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Pulsed double resonance study of the 14N nuclear quadrupole coupling in glycine
Volume 9, number 1
CHJ3UC.4 L PWSICS
LETTERS
1 April 1971
PULSED DOUBLE RESONANCE STUDY OF THE IGN NUCLEAR QUADRUPOLE COUPLING IN GLYCINE R. BLINC, M. htiL1, R. OSREDKAR, A. PRELESNIK, I. ZUPANC!Ic LrnivrsUy
of LjubZjam.
bstihte
‘-I.Ste_ran ‘, L)ubljana.*.iugaslavia and
Department
of Bixhemistq.
L. EHRENBERG Lhirersity of Stockholnr. StockI:oim. Sweden Received 4 January 1971
The technique of nuclear magnetic pulsed double resonance in the rotating frame has been used to
determine the quadrupole interactions of 14N in a single crystal of glycine nt 140°C. The 1% quldrupole coupling constant is found to be very small e%Q/fz= ;-%5kHz.whereas the ssymmetry paczmeter is relatively large ?J= 0.61. The direction of the Largest principal asis of the electric field gradient tensor
does rootcoincide with the C-N direction as determined by X-ray data at room temperature.
Though it is known that 14N qua&pole resonance spectra might be very valuable for the determination of the structure of biologically important moIecules this method has not been much used so far. This is due to the experimental difficulties connected wi&ththe low frequency of the 14N quadrupole resonances and the resulting poor signal-to-noise ratio. Nuclear double resonance [l] methods, which use the strong resonance of one nuclear species - say protons - to detect the weaker resonance of another species in our case 14N - might represent a satisfactory solution to this problem, if the dipolar spin-lattice relaxation times (TlD) of the protons are long enough. The optimum signal-to-noise ratio S with which the nitrogen spectra can be measured by this method may be e.xpressed as
(1) for TlD > w-1. Here (S)B is the signal-to-noise ratio of the proton spectra and W is the double resonance rate which is proportional to the strength of the dipolar coupling between the two nuclear spin reservoirs. Eq. (1) shows that if (TlD)H is long enough the I4N spectra can be measured with a signal-to-noise ratio which is not much smaller than the one of the protonic system. The increase in sensitivity over pure nuclear quadrupcle resonance spectroscopy is of the order.of TlD/T2 i.e. about lo3 - 104 in favourable cases.
ln order to test this method which has been described in detail elsewhere [2]% we decided to study the 14N (I= 1) quadrupole resonance spectra of a single crystaL of glycinne, NHZCH2COOH, which is the simplest of the ZOamino acids entering the structure of proteins. An effort to detect the pure nuclear quachpoIe resonance (NQR) spectra OC14N in glycine by standard
methods, was not successfuL. A study of the 14N quadrupole interactions in this compound was however reported by Anderson et al. [3] using quadrupole perturbed 14N nucIear magnetic resonance. The i4N quadrupoEe CoupIing constant (e2qQih = 12OOkEk) they reported was however much lower than the ones found in other amines
[4] (3.7 - 4.2 MHz) so that a check by a different experimental method seemed worthwhile. In view of (TlD) considerations we decided to work above room temperature. The temperature where most of the dais were t&en, was 140°C and B. = 428OG. The crystal structure of gLycine was determined by Albrecht and Corey [5]. The unit cell of cY-glycine is monoclinic - space group P21 ,1with four molecules per unit cell. The nearly flat glycine moltxules have the “titter-ion” structure, NH~CH2CC10’, and form discrete layers held together isy N-A--Q hydrogen bonds. Two of these bonds which connect nitrogen and oxygen atoms in the same layer are relatively strong (2.76 A and 2.88 A) whereas the third one - between Feighbouring layers - is weak (2.93 and 3.05 A). 85
Vulumti
9. number
1
CHEMICAL
Oauble HMR
I
I 0 Fig.!.. Angular
, 20 dependence
t b0
I
EO
PHYSICS LETTERS
1 April
1971
ii'-N'm Glycme SmgleCrystal,alt$; T:135+2YI
I
L
eo
100
of the 14N quadrupole
1
121
splitting
I
IL0 in glycine
I
I
160
‘8$-i* ]
for 8 I
HOandH, = 4280 G.
DoubleNMR ll'-W'L inGlycw Smgle Crystal, b*l H,,, T=1359"C
Fig.Z.Angular
dependence
of the “$I qundrupole splitting in glycine for a rotation about an axis perpendicular md making an angle of 560 with b. (R, = ti8OG).
On the ba& of the crystal structure one may expkct that all 14N.atoms in the unit cell are xhemically equivalent. There shouXdbe, how.ww, two physicalIy non-equivalent 14N sites related by a two-foJd ax&.
to (I
The angular dependence of the 14N-proton douwe resonance spectra coIlfirmed these expectatiaw (figs. 1 and 2). Both the Am = 1 and then Am = 2 nitrogen transitions were observed, and the coz6pIete14N quadnpole coupling tensor was
Volume 9, number 1
CHEbIICAL PHYSICS LETTERS
Table 1 Quadrupole coupling tensor [inkHz] of laN in glycine with respect to the 8. b and 8% bcrystal axes system
a 8
-640
b
axb
SO
- 70
b
+&IO
540
*705
axb
- 70
l705
100
determined (table 1). After diagonalization, the 14N quadrupole coupling constant, e2qg/h = 745 f 2OkR2, the asymmetry parameter 11= 0.61i 0.03 and the direction cosines of the tensor axes were obtained. The direction cosines of the largest principal axis of the 14N quadrupole coupling tensor with respect to the a, b and (a xb) crystal axes, in particular, are (0.182, 0.814, 0.552). Two importantfactsshould be stressed: The first is that the 14N quadrupole coupIing constant found in this study is even lower than the one observed by Anderson et al. [3]. The difference is probably due to the fact that our measurements were done at a higher temperature. Another surprising result is the observation that the largest principal axis of the 14N quadrupole coupling tensor does not coincide with the C-N axis and in fact makes an angle of 60° with the direction of this bond. This seems todemonstrate that the motion of the RR; group in glycine
1 April 1971
cannot be descriied as a simple hindered rotation about the C-N bond. Some support for this conclusion is provided by the fact that the C-N bond is not normal to the pIane oE the three hydrogen atoms in the NE$ group [S] and that the asymmetry parameter for the deuterium [6] quadrupole coupling tensor of the ND$ group is about 0.4, whereas it should be zero for hindered rotation of the ND8 group around its triad axis. This research was supported in part by the Wallenberg foundation, Sweden and the Federal Council for the Coordfnation of Research, YugosIa~ia. The authors are grateful to F. LonCar for technical assiaace. REFERENCES [l] S. R. Hartman and E. L. Hahn. Phys. Rev. 128 (1962; 2042. [Z] R.BLinc. M. Mli. R.Osredkar. I_ ZupanEZ and L.Ehrenberg.ActaChem.Scand.. to he published. [3] L-0. Andersson, h¶.Gourdji, L.Guibe and W-G. Proctor.Compt.Rend.Acad.Sci.(Paris) 267 (1968) 803. [4] Y. Abe and S. Koyima. J. Phys.Soc. Japan 17 (1962) 720, [5] G. Arbrecht and R. B. Corey. J. Am.Chem.Soc. 61 (1939) 1087. [6] V. Saraswati and R. Vijayaraghavan. J. Phys. Sot. Japan 23 Q967) SSO.
8’7
CHJ3UC.4 L PWSICS
LETTERS
1 April 1971
PULSED DOUBLE RESONANCE STUDY OF THE IGN NUCLEAR QUADRUPOLE COUPLING IN GLYCINE R. BLINC, M. htiL1, R. OSREDKAR, A. PRELESNIK, I. ZUPANC!Ic LrnivrsUy
of LjubZjam.
bstihte
‘-I.Ste_ran ‘, L)ubljana.*.iugaslavia and
Department
of Bixhemistq.
L. EHRENBERG Lhirersity of Stockholnr. StockI:oim. Sweden Received 4 January 1971
The technique of nuclear magnetic pulsed double resonance in the rotating frame has been used to
determine the quadrupole interactions of 14N in a single crystal of glycine nt 140°C. The 1% quldrupole coupling constant is found to be very small e%Q/fz= ;-%5kHz.whereas the ssymmetry paczmeter is relatively large ?J= 0.61. The direction of the Largest principal asis of the electric field gradient tensor
does rootcoincide with the C-N direction as determined by X-ray data at room temperature.
Though it is known that 14N qua&pole resonance spectra might be very valuable for the determination of the structure of biologically important moIecules this method has not been much used so far. This is due to the experimental difficulties connected wi&ththe low frequency of the 14N quadrupole resonances and the resulting poor signal-to-noise ratio. Nuclear double resonance [l] methods, which use the strong resonance of one nuclear species - say protons - to detect the weaker resonance of another species in our case 14N - might represent a satisfactory solution to this problem, if the dipolar spin-lattice relaxation times (TlD) of the protons are long enough. The optimum signal-to-noise ratio S with which the nitrogen spectra can be measured by this method may be e.xpressed as
(1) for TlD > w-1. Here (S)B is the signal-to-noise ratio of the proton spectra and W is the double resonance rate which is proportional to the strength of the dipolar coupling between the two nuclear spin reservoirs. Eq. (1) shows that if (TlD)H is long enough the I4N spectra can be measured with a signal-to-noise ratio which is not much smaller than the one of the protonic system. The increase in sensitivity over pure nuclear quadrupcle resonance spectroscopy is of the order.of TlD/T2 i.e. about lo3 - 104 in favourable cases.
ln order to test this method which has been described in detail elsewhere [2]% we decided to study the 14N (I= 1) quadrupole resonance spectra of a single crystaL of glycinne, NHZCH2COOH, which is the simplest of the ZOamino acids entering the structure of proteins. An effort to detect the pure nuclear quachpoIe resonance (NQR) spectra OC14N in glycine by standard
methods, was not successfuL. A study of the 14N quadrupole interactions in this compound was however reported by Anderson et al. [3] using quadrupole perturbed 14N nucIear magnetic resonance. The i4N quadrupoEe CoupIing constant (e2qQih = 12OOkEk) they reported was however much lower than the ones found in other amines
[4] (3.7 - 4.2 MHz) so that a check by a different experimental method seemed worthwhile. In view of (TlD) considerations we decided to work above room temperature. The temperature where most of the dais were t&en, was 140°C and B. = 428OG. The crystal structure of gLycine was determined by Albrecht and Corey [5]. The unit cell of cY-glycine is monoclinic - space group P21 ,1with four molecules per unit cell. The nearly flat glycine moltxules have the “titter-ion” structure, NH~CH2CC10’, and form discrete layers held together isy N-A--Q hydrogen bonds. Two of these bonds which connect nitrogen and oxygen atoms in the same layer are relatively strong (2.76 A and 2.88 A) whereas the third one - between Feighbouring layers - is weak (2.93 and 3.05 A). 85
Vulumti
9. number
1
CHEMICAL
Oauble HMR
I
I 0 Fig.!.. Angular
, 20 dependence
t b0
I
EO
PHYSICS LETTERS
1 April
1971
ii'-N'm Glycme SmgleCrystal,alt$; T:135+2YI
I
L
eo
100
of the 14N quadrupole
1
121
splitting
I
IL0 in glycine
I
I
160
‘8$-i* ]
for 8 I
HOandH, = 4280 G.
DoubleNMR ll'-W'L inGlycw Smgle Crystal, b*l H,,, T=1359"C
Fig.Z.Angular
dependence
of the “$I qundrupole splitting in glycine for a rotation about an axis perpendicular md making an angle of 560 with b. (R, = ti8OG).
On the ba& of the crystal structure one may expkct that all 14N.atoms in the unit cell are xhemically equivalent. There shouXdbe, how.ww, two physicalIy non-equivalent 14N sites related by a two-foJd ax&.
to (I
The angular dependence of the 14N-proton douwe resonance spectra coIlfirmed these expectatiaw (figs. 1 and 2). Both the Am = 1 and then Am = 2 nitrogen transitions were observed, and the coz6pIete14N quadnpole coupling tensor was
Volume 9, number 1
CHEbIICAL PHYSICS LETTERS
Table 1 Quadrupole coupling tensor [inkHz] of laN in glycine with respect to the 8. b and 8% bcrystal axes system
a 8
-640
b
axb
SO
- 70
b
+&IO
540
*705
axb
- 70
l705
100
determined (table 1). After diagonalization, the 14N quadrupole coupling constant, e2qg/h = 745 f 2OkR2, the asymmetry parameter 11= 0.61i 0.03 and the direction cosines of the tensor axes were obtained. The direction cosines of the largest principal axis of the 14N quadrupole coupling tensor with respect to the a, b and (a xb) crystal axes, in particular, are (0.182, 0.814, 0.552). Two importantfactsshould be stressed: The first is that the 14N quadrupole coupIing constant found in this study is even lower than the one observed by Anderson et al. [3]. The difference is probably due to the fact that our measurements were done at a higher temperature. Another surprising result is the observation that the largest principal axis of the 14N quadrupole coupling tensor does not coincide with the C-N axis and in fact makes an angle of 60° with the direction of this bond. This seems todemonstrate that the motion of the RR; group in glycine
1 April 1971
cannot be descriied as a simple hindered rotation about the C-N bond. Some support for this conclusion is provided by the fact that the C-N bond is not normal to the pIane oE the three hydrogen atoms in the NE$ group [S] and that the asymmetry parameter for the deuterium [6] quadrupole coupling tensor of the ND$ group is about 0.4, whereas it should be zero for hindered rotation of the ND8 group around its triad axis. This research was supported in part by the Wallenberg foundation, Sweden and the Federal Council for the Coordfnation of Research, YugosIa~ia. The authors are grateful to F. LonCar for technical assiaace. REFERENCES [l] S. R. Hartman and E. L. Hahn. Phys. Rev. 128 (1962; 2042. [Z] R.BLinc. M. Mli. R.Osredkar. I_ ZupanEZ and L.Ehrenberg.ActaChem.Scand.. to he published. [3] L-0. Andersson, h¶.Gourdji, L.Guibe and W-G. Proctor.Compt.Rend.Acad.Sci.(Paris) 267 (1968) 803. [4] Y. Abe and S. Koyima. J. Phys.Soc. Japan 17 (1962) 720, [5] G. Arbrecht and R. B. Corey. J. Am.Chem.Soc. 61 (1939) 1087. [6] V. Saraswati and R. Vijayaraghavan. J. Phys. Sot. Japan 23 Q967) SSO.
8’7