ESR, ENDOR, and double ENDOR at 300°K of DLserine x irradiated at 340°K, a newradical speciesDon A. Hampton and Grace C. Moulton Citation: The Journal of Chemical Physics 63, 1078 (1975); doi: 10.1063/1.431449 View online: http://dx.doi.org/10.1063/1.431449 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/63/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in ESR and ENDOR studies of the main radical created by gamma irradiation at 300 °K in the imidazole crystal J. Chem. Phys. 59, 3365 (1973); 10.1063/1.1680480 ESR and ENDOR studies of DLserine irradiated at 4.2 °K J. Chem. Phys. 59, 2509 (1973); 10.1063/1.1680365 Electron Spin Resonance and Electron Nuclear Double Resonance Studies of DLSerine. II. Identification of FreeRadicals Produced in the Range 153–340°K J. Chem. Phys. 57, 2762 (1972); 10.1063/1.1678663 Electron Spin Resonance Studies of Single Crystals of dlSerine. I. X Irradiation at 77°K and 300°K J. Chem. Phys. 55, 2598 (1971); 10.1063/1.1676455 ESR Spectra of a GammaIrradiated Single Crystal of DLSerine J. Chem. Phys. 35, 764 (1961); 10.1063/1.1732014

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ESR, ENDOR, and double ENDOR at 300 'K of DL­

serine x irradiated at 3401<, a new radical species* Don A. Hampton t and Grace C. Moulton

Department of Physics. Florida State University. Tallahassee. Florida 32306 (Received 19 February 1975)

When DL-serine single crystals are x rayed at 300 0 K or below. two stable radicals are observed at room temperature. One of these. IV. is of greater intensity than the other, V, and was previously identified. By irradiating at a slightly elevated temperature, ~ 340 oK, the concentration of V becomes more intense than IV. In this work we report on a detailed study of V using ESR, ENDOR, and double ENDOR, all at room temperature. We have determined that V is

H NH3 I I

'C-C-C=O

L~J ' H

and have obtained five hyperfine tensors representing one a-H, two I3-H's, the rotating amino H's, and a fourth H which is probably in a neighboring molecule. We have also observed the ENDOR transition of the nitrogen. The a-H tensor is somewhat unusual in that the isotropic term (-41.3 MHz) is considerably less than for a 2p1T electron and of the same sign, but the anisotropic term is approximately the same (+31.9, +3.5, -35.4 MHz). This is consistent with our model since the bonding to the a-carbon is strained and would not be expected to be pure s p 2.

INTRODUCTION

The amino acid DL-serine has proved to be a very in­teresting compound to study. There have been a num­ber of papers published identifying several free radicals and free radical reactions which occur after irradiation of serine single crystals_ 1-5 The free radicals that are stable at room temperature and the free radical reac­tion pathways seem to be unusually sensitive to previous treatment. There are two free radicals which are stable at room temperature, which we will designate as Nand V after Castleman and Moulton_ Z Radical IV has been unequivocably identified as COO-CHNH sCHOH, produced when the deamination radical decays by abstracting a hydrogen atom from another molecule_ Z The other radical, V, is somewhat unusual and was first tentative­ly identified by Almanov et al. 3 as CH3C(OH)COOH, and later by Castleman and Moultonz as COO-CNH ;CHzOH_ This radical is produced as a minor product if a crys­tal is irradiated at 77 OK and warmed to room tempera­ture, as an almost equal product if the crystal is x rayed with 3 MeV x rays at 298 OK, and as the major product (and N a minor product) if it is x rayed at a slightly elevated temperature, about 340 OK.

We have undertaken a thorough study of this radical at room temperature using electron spin resonance (ESR), electron nuclear double resonance (ENDOR), as well as electron nuclear double-double resonance (ENDOR/2).6 Using ENDOR/2 we were able to identify with certainty that four ENDOR transitions belong to radical V, and also the relative signs of the tensors of each of these four hyperfine tensors which were posi­tively identified as belonging to this radical_ We also identified, using ENDOR/2, the ENDOR lines which be­longed to the same magnetic site_ We conclude that V is

1078 The Journal of Chemical Physics. Vol. 63, No.3, 1 August 1975

H NH+ I I 3

'L~J=O' H

which must be in a somewhat strained configuration_ In this paper we will justify this conclusion.

EXPERIMENTAL PROCEDURE

The normal DL-serine crystals were grown from sat­urated aqeous solutions in a constant temperature bath. Deuterated crystals in which amino and hydroxyl hydro­gens were replaced by deuterium were grown from solutions of DL- serine in DaO in proportions such that at least 95 percent of these hydrogen atoms were replaced with deuterium. Crystals were also grown of ex-d-l DL-serine by slow evaporation of aqueous solutions of ex-d-l deuterated DL-serine powder obtained from Bio Rad Laboratories. We also had some crystals of DL­serine which had only the amino protons replaced by deuterons and not the hydroxyl protons. We will desig­nate these as ~D3 deuterated. We observed that in crystals which were grown and stored for a least two years, either unirradiated or irradiated, the dueterons of the O-D were replaced by protons, but the amino deuterons remained_ We determined this by first look­ing at some crystals which had been irradiated in 1971 and stored. We noted that the spectrum of radical N for both the old deuterated crystal and normal crystals were the same, whereas in a freshly prepared deuter­ated crystal the 21. 2 G doublet was replaced by a - 3 G triplet. If we irradiated some of the old deuterated

Copyright © 1975 American I nstitute of Physics

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D. A. Hampton and G. C. Moulton: ESR, ENDOR of DL-serine 1079

crystals which had not previously been irradiated, again we got the same ESR spectrum for N as in the normal crystal. It was determined that the ammonia group re­mained deuterated since the 77 OK anion radical is the same for either old or fresh deuterated crystals. The anion abstracts a deuterium atom from the ND 3 group.

The crystals are monoclinic with unit cell dimensions a= 10. 72 A, b = 9.14 A, e = 4. 825 A, and 13=106 ° 27'.7 There are four molecules per unit cell, which are re­lated in pairs by a center of symmetry, giving two magnetically inequivalent sites. The measurements were made with respect to an orthogonal axis system using the crystallographic b and e axes and an axis a * perpendicular to band e. The crystals were irradiated at approximately 340 OK to a total dose of several megarads with either 50 kV or 3 MeV x rays.

The spectrometer used was an x-band spectrometer with a cylindrical cavity operating in the TEoll mode. The cavity side walls were silver plated Plexiglas with silver plated brass end plates. The modulation coils were outside the cavity and could be rotated when the magnet was rotated. The rf coil for the ENDOR mea­surements was a three-turn coil inside the cavity mounted on the crystal holder. An rf oscillator and an EN! model 320 L broad band rf power amplifier sup­plied the rf power. A field-tracking NMR spectrometer was used for measuring the free-proton resonant fre­quency. For the ENDOR/2 measurements a second rf oscillator which was frequency modulated at 140 Hz and a second lock-in amplifier tuned for second harmonic detection were used. The two rf oscillators were iso­lated from each other by a buffer amplifier and fed to the rf coil in the cavity. All the ENDOR and ENDOR/2 measurements reported here were made at room tem­perature. The rf power at the sample was low enough to avoid obtaining negative ENDOR signals for the ENDOR/2 experiments.

GENERAL OBSERVATIONS

Four different hyperfine tensors have been deter­mined from the ENDOR data and have been shown to be­long to the same radical from the ENDOR/2 measure­ments. These include a large J3-hydrogen coupling, a smaller J3-hydrogen coupling, an a-hydrogen coupling, which is somewhat unusual, and a small coupling to the NH3 hydrogens. We observed ENDOR lines for the ni­trogen itself, but were not able to follow the lines through enough orientations to obtain a tensor. We also obtained a hyperfine tensor for a fifth proton which we believe to be part of radical V, however, we were not able to detect the ENDOR/2 transition for it. Because of complications, we were only able to observe ENDOR/ 2 transitions at or near Holl a*. This tensor has a very small value at this orientation and the ENDOR signal is weak. Therefore, we cannot unequivocably state that this latter proton belongs to radical V. However from ESR data at Ho II b, where this tensor has a larger value, this coupling is needed to generate the ESR spectra from the ENDOR tensors. The a-hydrogen coupling has the usual anisotropic principal values expected for a 2P7T unpaired electron, but the isotropic term is appreciably

14.80 (MHz)

FIG. 1. Second derivative x-band ESR spectrum of DL-serine x irradiated at 340 0 K and observed at 300 0 K for Holl b.

smaller than expected (- 41. 3 MHz compared to - - 63 MHz). The sign has been determined by ENDOR/2 to be opposite to that of the large J3-hydrogen coupling and therefore must be negative. This is probably a strained configuration which is not quite pure 2p7T. It is probably not a (j radical however, even though the magnitude is what would be expected for a (j electron. The sign is opposite to what has been proposed for a (j radicaI8-

14

and the g tensor is more typical of a 7T radical than a (J

radical. All five of these couplings and a small N cou­pling are necessary and sufficient to construct the ESR spectra from the ENDOR data. The ESR spectra were reconstructed from the ENDOR tensors for Ho II b and Ho II a*.

EXPERIMENTAL RESULTS

The ESR spectra for radical V at most orientations consist of many overlapping lines. In Fig. 1 is shown an ESR spectrum for Ho II b. In Fig. 2(a) are shown the ENDOR lines for all six couplings for this radical for Ho II a*. The typicallinewidths are 300 kHz. The lines at 71.17 and 41.69 MHz are the high and low frequency transitions, respectively, for a large J3-proton. The line at 36.63 MHz is due to a highly anisotropic a-pro­ton while those at 26.47 and 16.94 MHz are due to small J3-protons. The latter transition was only seen in this orientation as a negative ENDOR signal by the applica­tion of intense rf power. In other orientations only positive signals were obtained. The lines at 17.68 and 16.07 MHz are much narrower lines than the other pro­ton lines and represent, we believe, the interaction of the protons of intra- and intermolecular rotating am­monia groups. The linewidth of the 17.68 MHz transi­tion is only 55 kHz and quite intense. The two lines at 4.88 and 4.51 MHz are due to a nitrogen interaction showing quadrupole splitting. There were many other lines present also. For orientations of the magnetic field off axis, many more lines occasionally appeared and disappeared. Some of these lines had linewdiths of only 1 kHz. Attempts to follow any of these other in­teractions were unsuccessful. The nitrogen interaction was also seen along the e crystal axis. A single line was detected at - 3. 6 MHz; no quadrupole splitting was detected at this orientation.

Rotation data for the external magnetic field in the be crystallographic plane are shown in Fig. 3. From the three planes of ENDOR data, the principal values

J. Chern. Phys., Vol. 63, No.3, 1 August 1975

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1080 D. A. Hampton and G. C. Moulton: ESR, ENDOR of DL-serine

(0) DA IrfI 4.4 4.6 4.8 5.0 16 17 18 26 27

0 41 42

(MHz)

M (b)

36 37

of the hyperfine coupling constants and associated er­rors were obtained by a least squares fit computer pro­gram, correct to first order in ENDOR, and are given in Table I. The principal values of interactions 2 and 4 were obtained by determining the signs of the off-di­agonal tensor elements in the laboratory frame from out of plane ENDOR rotation data. The magnetic field was initially orientated 32 0 from the b crystal axis in the ab plane and was rotated toward the c axis. For each in­teraction there are four principal direction tensors re­lated by pairs. Only one pair corresponds to the two magnetically distinct sites in the unit cell.

All the interactions except 4 are more or less typical of i3-protons, i. e., the dipolar term is relatively small. The characteristics of the orbital containing the un­paired spin can be inferred from the a-proton coupling tensor 4. For unpaired spins in 1T molecular orbitals, or p atomic orbitals, the isotropic component is nega­tive and its magnitude is approximately equal to the range of the anisotropic coupling, whereas for elec­trons in orbitals containing appreciable s character (or in-plane radicals) the isotropic component is -60 per­cent. This difference is due to two competing mecha­nisms: the negative configuration interaction, as in the 1T radical, and the positive spin delocalization interac­tion. The ansiotropic variation for spin densities near 1 is approximately the same whether it is a 1T or a radi­cal. The absolute sign of the isotropic component of the hyperfine tensor for in-plane radicals is not definitely known but it has been most frequently proposed to be positive. 6- 14 The magnitude of the a-proton tensor is representative of a a electron radical. Our initial guess was therefore that of an in-plane radical, but this we do not now believe is correct. The symmetry of in­plane radicals, at least involving carbon atoms of the type

dictates that at least one of the principal directions from each interacting proton should be approximately parallel. A comparison of the principal directions shows that there is no symmetry axis, a situation that is en­countered frequently in radicals where the unpaired spin occupied a highly localized atomic orbital.

~}r FIG. 2. (a) X-band ENOOR trans ition for DL-serine x irra-diated at 340 0 K and observed at 300 0 K for Ho II a*. (b) X-band ENOOR/2 transitions for

71 72 DL-serine x irradiated at 340

)n 340 oK and observed at 300 oK forHo II a*.

71 72

The hyperfine couplings obtained for the H of the NH3 group may at first seem a little large since the H's of the intramolecular group are three bond lengths away. However, if one looks at the crystal structure data and the actual interatomic distances, these couplings are not unreasonable. For instance, the intramolecular C3H5 distance is 2.63 A as compared to the CsN distance of 2.44 A,7 where Cs is the site of our unpaired electron and H5, Hs , and H7 are the NHs hydrogens. In addition, there is an NHs group from an adjacent molecule which is close to Cs , i. e., the intermolecular distance C3HS = 2. 66 A. Moreover, when radical V is formed and the structure closed, one would expect from the stoichiome­try of the crystal that C3 would move even closer to the neighboring NH3 group. The values for this hyperfine

5

o 10 20 30 40 50 60 70 80 90 H II b ROTATION ANGLE H II c

FIG. 3. Plot of the ENOOR transition frequencies as a func­tion of the orientation of Ho in the be crystallographic plane. The x's indicate the depolarization line.

J. Chem. Phys., Vol. 63, No.3, 1 August 1975

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D. A. Hampton and G. C. Moulton: ESR, ENDOR of DL·serine 1081

TABLE I. Hyperfine and g tensors.

Hyperfine and K tensors Direction cosines for radical V

principal values in a*bc system

(MHz) Errors In n

1. r-H,(N) +6.56 0.9975 0.0148 0.0684

+ 12.08 0.0348 0.7434 - O. 6680

-13.03 - O. 0607 0.6687 0.7410 + 1.87 (± O. 67) isotropic val ue

Il-H 4.85 0.9944 ± O. 0659 - O. 832

32.53 'f0.0982 , O. 8689 0.4852

~ 0.0403 ±0.4906 0.8704 14.50 (+ 0.15) isotropic value

(3-11 22.36 0.80.,4 0.0743 - O. 5881 35. G:l 0.3649 0.7198 O. 5906 20.82 0.4671 -0.G902 0.5526

26.27 (±0.10) isotropic value

o-H - 37.84 0.9302 - 0.191.3 - O. 3121

- 9.:36 - 0.0444 0.7850 -O.GI78.

-76.71 0.3643 0.5886 0.7217 -41.:n (± O. 25) isotropic value

r-II 113.14 0.8722 0.3591 - O. 3322 123.86 - O. 2360 0.9037 0.3572 111.64 0.4285 - O. 23:n 0.87:)0

116.21 (i O. 14) isotropic value

~-N 5.6-7.3

~ 2.0023 0.9380 - O. 2586 - O. 2308 2.0042 0.3109 0.9220 0.2306

~ 0.1532 - O. 2881 0.9453 2.0034 (± O. 0002) isotropic value

tensor agree very well with the values obtained by others15 for NH3 proton couplings for intramolecular protons three bond lengths away from the unpaired spin and intermolecular NH3 proton couplings. Tensor 1 could be due to a proton from an intramolecular or in­termolecular NH3 group.

To obtain more information about this radical an ENDOR/2 experiment was performed to determine: (1) the absolute sign of the hyperfine coupling constants, and (2) those ENDOR lines that were definitely involved in the radical responsible for the a-proton. A positive a-proton isotropic component would be supportive evi­dence for an in-plane radical. The ENDOR/2 spectrum is shown in Fig. 2(b) for H parallel to the a* crystal axis. In this experiment an rf oscillator, frequency modulated, was tuned to the 17. 68 MHz ENDOR line and a second derivative of the positive ENDOR signal was obtained at low rf powers. While sitting on the derivative peak, another rf oscillator, operating at rf powers low enough to produce positive ENDOR signals, was swept through the other transitions. Any ENDOR/2 signal seen would indicate that its existence was due to the same radical that was responsible for the 17.68 MHz line. ENDOR/2 signals were detected at 71.17, 36.63, and 26.47 MHz. Since the interaction at 71.17 MHz was due to a .a-proton which is known to have a pos­itive isotropic coupling constant, the couplings at 26.47 and 17.68 MHz are also positive at this orientation since negative ENOOR/2 signals indicate coupling constants of like sign while positive ones indicate constants of oppo­site sign. This result strongly supports the assignment of the 26.47 MHz line as due to a small .a-proton.

The coupling at 37. 63 MHz is negative, indicating only slight s character of the unpaired spin orbital. An ENDOR/2 Signal was not detected at 16.94 MHz while sweeping at low or high rf powers. These results show

that the four former lines definitely belong to the same radical. An element of doubt still exists about the 16.94 MHz ENDOR line since (1) it could only be seen in this orientation as a negative ENDOR signal and (2) the total sum of the five proton hyperfine couplings add almost exactly to the total ESR band c crystal axes spectra widths. This interaction along the a* axis is less than one gauss and is too small to be seen with ESR. Ex­perimental difficulties prohibited an ENDOR/2 experi­ment along any axis other than the a * axis.

From crystals of deuterated DL-serine, ESR spectra were taken in the three crystallographic planes. A g­tensor was calculated and is also given in Table I. The principal values are characteristic of 'IT electron radi­cals. For these radicals the minimum g value is di­rected along the axis of the orbital containing the un­paired spin. The intermediate principal hyperfine value is also directed along this orbital. The experimentally determined angle between these principal directions is 6 0

• ENDOR data were also taken on these crystals and the large .a-proton transition and the a-H transition were both observed.

ESR and ENDOR data were taken on the ND3 deuterated crystals and identical results were obtained for V on these crystals and the deuterated crystals. This tells us that the hydroxyl proton (deuteron) is not involved in V.

ESR and ENDOR data were also taken on a-d-l crys­tals. The ENDOR transition for the a-H was observed, but not the large .a-proton coupling.

Another ENDOR/2 experiment was performed on the normal crystals where the external magnetic field was orientated 5 0 off the a * c rys tal axis in the ab plane, In this orientation eight positive ENDOR lines could be seen due to site splitting at the frequencies 17.55, 17.77,26.14,26.86,35.64,37.55,70,94, and 71.49 MHz. The ENDOR/2 experiment was performed as be­fore while sitting on the second derivative peak on the 17.55 MHz line. ENDOR/2 signals were seen at the frequencies 26.86, 35.64, and 70.94 MHz. This ex­periment uniquely determined the relative signs of the off diagonal hyperfine tensor elements in the ab plane which belong to the same magnetically distinct site in the unit cell.

IDENTIFICATION OF THE RADICAL

While the a-proton hyperfine tensor of radical IV is typical for a 2p'IT electron radical, the a-proton tensor for radical V is somewhat unusual. The simplicity of the molecule, on the other hand, limits the number of possible locations of the unpaired spin. Models of in­plane radicals centered either at the .a-carbon or car­boxyl carbon are not consistent with the lack of sym­metry and number of interactions of the radical. The results of the ESR and ENDOR experiments on the sub­stituted serines help to give an accurate determination of the radical site in the molecule. As noted previously, the ENDOR transition for the a-H interaction is present in the deuterated, ND3, and a-d-l serine crystals, as well as in the normal crystals. However, the large .a-proton ENDOR transition is not present in the a-d-l

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1082 D. A. Hampton and G. C. Moulton: ESR, ENDOR of DL-serine

crystals, but only in the normal, deuterated, and ND3 . From these experiments it is determined that the large J3 -proton is bonded to the a-carbon. This fact means that most of the unpaired spin density is located either at the J3 -carbon or the carboxyl carbon. The fact that the unpaired spin interacts with an a-proton implies that there is no simple model to explain a radical cen­tered at the carboxyl carbon. In contrast, the J3-carbon is an attractive site since the loss of one of its hydrogen atoms would give at once an a-proton, the large J3-proton, a J3-nitrogen, and a rotating ammonia group. We conclude that the unpaired spin density is mostly localized at the J3 -carbon. Its orbital is largely p but must contain some s character, due probably to some constraint on the molecule. Therefore, radicals IV and V are both centered on the J3-carbon. We believe that the hydroxyl group is not present in the molecular fragment V. There is no hyperfine coupling to the hy­droxyl proton, as evidenced by the data on the substi­tuted serines, although inlVthere is a fairly large (21. 2 G isotropic) J3 hyperfine coupling to this proton and the un­paired spin is on the same carbon atom in the two radicals.

Experiments involving irradiation of DL-serine at various temperatures show that the yield of V is a func­tion of the thermal energy of the molecule. The room temperature ESR spectra after 77 OK irradiation indi­cate yields of IV> V. Irradiation at elevated tempera­tures of 40 to 50°C, indicate yields of V>IV. Whether the formation of IV and V is competitive or IV is the precurser of V is difficult to establish. We propose the following scheme for the production of V from IV.

NH+ I 3

HO-C-C-COO' (IV) I I H H

NH+ , I 3

HO' + ·C-C-COO' I I H H

H NH+ I I 3

'C-C-C=O.

L:J I I I

H

(V)

After formation of IV from its precursor, thermal motion is sufficient to break the CO hydroxyl bond. The fragment then rebonds in a closed structure which would be somewhat strained, consistent with the a-H hyperfine tensor principal values. The J3-H coupling denoted as 3 in Table I is probably hydrogen bonded from the adjacent molecule. The large J3 -H (5 in Table I) is the H bonded to the a-carbon, and 1 represents the rotating ammonia hydrogens. Interaction 2 is probably an interaction with a neighboring, hydrogen bonded proton.

ACKNOWLEDGMENT

The authors wish to thank Dr. J. E. Leffler of the FSU Chemistry Department for his helpful discussions about the chemical feasibility of this radical.

*This work was supported in part by a grant from the Commit­t tee on Faculty Research Support of Florida State University. Present address: University of Virginia, Charlottesville, VA 22904.

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