3174
The Journal of Physical Chemistry, Voi. 83, No. 24, 1979
Tria, Hoel, and Johnsen
Kinetics of Radical Decay in Crystalline Amino Acids. 7. Monohydrates John J. Tria, Donna Hoel, and Russell H. Johnsen" Chemistry Department, Florida State University, Tallahassee, florida 32306 (Received June 18, 1979) Publicat'on costs assisted by the U S . Department of Energy
The kinetics of radical decay in X-irradiated single crystals of L-arginine hydrochloride monohydrate, L-asparagine monohydrate, and L-histidine hydrochloride monohydrate have been studied. At temperatures sufficiently low that no measurable dehydration occurs, radical decay fits a model which includes distinct activation energies for the combination of nearby radicals and for diffusion of distant radicals to within a critical reaction distance. At temperatures sufficiently high that significant dehydration occurs on the time scale of radical decay, two types of behavior were observed. In L-asparagine monohydrate and L-histidine hydrochloride monohydrate, radical decay became rapid only upon completion of dehydration, whereas in L-arginine hydrochloride monohydrate radical decay became rapid at the start of dehydration and was completed well before dehydration was complete. A t intermediate temperatures the radical decay kinetics appear to be a combination of the diffusion-controlled process and the dehydration-assisted one. No radical conversions were observed during decay.
Introduction Many of the free radicals produced by the action of ionizing radiation on crystalline amino acids at low temperature have been identified and their transformations upon heating studied. In general, the radical produced at low temperature transforms by either inter- or intramolecular processes into other radicals as the temperature is raised.lB2 At one time the radicals present at room temperature were thought to be completely table.^ Earlier work by Stentz and Johnsen4 showed that the radicals produced in many amino acids in both crystalline and powdered form, previously thought to be stable, actually decay slowly at room temperature over a time scale on the order of months. In single crystals of DL-leucine, DL-valine, L-valine, and L-isoleucine, a t least part of the radical population was shown to decay by a second-order process. When the kinetic investigation was extended above room temperature,j free-radical decay in DL-valine and L-leucine was observed to occur with two distinct rates: an initial fast process of short duration with an activation energy of about 18 kcal/mol, and a second slower process with an activation energy 3-6 kcal/mol higher. Both processes fit second-order rate laws. The fraction of radicals decaying in the initial process was usually the same as the fraction previously reported to decay slowly at room temperature. The major fraction of decay occurred in the second, slower process and was assigned to a vacancy controlled bulk diffusion mechanism. A surprising aspect of these higher temperature experiments was the unique behavior of radical decay in L-arginine hydrochloride monohydrate single crystals. At temperatures below about 70 " C radical decay was similar to that in nonhydrated crystals of the other amino acids. At about 7 5 "C and higher, radical decay became very rapid after an initial slow decay period of about 15 min. Dehydration was found to occur rapidly at this temperature also. In the present study we have focused attention on the rapid radical decay which occurs at temperatures a t which the rate of dehydration is significant. Rates of dehydration were compared with rates of radical decay for single crystals of three different amino acid hydrates. Radical decay data a t lower temperatures where dehydration does not occur were also obtained in order to help distinguish the difference between the decay process below and above the dehydration temperature. Effects of total dose on radical 0022-3654/79/2083-3174$01.OO/O
decay and dehydration were also investigated.
Experimental Section 1.-Asparagine monohydrate and L-histidine hydrochloride monohydrate were obtained from Sigma Chemical, L-arginine hydrochloride was from Calbiochem, and D 2 0 (99.8% D) was from Thompson Packard. Single crystals of all three amino acid monohydrates were grown by evaporation from aqueous solutions a t room temperature. Crystals of deuterated L-asparagine monohydrate were grown from D 2 0 solution. Powdered samples were prepared by crushing single crystals with a Pyrex mortar and pestle. Samples were irradiated with 3-MeV peak X rays produced by the Florida State University 3-MeV Van de Graaff accelerator. Absorbed doses were about 15 Mrd unless otherwise stated. Doses were higher than in previous work in order to get a usefully large signal from small single crystals (0.02 8). With larger samples the water escaping from some crystals tended to dissolve those above them. G(R-)vs. dose studies indicate that no appreciable saturation was observed over the range 2-17 Mrd. In this range G(R.) is of the order of 0.5 spins/100 eV. Average interradical distance is calculated to be 32 A in the most concentrated case. At these temperatures all radical pairs have decayed prior to analysis. ESR spectra were obtained at X-band with a Varian E-12 spectrometer. Details of the sample preparation procedure, irradiation technique, and computer double integration of the first derivative ESR signals to obtain relative spin concentrations have appeared e l ~ e w h e r e . ~ ESR - ~ microwave power was kept around 1mW. In these systems this was adequate to avoid power saturation, which does not occur until approximately 3 mW at room temperature, and of course higher a t the higher temperatures used.* Dehydration of the amino acids was followed with a DuPont 950 thermogravimetric analyzer. Results L-Asparagine-H,O. The previously reported difference between radical decay behavior above and below the temperature of rapid dehydration indicated the importance of knowing the dehydration characteristics of the material being studied. Figure 1 shows that with a sufficiently slow heating rate (-4 OC/min) single crystals of the three com0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 24, 7979 3175
Radical Decay in Crystalline Amino Acids
I
.
P
6
t
TEMPERATURE, 'C
Flgure 1. Thermogravimetric anaiysis curves for single crystal samples of amino acid hydrates. Ail curves shown followed to complete dehydration of sample: curve A, ~-histidine.HCi.H,O; B, ~-asparagine.H,O; C, L-arginine.HCI-H,O, all recorded at 10 OC/min heating rate. Curve D, ~-arginineHCI-H,Oat 0.5 'C/min, shows loss of water of hydration in distinct stages.
I
I
I
I
I
I
I
80
40
I20
TIME, MINUTES
Flgure 3. Secondader kinetic plot for ~-asparagineH,O single crystal decayed at 85 OC. I = spin concentration in arbitrary units.
2.0
1.5
5a
0
0
+
*
LL
x
+ B +
.4
1
H
-'1.0 0
I
0
. 1
e
.
1
40
-
1
1
+
+
I
I
I20
80
I
I60
TIME, MINUTES
0.5
I
I
I
I
I
I
I
2
4
6
8
IO
12
14
TIME, MINUTES x
16'
Figure 4. (Curve A) The fraction of water remaining in a crystal of L-asparagineH,O dehydrating at 82 O C vs. time. (Curve B) The fraction of radicals remaining in an irradiated crystal of L-asparagine.H,O decaying at 80 O c vs. time.
Flgure 2. Second-order kinetic plot for ~-asparagineH,Osingle crystal decayed at 45 OC. I = spin concentration in arbitrary units.
pounds studied do not lose water of hydration at a significant rate until a critical temperature is reached where water loss becomes rapid. (The unusual shape of the arginine dehydration curve (Figure lC,D) is discussed below. Provided the temperature is low enough to avoid dehydration the results obtained are consistent with those previously obtained in other systems. These results are interpreted as indicating a two-step process for radical recombination involving a diffusion-controlled process at long times and a barrier to recombination at short times. This is the model which is described by the Waite equationg with the radiation boundary condition. At very short times and very long times this equation yields two approximations which are of the form of ordinary secondorder kinetic equations. In Figure 2 the decay data yields, on a second-order plot, two regions which can be approximated by straight lines as indicated. Arrhenius plots with decay rates derived from these straight line plots yielded an activation energy, E*,for the fast process of 33 kcal/mol and an activation energy, ,311, for the slower process of 32 kcal/mol. This is discussed further below. At temperatures near 80 " C and above, where dehydration becomes rapid, a much €aster radical decay process comes into play, as shown by the increase of decay rate after 40 min in Figure 3. A more detailed look a t the relationship between water loss and radical decay is given in Figure 4 where curve A represents
:\ \
\ 0
'It 16 0
40
80
I20
TIME, MINUTES
Flgure 5. First-order kinetic plot for asparagine.H,O decayed at 85 OC.
the fraction of water remaining and curve B represents the fraction of radicals remaining at 82 O C as a function of time. Between 0 and 70 min typical rapid decay followed by a much slower decay process is observed. The break in curve B at 70 min marks the onset of the second rapid decay and coincides with the complete dehydration of the crystal. The decay of radicals in deuterated L-asparagine. D20 at the same temperature falls on curve B within experimental uncertainty. This fast decay associated with
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
3176
1.0
Tria, Hod, and Johnsen
i I
.
'Ot
i.
.
.8 0
.6
A "
0
H H '
A
8 + o
-4
0
.
A A
4 Q %
A
'a
A +Om+
0
A
0
*
+
A
0
A
A
,
+.*
+a
8
A
R
+
+
.2 TIME, MINUTES 1
I
I
I
40
80
I20
I60
TIME, MiNUTES
Flgure 6. The fraction of radicals remaining in Lasparagine.H,O decayed 11.8 Mrd; (0) 13.9 at 80 "C vs. time for various radiation doses: (0) 18.9 Mrd; (W)25.1 Mrd; (A)26.1 Mrd; (A)30.3 Mrd; (+) 34.8 Mrd; (0) Mrd.
dehydration, unlike the initial fast decay, generally fits first-order kinetics better than zero or second order, as seen in Figure 5, but the fit is not particularly good. Radical decay at 82 "C was also followed in a special sample which had liquid water near to but not in contact with the crystals in the sealed ESR tube. This sample lost approximately 15% of the spins in a few minutes, after which spin concentration remained constant for 2 h. When the temperature was raised to 90 "C the seal on the tube was broken by the increased pressure, but the number of spins did not change for an additional 30 min, after which a very rapid decay occurred. It is conjectured that the 30 min during which the radicals did not decay at 90 "C was the time needed for the excess water to escape. The relative stability of radicals in this sample, compared to a normal sample without excess water (see Figures 3-5) indicates that the presence of saturated water vapor in the sample tube prevented the occurrence of the rapid decay process normally observed a t this temperature by preventing dehydration of the crystalline sample. A sample of powdered L-asparagine.HzOdecayed at 78 "C in exactly the same manner as single crystal samples at the same temperature, and with nearly the same decay rate, losing about 60% of the spins in 120 min. In contrast, a sample of powdered L-asparagine.HzO which was predried to a nonhydrated state (by heating at 90 "C for 1.5 h) before irradiation exhibited much slower radical decay a t 78 "C,losing only 20% of the spins in 100 min, after which radical decay occurred at a nearly negligible rate for another 500 min. This experiment does seem to corroborate the importance of the water of hydration in the fast decay process. The exact nature of the radical formed in the dehydrated sample is not known. There are spectral differences, but whether they are due to radical structure or differences in the matrix was not determined. I t has been suggestedlO that a radical decay which appears to be second order may in fact be a superposition of many correlated radical recombinations, each radical reacting with a nearby radical produced by the same ion-
Figure 7. Second-rder kinetic plot for L-histMine+lCi-H,O single crystal decayed at 139 OC. I = spin concentration in arbitrary units.
izing electron. In this case a plot of fractional decay vs. time would be independent of radical concentration since each radical is predestined to combine with a radical in the same track. Samples having doses ranging from 11to 34 Mrd were heated to 80 "C and the decay followed. When the results are plotted (Figure 6) as fractional decay vs. time the samples in the low dose portion of the concentration vs. dose curve show an increased rate of decay with increased dose, as expected for a second-order decay. The fractional decay vs. time curves for those samples corresponding to higher total dose were not superimposable, but neither did they follow any pattern which correlated with dose or concentration of radicals. Although the interpretation of the curves for these high dose sample is unclear, the decay is not due to correlated radical recombination according to this test. The spectrum of the radical species was unchanged during decay at all the temperatures studied, indicating that the radical was not changing structure. At the very highest doses of course, considerable structural damage has occurred. Clearly this has a significant effect on the kinetic process. L-Histidine.HC1.HzO. The dehydration temperature for L-histidine*HCl.H,O is higher than for the other amino acids studied, requiring 30 min at 164 "C for all the water to be driven off. The dehydration process appears to be zeroth order and has an activation energy of 8.8 kcal/mol. Radical decay at four temperatures, 130,139,150, and 162 "C, was studied. Again complex decay curves are observed if the temperature is low enough to avoid dehydration. A second-order plot for the decay of histidine radicals at 139 "C is shown in Figure 7. E1 was determined to be 13 kcal/mol. Sufficient data to determine E11 were not obtained because at these temperatures the rapid decay process associated with dehydration took over quickly. At 162 "C only the rapid reaction was observed. The activation energy for this process is 37 kcal/mol. A comparison between the dehydration of histidine and radical loss at 150 "C is shown in Figure 8. The break in curve A at 40 min marks the onset of the rapid process and coincides with the complete dehydration of the crystal which is shown in curve B. This is the same behavior as displayed by asparagine. L-Arginine-HCl.H20. At temperatures well below the dehydration point radical decay is relatively slow and in-
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3177
Radical Decay in Crystalline Amino Acids
It:.
z
W
W 0
w
.a.
4
.6
0
m
hm
-
O
B 0
.,I
0
A
** u L
o
40
G
TIME, MINUTES
Figure 6. (Curve A) The fraction of radicals remaining in an irradiated crystal of L-histidine.HCI.H,O decaying at 150 OC vs. time. (Curve B) The fraction of water remaining in a crystal of L-histidine.HCbH,O dehydrating at 152 OC vs. time.
.
I I60
120
80
TIME, MINUTES
**
0
Figure 10. (Curve A) The fraction of radicals remaining in an irradiated crystal of L-arginine.HCI.H,O decaying at 66 OC. (Curve B) The fraction of water remaining in the same crystal.
+
+ u
z ' P .4 c
5 o 0 - v - - -
40
I
\.------
60
TIME, MINUTES
Figure 9. (Curve A) The fraction of radicals remaining in an irradiated crystal of L-arginine.HCI.H,O decaying at 75 OC. (Curve B) The fraction of water remaining.
volves, at least at longer time, a second-order rate process. At temperatures high enough that dehydration becomes significant, a fast radical decay process associated with dehydration occurs. Figure 9 shows that at 75 "C, just a few degrees above the temperature of rapid dehydration, radical decay proceeds slowly for a few minutes until water loss begins, after which the decay rate accelerates rapidly until all radicals are gone. Behavior of this protonated sample was identical with the previously published5 decay curve for exchange deuterated ~-arginine-DCl*D~O. This behavior is clearly different from that exhibited by histidine and asparagine. Careful comparison of water loss and radical decay at 66 "C (just below the temperature of rapid dehydration) shows that rapid radical decay begins concurrently with water loss and is complete long before dehydration is finished (Figure 10). Figure 11 shows the somewhat surprising effect of relatively low doses of radiation on the subsequent dehydration behavior of ~-arginine.HCl*H~O single crystals. Whereas the loss of water of hydration clearly occurs in two steps for unirradiated single crystals, increasing doses of radiation cause more and more of the water of hydration to be lost in the first step, until a t high enough doses all water is lost in a single step. The dehydration behavior of single crystals receiving the highest doses approaches that of a sample produced by crushing an unirradiated single crystal to form a coarse powder. The temperature at which first water loss occurs is not lowered by mechanical or radiation damage, but increasing damage causes water loss to be completed more rapidly a t lower temperatures. In the neighborhood of the rapid dehydration tempera-
-"I I
i
20
\______ I
60
I
I
I
I
I
1
TEMPERATURE,
,
l
140
100
1 >
,
I
,
I80
OC
Figure 11. Radiation dose effect on water loss in L-arginine.HCI.H,O single crystals. All curves followed to complete dehydration of sample: (curve A) Nonirradiated; (B) 4.0 Mrd; (C) 8.0 Mrd; (D) 11.5 Mrd; (E) 16.3 Mrd; (F) nonirradiated coarse powder from crushing single crystal.
ture the variation in behavior from one crystal sample to another can be significant. Both the length of time which passes before rapid radical decay and water loss occur (the "induction time") and the exact rates of these processes show significant sample-to-sample variation in this region. Care was taken to use samples of similar size and shape selected from the same batch of crystals for comparative studies, and several runs of each experiment were made to ensure the reported results are typical. As seen in Figure 12, when two samples of similar size and shape were cut from a large single crystal and were irradiated to doses of 1.8 and 17 Mrd, respectively (covering the dose range over which dehydration behavior changes markedly), relatively little difference was observed in radical decay at 66 "C. This dose independence of the decay behavior, taken with the marked dose effect on the dehydration behavior, suggests that the decay process is associated with only one type of water molecule in the crystal, that which is lost during the initial stages of dehydration.
Discussion The radical formed at room temperature in L-arginine.
3178
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
A
L
A
.6
%_
A
*'
.4
v,
A
l
e
': 210
01
3
4;
8'0
TIME, MiNUTES Figure 12. Radiation dose effect on radical decay at 66 O C for two L-arginine*HCI.H,O samples cut from same single crystal: (0)1.8 Mrd; (A)17 Mrd.
HC1.H20 is not definitely characterized but is believed to result from H atom loss5 HZ N
\
l
C-N-C-CH,-CH,-CH
//
I
1
coo-
\
+NH, H H
c1NH,'
L-Asparagine.HzO has been shown'l to yield an H-loss radical as the major radiolysis product O8
NH,
/
'NH, I
#
I
\
c-c-c-c I
H H
0 0-
with a much smaller yield of the deamination product also occurring O NH,
f
I
i
0
H l
c-c-c-c I l
H H
1
\
0-
L-Histidine.HCl.H,O yields a radical formed by addition of a H atom to Cz of the imidazole ring.12 HN
NH
/ + I 7% C-CH
C1
NH,'-CH-COO-
All of the radicals under consideration thus retain the basic size and shape of the parent molecule and would be expected to exhibit diffusion rates and activation energies for diffusion similar to what would be expected for the parent molecules. Radical decay processes in the three systems appear to fall into two categories, those operative below and above the temperature of rapid dehydration, respectively. For the three hydrated amino acids studied, radical decay well below the dehydration temperature follows no simple decay law, but shows two straight line regions on a secondorder decay plot. We have previously interpreted similar radical decay behavior in certain organic crystals6p7in terms of a model due to Waiteg which involves an activation energy for combination of nearby radicals, EI, and an activation energy for diffusion of distant radicals to a critical reaction distance, En. A random initial spatial distribution of radicals is assumed. EIIreflects the nature of the dif-
Tria, Hoel, and Johnsen
fusion process (bulk self-diffusion or a defect-controlled process) and in the present case seems to correspond to the major radical decay process studied in single crystal and polycrystalline samples of nonhydrated amino acids and attributed to vacancy controlled bulk d i f f u ~ i o n . ~ t ~ For the present case, the values of EI = 33 kcal/mol and E11 = 32 kcal/mol for arginine are reasonable in terms of this model and are similar to values reported for radical decay in other (nonhydrated) amino acid samplesa5The current and previous radical decay data in amino acids shows little difference between EI and En in a given lattice. In the azoxyanisole homologues previously studied6!'EI was usually 0.4-0.7 of EII, and was interpreted as as activation energy for rotation of a radical about the long molecular axis in order to achieve an orientation favorable for combination with an adjacent radical. The near equivalence of EI and Errfor the amino acid radicals studied may be related to the ionic nature of the amino acid lattices and the hydrogen bond network which contributes to stability of the lattice. It may be that in these more tightly bound lattices the process corresponding to EI may be more correctly viewed as "detrapping" of the radical. The rapid radical decay associated with dehydration does not fit into this model. The most interesting aspect of this rapid decay is that for the case of arginine it begins as soon as dehydration starts and is completed long before dehydration is complete, while as asparagine and histidine rapid radical decay does not begin until after dehydration is essentially complete (compare Figures 4,8, and 9). In all three compounds studied the water of hydration is involved in a complex network of hydrogen bonds which helps hold the lattice together. The question we seek to answer is whether the loss of water of hydration causes rapid decay by enhancing the radical diffusion process or the combination processes, or whether dehydration and decay are related in some other way. Since rapid decay in asparagine and histidine begins near completion of dehydration, the increase in decay rate does not seem to be due to an increase in defect-aided diffusion since most of the defects formed by dehydration are present before the rapid process begins. This conclusion is supported by the fact that decay in a powdered (hydrated) sample of asparagine (presumably containing many more defects than a single crystal) is similar to decay in a single crystal. It thus appears that the last few water molecules left in the lattice are associated with the radical sites in a way which reduces radical reactivity (perhaps by reducing mobility) and at the same time results in these water molecules being more tightly bound in the lattice than water molecules not associated with a radical site. This seems to imply mutual trapping between a certain set of water molecules and the radicals. In contrast, rapid decay in arginine begins almost immediately with the onset of dehydration and is complete long before all water is lost. Thus, if this rapid decay is just due to the low temperature processes made easier by dehydration-produced defects, then only a few such defects are needed. However, we see in Figure 12 that a dose sufficient to significantly enhance the rate of water loss has relatively little effect on overall radical decay at 66 " C a temperature at which decay is normally slow, so a simple defect enhancement of the lower temperature decay processes is not a sufficient explanation. Figure 11shows that grinding a sample has the same effect on dehydration that irradiation to a high dose has. It thus appears that creation of lattice defects, either by grinding or irradiation, has a greater effect on water loss than on radical decay in arginine.
Radical Decay in Crystalline Amino Acids
It would be useful to be able t o relate the differences in rapid decay behavior between arginine on the one hand and asparagine and histidine on the other to differences in crystal structure. In ~-asparagine.H,Ol~and Lhi~tidine.HCl.H~O there ~ ~ is a total of seven hydrogen bonds per hydrated molecule, three of which involve the water molecule. On the other hand ~-arginine.HCleH~Ol~ has ten hydrogen bonds per hydrated molecule, four of which involve the water molecule. However, the hydrogen bonds in arginine are generally weaker than those in the other two crystals. Furthermore, asparagine and histidine form orthorhombic lattices with only one type of hydrated molecule per unit cell, while arginine forms a monoclinic lattice with two types of hydrated molecules per unit cell, each with a somewhat different relationship between the water of hydration and the amino acid molecule. Examination of our dehydration curves shows that the water of hydration is lost in a single step for histidine and asparagine, but in two nearly equal steps in the case of arginine. Whether the two steps correspond to the two types of molecules in the unit cell is uncertain, but the lower temperature of first water loss corroborates the conclusion from the X-ray crystallographic data that the hydrogen bonding network in arginine is weaker than in the other two lattices. It would appear that this behavior is connected in some way with the differences in rapid radical decay noted above. A serious difficulty in relating differences in hydrogen bonding to differences in rapid radical decay arises from the following consideration: although ESR studies have shown that in L-asparagine.H,O and Id-histidine.HC1.H,O there is little change in the molecular structure upon radical formation (and therefore probably little change in the hydrogen bonding), once dehydration begins we can no longer be sure of the nature of hydrogen bonding in the neighborhood of a radical site, since loss of water may occur near radical sites. In any case, after most of the water of hydration has been lost (as it is before rapid decay begins in asparagine and histidine) it is certain that the lattice environment and hydrogen-bonding network around the remaining radicals must be significantly altered.
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3179
While the exact nature of the hydrogen bonding network around a hydrated (or previously hydrated) radical is thus difficult to establish, the most significant hydrogen bonding may occur between the radical site and the water molecule. Joshi and Johnsen5 proposed in the case of arginine.HC1.H20 that the radical site is stabilized by hydrogen bonding to the nearby water molecule. Dehydration was postulated to destabilize the radical and increase its mobility, possibly by enabling hydrogen atom transfer along neighboring lines of molecules. Such stabilization by hydrogen bonding is not possible in asparagine and histidine because the radical site is not favorably positioned with respect to a water molecule. Unfortunately, the evidence gathered to date is not conclusive regarding the role of water of hydration in the rapid decay process, Acknowledgment. The authors thank Mr. D. E. Lott for performing the irradiations. The support of D.O.E. is also gratefully acknowledged. This is OR0 Document 2001-46.
References and Notes (1) G. Saxebd, Int. J . Radiat. Biol., 24, 475 (1973). (2) E. Westhof, W. Flossman, and A Muller, Mol. Phys., 28, 151 (1974). (3) W. Snipes and P K. Horan, Radiat. Res., 30, 307 (1967). (4) F B. Stentz, E. D.Taylor, and R. H. Johnsen, Radiat. Res., 49, 124 ( 1972). (5) A. Josh1 and R. H. Johnsen, J . Phys. Chem., 80, 46 (1976). (6) J. J. Tria and R. H. Johnsen, J Phys. Chem., 81, 1274 (1977). (7)J. J. Tria and R. H. Johnsen, J . Phys. Chem., 81, 1279 (1977). (8) A. M. H. Rezk and R. H. Johnsen, Int. J. Radiat. Biol., 34,337(1978). (9) (a) T. R. Waite, Phys. Rev., 107, 463 (1957);(b) ibid., 107, 471 (1957). (101 J. E. Willard. Science. 180. 553 (1973). ill) D. M. Close, G. W. Fouse, and W'. A. Bernhard, J . Chem. Phys., 66, 1534 (1977). (12) H. C. Box, H. G. Freund, and K. T. Lilga, J. Chem. Phys., 46, 2130 (1967). (13) (a) M. Ramanadham, S . K. Sikka, and R. Chidambararn, Acta Crysfabgr., Sect. 8 , 28, 3000 (1972);(b) J. J. Verbist, M. S. Lehrnann, T. F. Koetzle, and W. C. Hamilton, ibid., 28, 3006 (1972). (14)(a) K. Oda and H.Koyarna, Acta Crystalkgr., Sect. B, 28, 639 (1972); (b) J. Donohue and A. Caron, Acta Crystaliogr., Sect. A , 17, 1178 (1964);( c )J. Donohue, L. R. Lavine, and J. S. Rollett, ibld., 9, 655 (1956). (15) (a) J. Dow and L. H. Jensen, Acta Crystallog, Sect. B , 26, 1662 (1970);(b) S.K. Mazumdar and R. Srinivasan, 2.Kristaliog., 123, 186 (1966).
