
i j
jjij
= ∑ ∑ , (5)
hHgH nnn /)( βυ = , (6)
and A
ij
are the components of superhyperfine A-tensor
in g-tensor axes frame, h
i
(i = 1-3) denote the direction
cosines of magnetic field vector, ig are the principal
values of g tensor, g
n
is the nuclear g factor, β and β
n
are the Bohr magneton and nuclear magneton, respec-
tively, h is the Planck�s constant, ν
mω is the microwave
frequency of EPR spectrometer.
The expressions for resonant field (2) and resonant
frequencies (3) contain many parameters and some sim-
plifications can be made.
Superhyperfine tensor has been considered to have
axial symmetry along the direction on nucleus, r
n
. Then,
in /// ,, zyx axes frame with ||/z r
n
it can be presented
as following:
+
−
−
=
ba
ba
ba
A
200
00
00
' , (7)
where
'
3
1
SpAa = , (8)
)(
3
1
'2'233 '' AAb −= , (9)
and a is a Fermi constant of isotropic superhyperfine
interaction, b is a constant of dipole-dipole interaction
determining the anisotropic superhyperfine interaction
[25].
We shall restrict ourselves to the axial symmetry of
g -tensor in EPR. In this case we can consider that ϕ
n
= 0
and yy ||/ . Then, the superhyperfine tensor in g -tensor
axes frame can be written as:
−+
−
−+
=
)1cos3(0sincos3
00
sincos30)1sin3(
2
2
nnn
nnn
bab
ba
bba
A
θθθ
θθθ
.(10)
Components A
ij
of A-tensor and ones A
i�j�
of A�-ten-
sor are related by transformation of the axes frame.
Superhyperfine interaction between CO
2
- and surround-
ing nuclei is smaller than the EPR linewidth, therefore it
can be neglected in (2). Besides, the anisotropy of g -
tensor of CO
2
- radical in tooth enamel is small [10] and
g can be considered as an isotropic value in (3). There-
fore, the spectrum of powder ENDOR can be written as:
∫ ∫ −=
π
π θθυ
0
0
0
2/ ))((sin),( HHLcHI
[ ] ϕθϕθυυϕθυυ ddHNHN )),,(()),,(( 00
−+ −+− , (11)
[ ] 2/1222
||
2
0
cos)(
)(
θβ
υ
θ ω
⊥⊥ −+
=
ggg
h
H m
, (12)
=± ),,(0 ϕθυ H
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
87SQO, 2(1), 1999
[ ]
2/1
2222 2sin
16
9
)1cos3(
2
1
)(
+−+±= γγυ bba
h
Hn ,(13)
nn θθϕθθγ coscoscossinsincos += , (14)
where γ is the angle between magnetic field and r
n
direc-
tion.
In our case b << ν
n
, therefore, the expression (13)
can be represented as a series expansion retaining only
the first term:
±=±
h
Hg
H nn β
ϕθυ ),,(0
−++−+ 1)
2
sin)cos(
2
cos)(cos(3
2
1 222 ϕθθϕθθ nnba
h .(15)
ENDOR signal defined by (11) is the spectrum aver-
aged over the orientations of magnetic field at θ = const
(see Fig. 1). If ENDOR frequencies are determined by
(15) then the spectrum (11) consists of three doublets cen-
tered at ν
n
. Spectral positions of the doublets peaks are
determined by:
[ ]{ }1)(cos3
2
1 2
1 −−+±=± θθυυ nn ba
h
,
)0(min == ϕγγ , (16)
[ ]{ }1)(cos3
2
1 2
2 −++±=± θθυυ nn ba
h
,
)(max πϕγγ == , (17)
2
),(
2
1
3
πγυυ =−±=± ba
hn . (18)
Note, that ν
3
± is observed only for nθπθ −>
2
. These
expressions have been deduced from (11) assuming that
the lineshapes are δ-function-like for both EPR and
ENDOR lines. Expressions (16)-(18) do not contain the
relative intensities of ENDOR doublets and can be used
to obtain superhyperfine parameters in the first-order
approximation only. Their accurate values can be deter-
mined from computer fitting of the experimental spec-
trum and the one calculated on the base of (11) or (1).
Such analysis has shown that the intensities of υ
3
± dou-
blet exceed considerably the ones of other lines. Thus,
υ
3
± doublet determines ENDOR spectrum if the later
takes place. This complicates the study of powder
ENDOR because the υ
3
± doublet has no angular depen-
dence.
It is convenient to introduce the value +− −= υυδ
which does not depend on the shift of υ
n
at H scan and
describes the orientation dependence of ENDOR spec-
trum.
4. Experimental results
4.1 EPR
EPR spectra of tooth enamel have been studied in de-
tail, so we discuss only the data that will be necessary to
understand ENDOR results.EPR spectrum observed by
us in the unheated samples is determined mainly by the
signal from CO
2
- radicals. This signal has been well de-
scribed by axial g -tensor with g
⊥ = 2.0021, ||g = 1.9975
and cg |||| ( c is the hexagonal axis of hydroxyapatite)
that agrees with our earlier publication [10]. There was
no reason to use a rhombic g tensor.
The sample annealing resulted in the decrease of the
overall intensity of EPR spectrum. It can be explained
by complete destruction of disordered CO
2
- radicals [17]
and particular disintegration of bulk CO
2
- radicals. Be-
sides, the relative intensities of weak signals changed
slightly and the new spectrum components revealed. As
to new signals, it should be noticed that EPR spectrum
consists of seven components centered at g = 2.0003 and
separated by 2.18 mT. The central line of this signal is
masked by more intensive signal from CO
2
- radical. This
spectrum has been assigned to (CH3)-C-R radical acti-
vated in organic matter by annealing [11]. EPR spectrum
Fig. 1. Definition of the different polar angles describing the
magnetic field vector and the direction of the interacting nu-
cleus in the g -tensor axes frame. z�� axial axis of the superhy-
perfine tensor.
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
8 8 SQO, 2(1), 1999
at g = 2.006 and ∆H
pp
= 0.1 mT was observed too. It
has been attributed to SO
2
- radical [26]. ENDOR sig-
nals caused by (CH
3
)-C-R and SO
2
- radicals were not
observed. EPR spectra of irradiated tooth enamel be-
fore and after the annealing are shown in Fig. 2. The
main EPR signal is seen to change slightly under the
annealing.
4.2. ENDOR
ENDOR spectrum of unheated powder of tooth enamel
has been recorded at T = 77 K and T = 4.2 K. It consists
of the singlet at Larmor frequency of 31P nuclei and dou-
blet at Larmor frequency of 1H nuclei. The intensity of
phosphorous line changes according to EPR lineshape
while its linewidth does not vary at different magnetic
fields within the experimental errors. Notice that field
dependence of an ENDOR intensity has been called
ENDOR-induced EPR (EI-EPR). The changing of 1H
doublet intensity distinguishes slightly from EPR line-
shape on the lowfield wing of EPR line. Besides, 1H dou-
blet splitting increases considerably on the EPR line
wings (Fig. 3). Similar behavior of the 1H doublet field
dependence has been also observed by Galtsev [19]. Re-
ducing the measurement temperature from 77 to 4.2 K
resulted in the increase of the overall intensity of EN-
DOR spectra only.
The doublet structure of the proton ENDOR signal
vanished after annealing. However, a new structure was
revealed for both 1H and 31P signals at liquid helium tem-
perature and below. This structure was caused by inter-
action of CO
2
- radical with nuclei of hydroxyapatite
lattice. It is observed for all settings of magnetic field
within EPR spectrum and centered at Larmor frequen-
cies of 1H and 31P nuclei.
The structure of 31P ENDOR spectra consists of nine
doublets (see Fig. 4). The δ values for all doublets are
represented in Table I. The peaks of each ENDOR dou-
336 340
b
a
E
P
R
s
ig
n
a
l,
a
rb
.u
n
.
Magnetic field, mT
Fig. 2. EPR spectrum of irradiated tooth enamel powders near
g = 2.0, T = 300 K: a � unheated sample, b � sample dried at
T = 2500C during 45 minutes.
5 6 12 13 14 15 16
1
H
31
P
H= 329.8 mT
H=330.7 mT
H=331.3 mT
E
N
D
O
R
s
ig
n
a
l,
a
rb
.u
n
.
Radiofrequency, MHz
Fig. 3. ENDOR spectra of unheated sample of tooth enamel
for different settings of magnetic field, υ
mω= 9252 MHz.
5,5 6,0 ν, MHz
θ = 53
0
H=332.1 mT
9
8
7
6
5
4
3
2
1
x4E
N
D
O
R
s
ig
n
a
l,
a
rb
.u
n
.
Fig. 4. 31P ENDOR spectrum of annealed powder of tooth
enamel. ENDOR doublets caused by nuclei of different shells
are labeled by 1-9.
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
89SQO, 2(1), 1999
blet can be formed by nuclei of one shell as well as by the
contributions of nuclei involved into several shells. The
structure of 31P ENDOR spectra can be arbitrarily sub-
divided on a three groups of lines. The first group
contains the doublets 1, 3, 6, 7 which depends on setting
of the magnetic field. The second group involves dou-
blets 2, 4, 5. Their spectral positions do not shift when a
setting of the magnetic field is changed. Finally, the lines
being near Larmor frequency are attributed to the third
group. These are 8 and 9 doublets; their lineshapes and
intensities are modified at different settings of the mag-
netic field. But their spectral shifts are small and can not
be determined accurately.
The structure of 1H ENDOR spectra is less distinc-
tive than the one 31P although seven doublets can be ex-
uded. The typical 1H ENDOR spectrum is represented
in Fig. 5. Its lineshape is modified while the setting of
the magnetic field is changed, however, the shifts of the
peaks can not be determined accurately. The values of δ
are represented in Table II. Note that intensive struc-
tureless signal centered at υ
n
revealed in both 1H and 31P
ENDOR spectra too. It can be attributed to centers
located in sites with high concentration of defects. The
contribution from distant nuclei is also possible.
5. Analysis of the spectra and discussion
The admixed hydrogen atoms other than ones involved
into hydroxyapatite lattice are present in unheated tooth
enamel. They occur in crystallites in the forms of H
2
O
molecules, admixed OH groups and H-centers of differ-
ent types. These atoms form the large amounts of hy-
droxyapatite defects that lead to disappearance of supe-
rhyperfine structure in ENDOR spectra. Besides, the hy-
drogen arranged randomly appears to be responsible for
Table I.
31
P superhyperfine coupling constants of the CO
2
-
radical in tooth enamel.
Doublet 1 2 3 4 5 6 7 8 9
δ, kHz 920-
860
530 380-320 220 170 130-110 60-40 40-30 11
h/a , kHz -310
±20
-165 a
±20
-90±20 -49 a
±15
-33 a
±10
-20±10 0±10 0
h/b , kHz 630
±20
385 ±20 260 ±20 183 ±15 147 ±10 100 ±10 60 ±10 56- 38b
θn, deg. 28±4 90±4 40±4 90±4 90 ±4 60 ±4 65 ±4
rn, nm 0.37
±0.01
0.44
±0.03
0.50
±0.01
0.56
±0.03
0.60
±0.03
0.68
±0.01
0.81
±0.01
l
nθ , deg 33 90 47 90 90 59 65
l
nr , nm
0.41 0.47 0.50 0.55 0.63 0.68 0.80 0.83-
0.94
0.97 �
shell I II III IV V VI VIII IX-XIV XV,.
a � values calculated with using the expression (19)
b � values calculated with using the expression (20) and l
nr
Fig. 5. 1H ENDOR spectrum of annealed powder of tooth
enamel. ENDOR doublets caused by nuclei of different shells
are numbered by 1-7.
13,5 14,0 14,5
θ = 800
H=331.9 mT
7
6
5
4
3
2
1
ν, MHz
x3
EN
D
O
R
si
gn
al
, a
rb
.u
n.
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
9 0 SQO, 2(1), 1999
the doublet structure of proton ENDOR signal. The sim-
ilar spectrum lineshape has been described in the work
[24] devoted to matrix ENDOR. The peaks positions of
such spectrum are determined by competition between
signal increase from distant nuclei due to the growth of
their amounts and signal decrease due to the diminution
of the transition probabilities and the relaxation mech-
anisms.
The change of the doublet splitting of the proton
ENDOR (Fig. 3) and deviation of EI-EPR lineshape
from EPR one can be explained by the presence in the
ENDOR signal of two components corresponding to two
paramagnetic centers. The first component is a narrow
intensive doublet with the splitting ∆
1
= 100-140 kHz. It
dominates in the central part of the EPR spectrum. The
value of ∆
1
is changed within the mentioned region for
different samples but it is constant for a certain sample
between g = 1.9975 and g = 2.0021. Thus, the narrow
component can be attributed to the main EPR signal
caused by the bulk CO
2
- radical. The second component
is the wider doublet with splitting ∆
2
= 260 kHz. It dom-
inates on the wings of the EPR spectrum. The nature of
a center that causes this signal is not clear. The above-
mentioned disordered CO
2
- radical is probably respon-
sible for it. The relaxation characteristics of the disor-
dered radicals appear to be more favourable for EN-
DOR observations. This increases the contribution of
these centers in EI-EPR signal and results in different
lineshapes of EPR and EI-EPR spectra. Such relaxation
amplification of ENDOR signal is probably absent in
phosphorous ENDOR. ENDOR spectra of the annealed
tooth enamel samples have been analyzed using the the-
ory of powder ENDOR. The a , b and θ
n
parameters
have been estimated using (16) - (18) and the assump-
tion that ( ) ( )[ ] 2/122
||
22
⊥⊥ −−= ggggθ where
Hhg m βυ ω /= at H = H
0
. Accurate values of parame-
ters have been determined by computer fitting of the
spectra calculated according to (11) to experimental ones.
In these simulations the gaussian with ∆H = 0.25 mT
and lorentzian with ∆υ = 10-30 kHz for different shells
have been used as EPR and ENDOR lineshapes, respec-
tively.
Note that parameters variation did not resulted in
the accurate agreement between calculated and experi-
mental dependencies. Experimental curves were more
smooth than the calculated ones. The rhombic g-tensor
with g
x
= 2.0030, g
y
= 2.0015, g
z
= 1.9970 represented in
[16] and the expression (1) have been used to simulate
the ENDOR spectra, too, but improvement has not been
obtained. Thus, the mentioned discrepancy appears to
be due to the neglection of relaxation orientation de-
pendencies in (1) and (11).
The analysis of the expressions (16)-(18) and (11) has
shown that angular variation of the second group (dou-
blets 2, 4, 5) of phosphorous ENDOR is characteristic
for nuclei with θ
n
= 900. Powder ENDOR spectra of such
nuclei consist of dominating doublets (18) which have
no orientation dependencies. In this case the value ba −
can be determined at once from the experiment. To esti-
mate the constants a and b separately the following ap-
proximation can be made:
kreaa −= 0 , (19)
where 0a and k are the parameters which can be deter-
mined using the expression (19) for nuclei of first group
(doublets 1, 3, 6, 7).
The anisotropic superhyperfine interaction has been
considered as pure dipole-dipole interaction. Then
constant b can be written as:
3r
gg
b nn ββ
= , (20)
where r ≡ r
n
corresponds to the distance between CO
2
-
radical site and nucleus n.
The values θ
n
and r
n
obtained from ENDOR spectra
for different nuclei allow to determine the location of CO
2
-
radical in crystal lattice. The values l
nθ and nr for differ-
ent possible sites of CO
2
- in hydroxyapatite lattice have
been calculated using the model of its structure (Ref. 28)
and compared with θ
n
and r
n
. It has been found that
only CO
2
- in B site (phosphorous substitution) at cg ||||
agrees well with the experimental data. Some discrepancy
between experimental and lattice data for phosphorous
nuclei of shell I (see Table I) can be explained by their
shift towards the defect. Note that the ENDOR lines due
to 31P nuclei of VII shell were not observed. In accor-
dance with the computer simulation these lines have in-
tense angular dependencies and they are broadened
strongly. Besides, they are likely superimposed with lines
of shell V and VI.
A constant a can be considered to be equal to zero
for phosphorous of VIII shell and more distant nuclei.
Then the frequency region for lines caused by these nu-
clei can be estimated by means of (15) and (20) using
lattice data for l
nθ and l
nr . Such calculations permit to
explain the peaks 8, 9 in phosphorous ENDOR (see
Table I).
The values l
nθ and l
nr have been also used to simulate
proton ENDOR spectra. Constants b have been
calculated from l
nr . Constants a have been determined
by computer fitting of calculated ENDOR spectra to
experimental ones. The obtained values of a have not
been contradicted with the model of center when OH
vacancy is situated in the nearest surrounding of CO
2
-
radical. This finding agrees with the well-known model
[29] of carbon incorporation in hydroxyapatite structure:
OHCa VCOVOHPOCa ++↔++ −−−+ 3
3
3
4
2 , (21)
where CO
3
3- molecular ion is a precursor of CO
2
- radical.
The CO
2
- site and surrounding apatite structure are
represented in Fig. 6-8. The neighbour nuclei 31P and 1H
are numbered in accordance with Tables I, II.
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
91SQO, 2(1), 1999
6. Conclusion
The signal caused by CO
2
- dominates in EPR spectrum
of irradiated tooth enamel. The contributions of other
centers have been revealed too, but their intensities are
smaller. Two types of CO
2
- radicals are observed in un-
heated samples. One of them is the bulk center. It causes
the angular variations of EPR spectrum in a tooth plate.
The centers of the second type are disordered and cause
EPR spectrum of powder line shape. They appear to be
located on the surface of crystallites.
The singlet at Larmor frequency of 31P nuclei and
doublet at Larmor frequency of 1H nuclei are observed
in ENDOR spectrum of unheated tooth enamel. The
value of doublet splitting depends on the settings of mag-
netic field and is equal to 100-140 kHz and 260 kHz in
the central part and wings of EPR line, respectively. The
doublet structure of proton ENDOR can be explained
by the theory of matrix ENDOR assuming the presence
of a large amount of admixed hydrogen atoms. The modi-
fication of proton ENDOR line at different settings of
magnetic field appears to be connected with the exist-
ence of two types of CO
2
- radicals. These radicals cause
the proton ENDOR signal with various splittings, but
their contributions are different in the central part and
on the wings of EPR line.
The annealing at 250 0C results in the decrease of the
overall intensity of EPR spectrum due to complete de-
struction of the disordered CO
2
- radicals and particular
disintegration of the bulk CO
2
- radicals. Small variation
of EPR lineshape due to the redistribution of the inten-
sities of weak signals and appearance of the new signal
from centers activated by annealing is also observed. The
structure of phosphorous and proton ENDOR centered
at Larmor frequencies of 1H and 31P nuclei reveals after
annealing. Its analysis allows to determine the constants
of superhyperfine interaction of CO
2
- radical with neigh-
bour nuclei and to conclude that CO
2
- ion substitutes
phosphorous in hydroxyapatite lattice of tooth enamel
(B site). This substitution is found to be accompanied by
the creation of OH vacancy in the nearest surroundings
and shift of phosphorous nuclei of shell I towards the
Fig. 6. Fragment of hydroxyapatite structure. δγβα ,,,
are the hexagonal axes.
Fig. 7. Hydroxyapatite structure close to the hexagonal c-axis
and proposed site for CO
2
- radical. The numbers I-X are shown
the location of protons of different shells.
Fig. 8. The locations of phosphorous nuclei of different shells
relative to CO
2
- radical.
S. Ishchenko et al.: ENDOR study of irradiated tooth enamel
9 2 SQO, 2(1), 1999
defects by dr = 0.04 nm. The experimental angular de-
pendencies of ENDOR lines are more smooth than the
calculated ones. This discrepancy appears to be due to
the neglection of an orientational dependence of
relaxation times in the theory. All EPR and ENDOR
spectra have been described by axial g -tensor with ||g =
= 1.9975 and ⊥g = 2.0021 at cg |||| .
Acknowledgments
The authors wish to thanks Prof. A. B. Roitsin for con-
sultations on the theory of powder ENDOR and Dr.
S. V. Virko for assistance in computer calculations.
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