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Study of the first stable l -alanine paramagnetic center by 2D-HYSCORE spectroscopy: Detection of 14N hyperfine and quadrupole splitting
Chemical Physics Letters 415 (2005) 161–164 www.elsevier.com/locate/cplett
Study of the first stable L-alanine paramagnetic center by 2D-HYSCORE spectroscopy: Detection of 14N hyperfine and quadrupole splitting B. Rakvin a
a,*
, N. Maltar-Strmecˇki
b
Department of Physical Chemistry, Rud-er Bosˇkovic´ Institute, P.O. Box 180, 10002 Zagreb, Croatia b Faculty of Veterinary Medicine, University of Zagreb, P.O. Box 466, 10002 Zagreb, Croatia Received 18 July 2005; in final form 25 August 2005 Available online 23 September 2005
Abstract Two-dimensional hyperfine sublevel correlation (2D-HYSCORE) spectra of the c-irradiated crystal of L-alanine were used to detect possible contribution of 14N hyperfine splitting to the spectrum of stable radical centers at room temperature. The 14N hyperfine and quadrupole tensors were evaluated and assigned to an abstracted NH3 molecule, which appeared as a product of the first stable radical formation. The obtained results have been discussed as potentially new elements in description of the environment of the first stable Lalanine radical in the crystal lattice. 2005 Elsevier B.V. All rights reserved.
1. Introduction The first stable L-alanine radical, CHCH3COOH, henceforth, SAR1, has been studied over many years [1– 20] and remains the most highly studied paramagnetic center of a simple amino acid in the solid form. It has been examined by various electron paramagnetic resonance (EPR) techniques including continuous wave (CW-EPR) [1–5], electron nuclear double resonance (ENDOR) [7– 9,15], and other nonlinear EPR methods [6,10–14,16–20]. However, molecular environment of the radical consisting of an unusually large shift of the central carbon atom, Ca, containing 0.75 unpaired spin density, as well as the dynamic processes associated with the radical motion, are still not fully understood. In a recent theoretical modeling of such centers [21], SAR1 center was found restricted [22] only to the center itself, without any interaction with surrounding atoms. Moreover, internal rotations of the CH3 and NHþ 3 groups accompanied with molecular zwitterionic
*
Corresponding author. Fax: +385 1 4680 45. E-mail address: [email protected] (B. Rakvin).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.138
form in the crystalline state produce even more complex local environment of SAR1 for theoretical description at room temperature. Thus, to obtain complete description of the center, more accurate estimation of the local position of the center and more detailed evaluation of intermolecular interaction within the lattice are essential. Recently, a detailed assignment of the room temperature spectrum from an irradiated L-alanine crystal revealed three types of paramagnetic centers with the largest contributions (60%) from SAR1 center [15]. It is well known that, within a temperature interval from 300 to 200 K, the single crystal CW-EPR spectrum of SAR1 consists of a prominent quintet of relative intensity 1:4:6:4:1 when the magnetic field is applied parallel to the crystalÕs c-axis. It happens due to hyperfine coupling from four equivalent protons. Below 80 K, the CH3 motion is frozen on the EPR time-scale that leads to the CW-EPR spectrum consisting of splittings from four inequivalent protons (b1 = 0.51 mT, b2 = 2.76 mT, b3 = 4.61 mT and a = 2.62 mT, along the c-axis). The environment of SAR1 was studied by ENDOR spectroscopy at 77 K [7,8]. The obtained results based only on the distant proton splitting indicated that, as the C–N
162
B. Rakvin, N. Maltar-Strmecˇki / Chemical Physics Letters 415 (2005) 161–164
bond was ruptured and SAR1 was formed, the Ca atom ˚ . This finding was unexpected, shifted by a distance of 1.5 A since atomic position usually remains in the vicinity of the original position in the irradiated crystal. However, earlier low temperature study [7,8] and a more recent ENDOR study at room temperature [15] found no evidence of possible small dipolar coupling with 14N due to abstracted NH3 group, which was expected to be positioned in the vicinity of the SAR1 center. Possible detection of NH3 may provide additional information that could help to explain the unusually large shift of SAR1 center in the L-alanine lattice. In the present study, two-dimensional hyperfine sublevel correlation (2D-HYSCORE) spectra of SAR1 center were used to deduce possible 14N hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI) at room temperature. The evaluated tensors of HFI and NQI provided new experimental elements for better characterization of SAR1 center in the lattice. 2D-HYSCORE pulse sequences were used because their sensitivity in the region of low Larmor frequencies, such as that for 14N (mN = 1.07 MHz at X-band frequency), is larger than that of ENDOR spectroscopy. 2. Experimental Single crystals of L-alanine were obtained by dissolving commercially available L-alanine (Sigma) in water and allowing the solution to slowly evaporate over time. The slow cooling of saturated D2O solution in a closed cryostat yielded the deuterium-exchanged crystals. The single crystals were c-irradiated with a dose of 30 kGy from a 60Co source to create alanine-based organic radicals stable at room temperature. Pulsed-EPR experiments were performed on a Bruker E-580 FT/CW X-band EPR spectrometer at room temperature. A pulse sequence of p/2 s p/2 t1 p t2 p/2 s-echo, where the echo intensity was measured as function t1 and t2, was used to obtain 2D-HYSCORE spectra. Microwave pulse amplitudes of 16 and 72 ns were used for p/2 pulse and s, respectively. The times t1 and t2 were increased by a 64 ns step. Fourier transformation was obtained after subtracting the background decay with a
square function and tapering with a Hamming window. Phase-cycling procedures were applied to minimize measurement artifacts from unwanted echoes. Field-sweep echo-detected EPR spectra (p/2 s p s-echo) obtained with the same amplitude of microwave pulse and s as described above exhibited dominant contribution from the SAR1 center. Thus, the contribution of other two radicals present in the room temperature spectrum to the 2D-HYSCORE spectrum taken on the most intense central line was not expected to be significant. Literature data were used for numerical calculations to simulate 2D-HYSCORE spectra [23,24]. 3. Results and discussion Typical 2D-HYSCORE spectrum of SAR1 for the crystal b-axis oriented along the magnetic field is shown in Fig. 1a. The center peak at a frequency of 1.07 MHz in the (+, +) frequency quadrant on a diagonal position corresponds to 14N Larmor frequency, mN. Two separate peaks present in the (, +) quadrant and perpendicular to diagonal direction represent an effective 14N nucleus splitting due to the hyperfine (I = 1) and quadrupole interaction, which is larger than 2 mN. The simulated line position for the corresponding hyperfine interaction, A, and nuclear quadrupole interaction, Q, tensors along the b-axis are shown in Fig. 1b. The deuterium-exchanged crystal oriented along the a-axis was used to confirm the origin of 14 N splitting and detect other possibly present nuclei with a low value of Larmor frequency. Indeed, in Fig. 2, two peaks positioned perpendicularly on the diagonal at the same mN, but with a smaller effective splitting than the splitting along the b-axis orientation, appeared inside (+, +) quadrant (effective splitting <2 mN). Additionally, two new peaks appeared perpendicularly to the diagonal in the (, +) quadrant (effective splitting >2 mD; mD is deuteron Larmor frequency). The obtained correlation peaks (5.81, 2.21 MHz) and (2.21, 5.81 MHz) could be assigned to 2H on the Ca position, since the expected splitting recalculated from HFI of proton was around 5.5 MHz for the magnetic field along the a-axis [15]. In the further consideration, only the 14N effective splitting was examined in all three mutual planes.
Fig. 1. Contour plots of 2D-HYSCORE frequency-domain spectra of c-irradiated L-alanine with magnetic field along the b-crystal axis: (a) experimental spectrum, (b) simulated spectra obtained by using determined hyperfine and quadrupole tensors listed in Table 1 for the same crystal orientation.
B. Rakvin, N. Maltar-Strmecˇki / Chemical Physics Letters 415 (2005) 161–164
163
Table 1 Q and A tensor elements (MHz) for 14N of abstracted NH3 in the vicinity of SAR1 center measured at room temperature Tensor
Isotropic values
Q
A
1.23
Principal values
Eigenvectors Æaæ
Æbæ
Æcæ
0.86 0.60 0.26
0.3736 0.2819 0.8837
0.7258 0.5044 0.4771
0.5776 0.8162 0.0162
1.38 0.41 0.97
0.5016 0.7621 0.4093
0.7503 0.6189 0.2325
0.4305 0.1904 0.8823
The estimated error margins of Q and A tensors are ±0.03 and ±0.1 MHz, respectively. Fig. 2. Contour plots of 2D-HYSCORE frequency-domain spectra of c-irradiated deuteron exchange L-alanine with magnetic field along the a-crystal axis.
In order to analyze experimental data, the following Hamiltonian was used to describe electron–nitrogen magnetic interactions in external magnetic field [23–25]: ^ þS ^ A ^I þ ^I Q ^I c ^ ¼ bB g S H hB ^I.
ð1Þ
By assuming a strong field approximation and isotropic g-value to first-order, the energy levels described by Hamiltonian (1) were obtained [25] 1 3 En ¼ An mI þ Qn m2I 1 mN mI ; ð2Þ 2 2 where An and Qn are effective HFI and NQI splitting along the direction of the magnetic field. The relation (2) can be used to calculate the first-order frequencies of the nitrogen nuclear spin transitions for corresponding orientation of magnetic field. The effective values for An and 3Qn parameters were extrapolated from the measured line position for different angular orientations of magnetic field (Fig. 3). Tensors A and Q were evaluated by taking advantage of the combined features of the first-order frequencies [25] and by using the standard procedure [26,27] for fitting; their values are listed in Table 1. A recent detailed investi-
gation [15] has confirmed that SAR1 center exhibits a p-type planar structure. This structure is expected to have both the eigenvector of the intermediate principal value for an a-proton coupling tensor and the eigenvector of gmin along the lone electron orbital (LEO) [28,29]. Indeed, for SAR1 these directions are nearly parallel and deviate less than 3. Values in Table 1 show that Q tensor does not exhibit an axial symmetry and its maximum value is pointed in the direction of gmin (about 1 deviation), i.e. along the LEO, since the largest electric field gradient is expected in this direction [27]. Furthermore, the maximum of 14N dipolar coupling is also nearly parallel to the maximum of Q tensor (about 12 deviation). From the dipolar coupling and spin density of 0.75 at Ca position, the effective dis˚ between 14N and Ca is obtained. Our tance r = 1.85 A experimental data suggest that the nitrogen atom in NH3 molecule is pointed towards LEO and nearly perpendicular to the plane of the SAR1 center. It can be expected that, due to deviation from the axial symmetry elements of Q and A tensors, the abstracted NH3 group probably performs fast internal hindered dynamics at monitored room temperature. It should be noted here that the previous model [7,8] assumed that abstracted NH3 molecule was located in the vicinity of the original position in the undamaged crystal. However, that model was based on low temperature ENDOR data accompanied with deuteron
Fig. 3. Angular variation of the splitting frequency of the 14N lines for: (a) effective quadrupole splitting 3Qn and (b) effective hyperfine splitting An. The solid lines are calculated by employing the best fitting procedure to relation (2) and tensors parameters listed in Table 1 are evaluated.
B. Rakvin, N. Maltar-Strmecˇki / Chemical Physics Letters 415 (2005) 161–164
164
exchange effect besides the shift of Ca atom. The original position of NH3 in undamaged crystal lattice shows large deviation from the presently suggested position. On the other side, for the second stable L-alanine center, a small change of the original position of NH3 group is expected and 14N coupling is significantly larger [15,21] than that for SAR1. The recent room temperature ENDOR study [15] has shown that a detected dipolar proton coupling on the ˚ is related to the intermolecular effective distance of 2.9 A interaction between Ca of SAR1 and methyl group of an undamaged neighboring molecule. To realize this assump˚ tion, one needs to include a shift of the SAR1 for 1.5 A ˚ and an effective distance of 3.13 A would be obtained from the Ca position. On the basis of presently obtained data, one can speculate that detected proton coupling at room temperature arises from the fast rotating NH3 protons, which ˚ from the are expected at an average position around 2.8 A Ca position. Thus, in the present case, it is not necessary to introduce any major shift of Ca in order to justify dipolar coupling with lattice protons. The additional consequences of this speculation also involve fast proton exchange between ND3 and CH3 neighboring groups in partially deuterated crystals [30] before final transformation in SAR1. In conclusion, the obtained data strongly support that abstracted NH3 molecule is present in the near vicinity of SAR1 center. This finding may provide a new impetus for additional experimental and theoretical investigations of the SAR1 center in the solid state form of simple amino acids. References [1] [2] [3] [4]
I. Miyagawa, W. Gordy, J. Chem. Phys. 32 (1960) 255. J.R. Morton, A.J. Horsfield, J. Chem. Phys. 35 (1961) 1142. A. Horsfield, J.R. Morton, D.H. Whiffen, Mol. Phys. 5 (1962) 115. I. Miyagawa, K. Itoh, J. Chem. Phys. 36 (1962) 2157.
[5] J.W. Sinclair, M.W. Hanna, J. Phys. Chem. 71 (1967) 84. [6] S.A. Dzuba, K.M. Salikhov, Yu.D. Tsvetkov, Chem. Phys. Lett. 79 (1981) 568. [7] S.-i. Kuroda, I. Miyagawa, J. Chem. Phys. 76 (1982) 3933. [8] K. Matsuki, I. Miyagawa, J. Chem. Phys. 76 (1982) 3945. [9] M. Brustolon, T. Cassol, L. Micheletti, U. Segre, Mol. Phys. 57 (1986) 1005. [10] K. Nakagawa, S.S. Eaton, G.R. Eaton, Appl. Radiat. Isot. 44 (1993) 73. [11] R. Angelone, C. Forte, C. Pinzino, J. Magn. Reson. A 101 (1993) 16. [12] M. Brustolon, U. Segre, Appl. Magn. Reson. 7 (1994) 405. [13] B.T. Ghim, J.-K. Du, S. Pfenninger, G.A. Rinard, R.W. Quine, S.S. Eaton, G.R. Eaton, Appl. Radiat. Isot. 47 (1996) 1235. [14] B. Rakvin, Appl. Radiat. Isot. 47 (1996) 1251. [15] E. Sagstuen, E.O. Hole, S.R. Haugedal, W.H. Nelson, J. Phys. Chem. A 101 (1997) 9763. [16] B. Rakvin, N. Maltar-Strmecˇki, Spectrochim. Acta 56 (2000) 399. [17] B. Rakvin, N. Maltar-Strmecˇki, P. Cevc, D. Arcˇon, J. Magn. Res. 152 (2001) 149. [18] N. Maltar-Strmecˇki, B. Rakvin, P. Cevc, D. Arcˇon, Appl. Magn. Reson. 22 (2002) 551. [19] J.R. Harbridge, S.S. Eaton, G.R. Eaton, J. Phys. Chem. A 107 (2003) 598. [20] B. Rakvin, N. Maltar-Strmecˇki, C.M. Ramsey, N.S. Dalal, J. Chem. Phys. 120 (2004) 6665. [21] E. Pauwels, W. van Speybroeck, M. Waroquier, J. Phys. Chem. A 105 (2004) 8794. [22] P. Lahorte, F. de Proft, G. Vanhealewyn, B. Masschaele, P. Cauwels, F. Callens, P. Geerlings, W. Mondelaers, J. Phys. Chem. A 103 (1999) 6650. [23] C. Gemperle, G. Aebli, A. Schweiger, R.R. Ernst, J. Magn. Res. 88 (1990) 241. [24] A.M. Tyryshkin, S.A. Dikanov, D. Golfard, J. Magn. Res. 105 (1993) 271. [25] E.J. Reijerse, A.M. Tyryshkin, S.A. Dikanov, J. Magn. Res. 131 (1998) 295. [26] A. Naito, C.A. McDowell, J. Chem. Phys. 81 (1984) 4795. [27] J.A. Weil, J.R. Bolton, J.E. Wertz, Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, Wiley, New York, 1994, p. 125, 137. [28] H.M. McConnell, C. Heller, T. Cole, R.W. Fessenden, J. Am. Chem. Soc. 82 (1960) 766. [29] H.M. McConnell, R.E. Robertson, J. Phys. Chem. 61 (1957) 1018. [30] K. Itoh, I. Miyagawa, C.S. Chen, J. Chem. Phys. 52 (1970) 1822.
Study of the first stable L-alanine paramagnetic center by 2D-HYSCORE spectroscopy: Detection of 14N hyperfine and quadrupole splitting B. Rakvin a
a,*
, N. Maltar-Strmecˇki
b
Department of Physical Chemistry, Rud-er Bosˇkovic´ Institute, P.O. Box 180, 10002 Zagreb, Croatia b Faculty of Veterinary Medicine, University of Zagreb, P.O. Box 466, 10002 Zagreb, Croatia Received 18 July 2005; in final form 25 August 2005 Available online 23 September 2005
Abstract Two-dimensional hyperfine sublevel correlation (2D-HYSCORE) spectra of the c-irradiated crystal of L-alanine were used to detect possible contribution of 14N hyperfine splitting to the spectrum of stable radical centers at room temperature. The 14N hyperfine and quadrupole tensors were evaluated and assigned to an abstracted NH3 molecule, which appeared as a product of the first stable radical formation. The obtained results have been discussed as potentially new elements in description of the environment of the first stable Lalanine radical in the crystal lattice. 2005 Elsevier B.V. All rights reserved.
1. Introduction The first stable L-alanine radical, CHCH3COOH, henceforth, SAR1, has been studied over many years [1– 20] and remains the most highly studied paramagnetic center of a simple amino acid in the solid form. It has been examined by various electron paramagnetic resonance (EPR) techniques including continuous wave (CW-EPR) [1–5], electron nuclear double resonance (ENDOR) [7– 9,15], and other nonlinear EPR methods [6,10–14,16–20]. However, molecular environment of the radical consisting of an unusually large shift of the central carbon atom, Ca, containing 0.75 unpaired spin density, as well as the dynamic processes associated with the radical motion, are still not fully understood. In a recent theoretical modeling of such centers [21], SAR1 center was found restricted [22] only to the center itself, without any interaction with surrounding atoms. Moreover, internal rotations of the CH3 and NHþ 3 groups accompanied with molecular zwitterionic
*
Corresponding author. Fax: +385 1 4680 45. E-mail address: [email protected] (B. Rakvin).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.138
form in the crystalline state produce even more complex local environment of SAR1 for theoretical description at room temperature. Thus, to obtain complete description of the center, more accurate estimation of the local position of the center and more detailed evaluation of intermolecular interaction within the lattice are essential. Recently, a detailed assignment of the room temperature spectrum from an irradiated L-alanine crystal revealed three types of paramagnetic centers with the largest contributions (60%) from SAR1 center [15]. It is well known that, within a temperature interval from 300 to 200 K, the single crystal CW-EPR spectrum of SAR1 consists of a prominent quintet of relative intensity 1:4:6:4:1 when the magnetic field is applied parallel to the crystalÕs c-axis. It happens due to hyperfine coupling from four equivalent protons. Below 80 K, the CH3 motion is frozen on the EPR time-scale that leads to the CW-EPR spectrum consisting of splittings from four inequivalent protons (b1 = 0.51 mT, b2 = 2.76 mT, b3 = 4.61 mT and a = 2.62 mT, along the c-axis). The environment of SAR1 was studied by ENDOR spectroscopy at 77 K [7,8]. The obtained results based only on the distant proton splitting indicated that, as the C–N
162
B. Rakvin, N. Maltar-Strmecˇki / Chemical Physics Letters 415 (2005) 161–164
bond was ruptured and SAR1 was formed, the Ca atom ˚ . This finding was unexpected, shifted by a distance of 1.5 A since atomic position usually remains in the vicinity of the original position in the irradiated crystal. However, earlier low temperature study [7,8] and a more recent ENDOR study at room temperature [15] found no evidence of possible small dipolar coupling with 14N due to abstracted NH3 group, which was expected to be positioned in the vicinity of the SAR1 center. Possible detection of NH3 may provide additional information that could help to explain the unusually large shift of SAR1 center in the L-alanine lattice. In the present study, two-dimensional hyperfine sublevel correlation (2D-HYSCORE) spectra of SAR1 center were used to deduce possible 14N hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI) at room temperature. The evaluated tensors of HFI and NQI provided new experimental elements for better characterization of SAR1 center in the lattice. 2D-HYSCORE pulse sequences were used because their sensitivity in the region of low Larmor frequencies, such as that for 14N (mN = 1.07 MHz at X-band frequency), is larger than that of ENDOR spectroscopy. 2. Experimental Single crystals of L-alanine were obtained by dissolving commercially available L-alanine (Sigma) in water and allowing the solution to slowly evaporate over time. The slow cooling of saturated D2O solution in a closed cryostat yielded the deuterium-exchanged crystals. The single crystals were c-irradiated with a dose of 30 kGy from a 60Co source to create alanine-based organic radicals stable at room temperature. Pulsed-EPR experiments were performed on a Bruker E-580 FT/CW X-band EPR spectrometer at room temperature. A pulse sequence of p/2 s p/2 t1 p t2 p/2 s-echo, where the echo intensity was measured as function t1 and t2, was used to obtain 2D-HYSCORE spectra. Microwave pulse amplitudes of 16 and 72 ns were used for p/2 pulse and s, respectively. The times t1 and t2 were increased by a 64 ns step. Fourier transformation was obtained after subtracting the background decay with a
square function and tapering with a Hamming window. Phase-cycling procedures were applied to minimize measurement artifacts from unwanted echoes. Field-sweep echo-detected EPR spectra (p/2 s p s-echo) obtained with the same amplitude of microwave pulse and s as described above exhibited dominant contribution from the SAR1 center. Thus, the contribution of other two radicals present in the room temperature spectrum to the 2D-HYSCORE spectrum taken on the most intense central line was not expected to be significant. Literature data were used for numerical calculations to simulate 2D-HYSCORE spectra [23,24]. 3. Results and discussion Typical 2D-HYSCORE spectrum of SAR1 for the crystal b-axis oriented along the magnetic field is shown in Fig. 1a. The center peak at a frequency of 1.07 MHz in the (+, +) frequency quadrant on a diagonal position corresponds to 14N Larmor frequency, mN. Two separate peaks present in the (, +) quadrant and perpendicular to diagonal direction represent an effective 14N nucleus splitting due to the hyperfine (I = 1) and quadrupole interaction, which is larger than 2 mN. The simulated line position for the corresponding hyperfine interaction, A, and nuclear quadrupole interaction, Q, tensors along the b-axis are shown in Fig. 1b. The deuterium-exchanged crystal oriented along the a-axis was used to confirm the origin of 14 N splitting and detect other possibly present nuclei with a low value of Larmor frequency. Indeed, in Fig. 2, two peaks positioned perpendicularly on the diagonal at the same mN, but with a smaller effective splitting than the splitting along the b-axis orientation, appeared inside (+, +) quadrant (effective splitting <2 mN). Additionally, two new peaks appeared perpendicularly to the diagonal in the (, +) quadrant (effective splitting >2 mD; mD is deuteron Larmor frequency). The obtained correlation peaks (5.81, 2.21 MHz) and (2.21, 5.81 MHz) could be assigned to 2H on the Ca position, since the expected splitting recalculated from HFI of proton was around 5.5 MHz for the magnetic field along the a-axis [15]. In the further consideration, only the 14N effective splitting was examined in all three mutual planes.
Fig. 1. Contour plots of 2D-HYSCORE frequency-domain spectra of c-irradiated L-alanine with magnetic field along the b-crystal axis: (a) experimental spectrum, (b) simulated spectra obtained by using determined hyperfine and quadrupole tensors listed in Table 1 for the same crystal orientation.
B. Rakvin, N. Maltar-Strmecˇki / Chemical Physics Letters 415 (2005) 161–164
163
Table 1 Q and A tensor elements (MHz) for 14N of abstracted NH3 in the vicinity of SAR1 center measured at room temperature Tensor
Isotropic values
Q
A
1.23
Principal values
Eigenvectors Æaæ
Æbæ
Æcæ
0.86 0.60 0.26
0.3736 0.2819 0.8837
0.7258 0.5044 0.4771
0.5776 0.8162 0.0162
1.38 0.41 0.97
0.5016 0.7621 0.4093
0.7503 0.6189 0.2325
0.4305 0.1904 0.8823
The estimated error margins of Q and A tensors are ±0.03 and ±0.1 MHz, respectively. Fig. 2. Contour plots of 2D-HYSCORE frequency-domain spectra of c-irradiated deuteron exchange L-alanine with magnetic field along the a-crystal axis.
In order to analyze experimental data, the following Hamiltonian was used to describe electron–nitrogen magnetic interactions in external magnetic field [23–25]: ^ þS ^ A ^I þ ^I Q ^I c ^ ¼ bB g S H hB ^I.
ð1Þ
By assuming a strong field approximation and isotropic g-value to first-order, the energy levels described by Hamiltonian (1) were obtained [25] 1 3 En ¼ An mI þ Qn m2I 1 mN mI ; ð2Þ 2 2 where An and Qn are effective HFI and NQI splitting along the direction of the magnetic field. The relation (2) can be used to calculate the first-order frequencies of the nitrogen nuclear spin transitions for corresponding orientation of magnetic field. The effective values for An and 3Qn parameters were extrapolated from the measured line position for different angular orientations of magnetic field (Fig. 3). Tensors A and Q were evaluated by taking advantage of the combined features of the first-order frequencies [25] and by using the standard procedure [26,27] for fitting; their values are listed in Table 1. A recent detailed investi-
gation [15] has confirmed that SAR1 center exhibits a p-type planar structure. This structure is expected to have both the eigenvector of the intermediate principal value for an a-proton coupling tensor and the eigenvector of gmin along the lone electron orbital (LEO) [28,29]. Indeed, for SAR1 these directions are nearly parallel and deviate less than 3. Values in Table 1 show that Q tensor does not exhibit an axial symmetry and its maximum value is pointed in the direction of gmin (about 1 deviation), i.e. along the LEO, since the largest electric field gradient is expected in this direction [27]. Furthermore, the maximum of 14N dipolar coupling is also nearly parallel to the maximum of Q tensor (about 12 deviation). From the dipolar coupling and spin density of 0.75 at Ca position, the effective dis˚ between 14N and Ca is obtained. Our tance r = 1.85 A experimental data suggest that the nitrogen atom in NH3 molecule is pointed towards LEO and nearly perpendicular to the plane of the SAR1 center. It can be expected that, due to deviation from the axial symmetry elements of Q and A tensors, the abstracted NH3 group probably performs fast internal hindered dynamics at monitored room temperature. It should be noted here that the previous model [7,8] assumed that abstracted NH3 molecule was located in the vicinity of the original position in the undamaged crystal. However, that model was based on low temperature ENDOR data accompanied with deuteron
Fig. 3. Angular variation of the splitting frequency of the 14N lines for: (a) effective quadrupole splitting 3Qn and (b) effective hyperfine splitting An. The solid lines are calculated by employing the best fitting procedure to relation (2) and tensors parameters listed in Table 1 are evaluated.
B. Rakvin, N. Maltar-Strmecˇki / Chemical Physics Letters 415 (2005) 161–164
164
exchange effect besides the shift of Ca atom. The original position of NH3 in undamaged crystal lattice shows large deviation from the presently suggested position. On the other side, for the second stable L-alanine center, a small change of the original position of NH3 group is expected and 14N coupling is significantly larger [15,21] than that for SAR1. The recent room temperature ENDOR study [15] has shown that a detected dipolar proton coupling on the ˚ is related to the intermolecular effective distance of 2.9 A interaction between Ca of SAR1 and methyl group of an undamaged neighboring molecule. To realize this assump˚ tion, one needs to include a shift of the SAR1 for 1.5 A ˚ and an effective distance of 3.13 A would be obtained from the Ca position. On the basis of presently obtained data, one can speculate that detected proton coupling at room temperature arises from the fast rotating NH3 protons, which ˚ from the are expected at an average position around 2.8 A Ca position. Thus, in the present case, it is not necessary to introduce any major shift of Ca in order to justify dipolar coupling with lattice protons. The additional consequences of this speculation also involve fast proton exchange between ND3 and CH3 neighboring groups in partially deuterated crystals [30] before final transformation in SAR1. In conclusion, the obtained data strongly support that abstracted NH3 molecule is present in the near vicinity of SAR1 center. This finding may provide a new impetus for additional experimental and theoretical investigations of the SAR1 center in the solid state form of simple amino acids. References [1] [2] [3] [4]
I. Miyagawa, W. Gordy, J. Chem. Phys. 32 (1960) 255. J.R. Morton, A.J. Horsfield, J. Chem. Phys. 35 (1961) 1142. A. Horsfield, J.R. Morton, D.H. Whiffen, Mol. Phys. 5 (1962) 115. I. Miyagawa, K. Itoh, J. Chem. Phys. 36 (1962) 2157.
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