- Email: [email protected]
Carbon relaxation analysis in proton coupled spin systems
14 July 1995
CHEMICAL PHY$lCS LEfiERS ELSEVIER
Chemical Physics Letters 241 (1995) 97-102
Carbon relaxation analysis in proton coupled spin systems Claudio Rossi, Nadia Marchettini,
Simone Bastianoni, Alessandro Donziti
Department of Chemistry, University of Siena, Pian dei Mantellini, 44, 53100 Siena, Italy
Received 17 March 1995; in final form 5 May 1995
Abstract Selective, non-selective and biselective carbon spin-lattice relaxation measurements were determined in methyl-salicylate DMSO-d, solution. Tbe frequency dependence of biselective relaxation measurements of protonated aromiitic carbons showed the effects of J-scalar modulation. The dipolar contribution induced by asymmetric selective proton inversion of the spin population of a single satellite peak could be useful for investigating of the Shimizu-Fujiwara-Machor-Maclean relaxation rate. Analysis of the ratios is also proposed for the calculation of dipolar relaxation mechanism effidiency.
1. Introduction Spin-lattice relaxation rate measurements of protonated carbons are widely used to monitor the dynamical properties of biomolecules in solution [1,2]. This is because the carbon spin-lattice relaxation rate, measured under conditions of broad-band proton decoupling, is dominated the ‘H- l3 C intramolecular dipole-dipole interactions. 13C isotopic spin dilution makes the mono-protonated carbon nuclei a good isolated two spin system for: (i) extracting dynamical information on molecular tumbling [3] and (ii) investigating nuclear relaxation in coupled spin conditions. Quatemary carbons spin-lattice relaxation rates contain a complementary set of information. The absence of a dominant dipolar relaxation term makes the relaxation of these carbons extremely sensitive to long range ‘H-13C interactions, which are particularly useful for structural investigations [4-61. Under broad-band proton decoupling conditions, a 0009-2614/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0009-2614(95)00565-X
single direct dipolar term ( p) is importay for determining the relaxation of mono-protonatetd carbons. Under these circumstances, the process of ~magnetization recovery about the longitudinal a&, after a perturbing pulse, is described by a single exponential. However, this relaxation behaviout does not express the total relaxation potentiality. It has been shown that in conditions of selective protan perturbation, faster carbon relaxation rates are observed due to additional cross-relaxation contributiotib [7-l 11. In this Letter, we report the effect of selective inversion of the spin population of covalemtly bonded protons on carbon relaxation. The ‘Hi13C scalar coupling (‘J) modulation effect on the cioss-relaxation contribution generated is discussed, together with the structural application of the techniques.
2. Experimental Methyl salicylate, Merck (Fig. 1) wa$ used as a model compound for investigating the effects of
C. Rossi et al. / Chemical Physics Letters 241 (1995) 97-102
98
3. Theory
Fig. 1. Structure and numbering
of methyl salicylate.
proton perturbation on the relaxation behaviour of carbon nuclei in scalar coupled spin systems. A 50% methyl salicylate, 50% DMSO-d, solution was used in our experiments. NMR spectra were recorded on a Varian XL-200 spectrometer operating at 200 and 50.3 MHz for proton and carbon nuclei, respectively. In all experiments the temperature of the sample was 27 f 05°C. A maximum experimental error of 5% was estimated for the R,, measurement. Methyl salicylate has four protonated aromatic carbons. Each carbon relaxes by a dipolar mechanism with the proton covalently bonded. Only a small dipolar contribution (less than 2%) is observed on protonated carbons from neighbouring protons. Hence we assumed each proton-carbon pair to be an isolated spin system suitable for studying the total dipolar relaxation process in coupled spin system. Carbon spin-lattice relaxation rates were measured in three different modes: selective CR:,), biselective (RF: > and non-selective (Rrz). The selective spin-lattice relaxation rate was measured using the standard (180”-T-90”-AT), ‘inversion-recovery’ pulse sequence under continuous broad-band proton decoupling conditions. Biselective and non-selective R,, were measured using the {[(180’),,,-(180’),,]-7-90”-AT), pulse sequence [8,9]. In these sequences a simultaneous 180” nonselective carbon pulse is applied together with a selective or a non-selective 180” proton pulse for biselective and non-selective experiments, respectively. In Rp: and Ryz experiments broad-band proton decoupling conditions were applied during the signal acquisition period in order to remove proton-carbon scalar coupling. The carbon spinlattice relaxation rates were calculated by three parameters exponential regression analysis of the longitudinal recovery curve using the initial slope approximation.
Nuclear spin relaxation of AX systems has been described by transition probabilities between the four energy levels (Fig. 2) associated with the A-X dipole-dipole interaction. The population time dependence in each level can be written [12]
z
N3
-w,
K Wl”
w,,
w, - wa w,,
w2
K”
WlC Wl” WlC - wa w,
4 d N2 =
N4/
WlC -K
Nl -Nf X
N,-N,o
(1)
,
N,-N,o N,-N;
where
w*= w,, + w,, + w,, w,=w,+w,,+w,,,
(2)
Operating a linear transformation of the populations, the latter are correlated to experimentally determined parameters, n+= (4
+Nz) + (Nj +N,),
nc = (4
-4)
+ (4
-4)
= [(2/~,)+40
n,=(N,-N,)+(N,-N,)=[ n,=(N,-N,)+(N,-N,)=[
i=H,C,
2 4
Fig. 2. Energy level diagram and transition probabilities isolated AX system of non-identical spin l/2 nuclei.
for an
C. Rossi et al. /Chemical
Physics Letters 241 (1995) 97-102
where Mu and MC are the total z magnetization, MAi (=MAu = MA,) are the differences in z magnetization between the two lines of the measured doublet. Eq. (1) can be simplified using the new variables described by Eqs. (3) and introducing the thermal equilibium values n: and nt
99
experimental results refer to parameters which are related to the direct and cross-relaxation terms by RYE = Cp,
(6)
RyS=Cp+a,
(7)
R:s=Cp+Ca.
(8)
4. Results and discussion dn,/dt
= pn,,
(4)
and pH are the direct relaxation terms, the cross-relaxation terms used in the ~ckl = gut Solomon notation [13],
where
pc
pc = 2w,,
+ w, + W”,
PH=2W,H+W*+W”, *CU
=CT "C=W2-W",
and where I_L is the Shimizu-Fujiwara-MackorMaclean [14,15] multiplet asymetry relaxation rate, /.L=2w,,+2w
IH,
(5)
Under our experimental conditions, the information about m is lost and the dipolar relaxation terms are expressed by the classical Solomon equations. Our
I 135
11
1 1’1 I30
1 ‘I’ Ia5
1’1
II 110
11 Ip
1
r
11
IIS
Fig. 3. Selective (A) and non-selective (B) carbon spin-lattice 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 14, 18, 24, 120 (s).
Fig. 3 shows carbon spin-lattice relaxation rate measurements obtained in selective and non-selective mode. Fig. 4 shows two biselective experiments obtained by selective proton spin population inversion at 7.76 and 7.30 ppm, respectively. Fig. 4 shows the effect of the cross-relaxation terms on dipolar coupled carbons from proton population inversion. In order to investigate the origin of cross-relaxation through selective ‘H spin-population inversion in detail, carbon spin-lattice relaxation rates of methyl salicylate were measured as a function of the selective proton perturbation frequency. The results for the four protonated carbons of the aromatic residue of methyl salicylate are shown in Fig. 5.
135
relaxation
11
1 II
(1
130
rate measurements.
111 II,
’ 11
II LD
11 wp
1
r 111
The T values used were: 0.04, 0.06, 0.08, 0.1,
C. Rossi et al. /Chemical
I
I,
I
,,I
I
11,
I
I30
II
I,
I1
I25
I
I,
I
111
I! Ic
Physics Letters 241 (199.5) 97-102
1 I IIS
Fig. 4. Biselective carbon spin-lattice relaxation experiments respectively. The T values used were the same as in Fig. 3.
11 I,,
“‘I”‘1
obtained by inversion
For each carbon, two symmetric maxima centred at the ‘H frequency of the covalently bounded proton were observed. This behaviour is due to the strong direct proton-carbon scalar coupling and to the low natural abundance of carbon-13. In fact, the proton spectrum terms aut to be as the sum of two contributions. In the first one, accounting for 99% of the total, the proton nucleus is bonded to the 12C isotope, no scalar modulation is induced and the signal is a singlet. The second contribution, 1% of the total, is a doublet separated by the JC_H scalar coupling. This doublet is known as ‘satellite peaks’. In spite of the fact that the second contribution accounts for 1% of the total signal, only the perturbation of the satellite peaks yields a cross-relaxation term on the coupled carbon. Significantly the asymmetric proton spinpopulation inversion is sufficient to generate the maximum allowed cross-relaxations contribution on the dipolar coupled carbon. In the case of loosely scalar coupled proton-carbon pairs, the J modulation effect is usually masked by the natural modulation of the proton perturbation frequency and the maximum cross-relaxation contribution is induced by spin-population inversion at the frequency of the most abundant 99% proton signal. The generation of a cross-relaxation contribution to the observed carbon spin-lattice relaxation rate by
II 423
no
1 “I”1
1 120 pp
c IIS
at 7.76 ppm (A) and 7.30 ppm (B) proton frequencies,
proton perturbation can be useful in the following situations: (i) To verify the strict application of the basic theory in the case of heteronuclear dipolar relaxation,
-I 6.05
7.65
1.25
Fig. 5. Biselective spin-lattice protonated carbons of methyl frequency.
6.85
relaxation salicylate
nl
6.45
rates of the aromatic in relation to proton
C. Rossi et al. /Chemical Table 1 Carbon relaxation
parameters
Physics Letters 241 (1995) 97-102
obtained for a 50% methyl salicylate-50%
DMSO-d,
101
solution at 300 K
Carbon No.
6 (ppm)
RrS (s- ‘)
RsE (s- ‘)
Few a
XOD
R (DD h (s_‘)
Tc (s)
7 2 4 6 5 3 1 8
170.12 161.29 135.46 129.74 118.91 117.21 112.21 51.85
0.037 0.068 1.88 1.42 1.27 1.32 0.039 0.67
0.032 0.053 0.67 0.50 0.46 0.47 0.033 0.25
1.16 1.28 2.82 2.85 2.79 2.80 1.18 2.65
0.39 0.43 0.94 0.95 0.93 0.94 0.52 0.89
0.012 0.023 0.64 0.48 0.43 0.45 0.017 0.22
3.4 2.3 1.9 2.0
complicated by simultaneous scalar coupling. In the present Letter it was shown (see Table 1) that RBS - I = 1.5% RSE I YC
(9)
for a fairly isolated two spin system such as the aromatic proton-carbon spin pair when the extreme narrowing conditions wOrc +z 1 apply. (ii) To detect additional relaxation contributions to quaternary carbons of relevance structural analysis. It must be considered that the RF: experiment, obtained by selective inversion of a proton spinpopulations induces cross-relaxation contributions on the carbon microenvironment of the perturbed proton. This allows complete definition of the conformation properties in solution. In Fig. 5 are reported the experimental RF: values from which the induced cross-relaxation contribution
(10) was calculated. The use of an explicit equation for cr with the effective correlation time value determined from Rs: allows the distances between any perturbed proton and all neighbouring dipolar coupled carbons to be calculated. Proton-carbon distances calculated on the basis of the experimental results for methyl salicylate were in excellent agreement with the theoretical canonical distance ‘.
’ This data is available
on request.
x 110-” x 110-” x lo-” x lo-”
(iii) To obtain the efficiency of the dipolar relaxation contribution to the carbon spin-lattice relaxation rate by analysis of the ratio RyS/Ry
= Fexp.
The fractional termined as
dipolar contribution
(11) xnn
is then de-
(l-2) F theor range from 1.5y,/ yc, in the extreme narrowing limit, to 1 in slow motion conditions. From analysis of the ratio Ryz/RSE we can calculate the efficiency of any dipolar contribution to ‘the carbon relaxation rate. Table 1 shows the non-selective and selective carbon spin-lattice relaxation rates of methyl salicilate, the F_, values, the fractional dilpolar term x,,n and the effective correlation time calculated using the Allerhand equation [ll. As shown in Table dipolar interaction is modulated by I the C13,-%s the longer correlation time, since the bond direction is parallel to the internal reorientational axis. This is also evident in Fig. 5, where the C,,, carbon has the most efficient biselective relaxation rate of all the aromatic protonated carbons. In our opinion the use of biselective carbon relaxation experiments in scalar coupled systems (where the effects of ‘J modulation appears) together with the measurements could be important for understanding carbon relaxation processes and for application in structural and dynamical analysis. The method is also useful for determinating the Shimizu-Fujiwara-Mackor-Maclean
102
C. Rossi et al. /Chemical
Physics Letters 241 (1995) 97-102
multiplet asymmetry relaxation rate on the basis of selective perturbation of a single ‘satellite peak’ proton spin population.
Acknowledgements
We gratefully acknowledge the financial support for this project by the Italian National Research Council (CNR) under grant CU 93.03555.CT.
References [1] A. Allerhand and R.A. Komoroski, J. Am. Chem. Sot. 95 (1973) 8228. [2] C. Rossi, Chem. Phys. Letters 193 (1992) 553. [3] C.L. Mayne, D.M. Grant and D.W. Alderman, J. Chem. Phys. 65 (197.5) 1684.
141 N. Niccolai, C. Rossi, V. Brizzi and W.A. Gibbons, J. Am. Chem. Sot. 106 (1984) 5732. C. Rossi, P. Mascagni, P. Neri and W.A. Gibbons, Biochem. Biophys. Res. Commun. 124 (1984) 739. b1 N. Niccolai and C. Rossi, Methods Enzymol. 176 (1989) 184. [71 I.D. Campbell and R. Freeman, J. Chem. Phys. 58 (1973) 2666. k31C. Rossi, J. Chem. Phys. 84 (1986) 6581. [91 C. Rossi, N. Marchettini, L. Pogliani, F. Laschi and N. Niccolai, Chem. Phys. Letters 136 (1987) 506. [lOI K.E. Kovtr and G. Batta, Progr. Nucl. Magn. Spectry. 19 (1987) 223. 1111C. Rossi, in: Encyclopedia of NMR. Selective Relaxation Techniques in Biological NMR, eds. D.M. Grant and R.H. Harris (Wiley, New York), in press. Ml C.L. Mayne, D.W. AIderman and D.M. Grant, J. Chem. Phys. 63 (1975) 2514. [131 I. Solomon, Phys. Rev. 99 (1955) 55. [141 H. Shimizu and S. Fujiwara, J. Chem. Phys. 34 (1961) 1501. ml E.L. Mackor and C. Maclean, J. Chem. Phys. 42 (1965) 4254.
El N. Niccolai,
CHEMICAL PHY$lCS LEfiERS ELSEVIER
Chemical Physics Letters 241 (1995) 97-102
Carbon relaxation analysis in proton coupled spin systems Claudio Rossi, Nadia Marchettini,
Simone Bastianoni, Alessandro Donziti
Department of Chemistry, University of Siena, Pian dei Mantellini, 44, 53100 Siena, Italy
Received 17 March 1995; in final form 5 May 1995
Abstract Selective, non-selective and biselective carbon spin-lattice relaxation measurements were determined in methyl-salicylate DMSO-d, solution. Tbe frequency dependence of biselective relaxation measurements of protonated aromiitic carbons showed the effects of J-scalar modulation. The dipolar contribution induced by asymmetric selective proton inversion of the spin population of a single satellite peak could be useful for investigating of the Shimizu-Fujiwara-Machor-Maclean relaxation rate. Analysis of the ratios is also proposed for the calculation of dipolar relaxation mechanism effidiency.
1. Introduction Spin-lattice relaxation rate measurements of protonated carbons are widely used to monitor the dynamical properties of biomolecules in solution [1,2]. This is because the carbon spin-lattice relaxation rate, measured under conditions of broad-band proton decoupling, is dominated the ‘H- l3 C intramolecular dipole-dipole interactions. 13C isotopic spin dilution makes the mono-protonated carbon nuclei a good isolated two spin system for: (i) extracting dynamical information on molecular tumbling [3] and (ii) investigating nuclear relaxation in coupled spin conditions. Quatemary carbons spin-lattice relaxation rates contain a complementary set of information. The absence of a dominant dipolar relaxation term makes the relaxation of these carbons extremely sensitive to long range ‘H-13C interactions, which are particularly useful for structural investigations [4-61. Under broad-band proton decoupling conditions, a 0009-2614/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0009-2614(95)00565-X
single direct dipolar term ( p) is importay for determining the relaxation of mono-protonatetd carbons. Under these circumstances, the process of ~magnetization recovery about the longitudinal a&, after a perturbing pulse, is described by a single exponential. However, this relaxation behaviout does not express the total relaxation potentiality. It has been shown that in conditions of selective protan perturbation, faster carbon relaxation rates are observed due to additional cross-relaxation contributiotib [7-l 11. In this Letter, we report the effect of selective inversion of the spin population of covalemtly bonded protons on carbon relaxation. The ‘Hi13C scalar coupling (‘J) modulation effect on the cioss-relaxation contribution generated is discussed, together with the structural application of the techniques.
2. Experimental Methyl salicylate, Merck (Fig. 1) wa$ used as a model compound for investigating the effects of
C. Rossi et al. / Chemical Physics Letters 241 (1995) 97-102
98
3. Theory
Fig. 1. Structure and numbering
of methyl salicylate.
proton perturbation on the relaxation behaviour of carbon nuclei in scalar coupled spin systems. A 50% methyl salicylate, 50% DMSO-d, solution was used in our experiments. NMR spectra were recorded on a Varian XL-200 spectrometer operating at 200 and 50.3 MHz for proton and carbon nuclei, respectively. In all experiments the temperature of the sample was 27 f 05°C. A maximum experimental error of 5% was estimated for the R,, measurement. Methyl salicylate has four protonated aromatic carbons. Each carbon relaxes by a dipolar mechanism with the proton covalently bonded. Only a small dipolar contribution (less than 2%) is observed on protonated carbons from neighbouring protons. Hence we assumed each proton-carbon pair to be an isolated spin system suitable for studying the total dipolar relaxation process in coupled spin system. Carbon spin-lattice relaxation rates were measured in three different modes: selective CR:,), biselective (RF: > and non-selective (Rrz). The selective spin-lattice relaxation rate was measured using the standard (180”-T-90”-AT), ‘inversion-recovery’ pulse sequence under continuous broad-band proton decoupling conditions. Biselective and non-selective R,, were measured using the {[(180’),,,-(180’),,]-7-90”-AT), pulse sequence [8,9]. In these sequences a simultaneous 180” nonselective carbon pulse is applied together with a selective or a non-selective 180” proton pulse for biselective and non-selective experiments, respectively. In Rp: and Ryz experiments broad-band proton decoupling conditions were applied during the signal acquisition period in order to remove proton-carbon scalar coupling. The carbon spinlattice relaxation rates were calculated by three parameters exponential regression analysis of the longitudinal recovery curve using the initial slope approximation.
Nuclear spin relaxation of AX systems has been described by transition probabilities between the four energy levels (Fig. 2) associated with the A-X dipole-dipole interaction. The population time dependence in each level can be written [12]
z
N3
-w,
K Wl”
w,,
w, - wa w,,
w2
K”
WlC Wl” WlC - wa w,
4 d N2 =
N4/
WlC -K
Nl -Nf X
N,-N,o
(1)
,
N,-N,o N,-N;
where
w*= w,, + w,, + w,, w,=w,+w,,+w,,,
(2)
Operating a linear transformation of the populations, the latter are correlated to experimentally determined parameters, n+= (4
+Nz) + (Nj +N,),
nc = (4
-4)
+ (4
-4)
= [(2/~,)+40
n,=(N,-N,)+(N,-N,)=[ n,=(N,-N,)+(N,-N,)=[
i=H,C,
2 4
Fig. 2. Energy level diagram and transition probabilities isolated AX system of non-identical spin l/2 nuclei.
for an
C. Rossi et al. /Chemical
Physics Letters 241 (1995) 97-102
where Mu and MC are the total z magnetization, MAi (=MAu = MA,) are the differences in z magnetization between the two lines of the measured doublet. Eq. (1) can be simplified using the new variables described by Eqs. (3) and introducing the thermal equilibium values n: and nt
99
experimental results refer to parameters which are related to the direct and cross-relaxation terms by RYE = Cp,
(6)
RyS=Cp+a,
(7)
R:s=Cp+Ca.
(8)
4. Results and discussion dn,/dt
= pn,,
(4)
and pH are the direct relaxation terms, the cross-relaxation terms used in the ~ckl = gut Solomon notation [13],
where
pc
pc = 2w,,
+ w, + W”,
PH=2W,H+W*+W”, *CU
=CT "C=W2-W",
and where I_L is the Shimizu-Fujiwara-MackorMaclean [14,15] multiplet asymetry relaxation rate, /.L=2w,,+2w
IH,
(5)
Under our experimental conditions, the information about m is lost and the dipolar relaxation terms are expressed by the classical Solomon equations. Our
I 135
11
1 1’1 I30
1 ‘I’ Ia5
1’1
II 110
11 Ip
1
r
11
IIS
Fig. 3. Selective (A) and non-selective (B) carbon spin-lattice 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 14, 18, 24, 120 (s).
Fig. 3 shows carbon spin-lattice relaxation rate measurements obtained in selective and non-selective mode. Fig. 4 shows two biselective experiments obtained by selective proton spin population inversion at 7.76 and 7.30 ppm, respectively. Fig. 4 shows the effect of the cross-relaxation terms on dipolar coupled carbons from proton population inversion. In order to investigate the origin of cross-relaxation through selective ‘H spin-population inversion in detail, carbon spin-lattice relaxation rates of methyl salicylate were measured as a function of the selective proton perturbation frequency. The results for the four protonated carbons of the aromatic residue of methyl salicylate are shown in Fig. 5.
135
relaxation
11
1 II
(1
130
rate measurements.
111 II,
’ 11
II LD
11 wp
1
r 111
The T values used were: 0.04, 0.06, 0.08, 0.1,
C. Rossi et al. /Chemical
I
I,
I
,,I
I
11,
I
I30
II
I,
I1
I25
I
I,
I
111
I! Ic
Physics Letters 241 (199.5) 97-102
1 I IIS
Fig. 4. Biselective carbon spin-lattice relaxation experiments respectively. The T values used were the same as in Fig. 3.
11 I,,
“‘I”‘1
obtained by inversion
For each carbon, two symmetric maxima centred at the ‘H frequency of the covalently bounded proton were observed. This behaviour is due to the strong direct proton-carbon scalar coupling and to the low natural abundance of carbon-13. In fact, the proton spectrum terms aut to be as the sum of two contributions. In the first one, accounting for 99% of the total, the proton nucleus is bonded to the 12C isotope, no scalar modulation is induced and the signal is a singlet. The second contribution, 1% of the total, is a doublet separated by the JC_H scalar coupling. This doublet is known as ‘satellite peaks’. In spite of the fact that the second contribution accounts for 1% of the total signal, only the perturbation of the satellite peaks yields a cross-relaxation term on the coupled carbon. Significantly the asymmetric proton spinpopulation inversion is sufficient to generate the maximum allowed cross-relaxations contribution on the dipolar coupled carbon. In the case of loosely scalar coupled proton-carbon pairs, the J modulation effect is usually masked by the natural modulation of the proton perturbation frequency and the maximum cross-relaxation contribution is induced by spin-population inversion at the frequency of the most abundant 99% proton signal. The generation of a cross-relaxation contribution to the observed carbon spin-lattice relaxation rate by
II 423
no
1 “I”1
1 120 pp
c IIS
at 7.76 ppm (A) and 7.30 ppm (B) proton frequencies,
proton perturbation can be useful in the following situations: (i) To verify the strict application of the basic theory in the case of heteronuclear dipolar relaxation,
-I 6.05
7.65
1.25
Fig. 5. Biselective spin-lattice protonated carbons of methyl frequency.
6.85
relaxation salicylate
nl
6.45
rates of the aromatic in relation to proton
C. Rossi et al. /Chemical Table 1 Carbon relaxation
parameters
Physics Letters 241 (1995) 97-102
obtained for a 50% methyl salicylate-50%
DMSO-d,
101
solution at 300 K
Carbon No.
6 (ppm)
RrS (s- ‘)
RsE (s- ‘)
Few a
XOD
R (DD h (s_‘)
Tc (s)
7 2 4 6 5 3 1 8
170.12 161.29 135.46 129.74 118.91 117.21 112.21 51.85
0.037 0.068 1.88 1.42 1.27 1.32 0.039 0.67
0.032 0.053 0.67 0.50 0.46 0.47 0.033 0.25
1.16 1.28 2.82 2.85 2.79 2.80 1.18 2.65
0.39 0.43 0.94 0.95 0.93 0.94 0.52 0.89
0.012 0.023 0.64 0.48 0.43 0.45 0.017 0.22
3.4 2.3 1.9 2.0
complicated by simultaneous scalar coupling. In the present Letter it was shown (see Table 1) that RBS - I = 1.5% RSE I YC
(9)
for a fairly isolated two spin system such as the aromatic proton-carbon spin pair when the extreme narrowing conditions wOrc +z 1 apply. (ii) To detect additional relaxation contributions to quaternary carbons of relevance structural analysis. It must be considered that the RF: experiment, obtained by selective inversion of a proton spinpopulations induces cross-relaxation contributions on the carbon microenvironment of the perturbed proton. This allows complete definition of the conformation properties in solution. In Fig. 5 are reported the experimental RF: values from which the induced cross-relaxation contribution
(10) was calculated. The use of an explicit equation for cr with the effective correlation time value determined from Rs: allows the distances between any perturbed proton and all neighbouring dipolar coupled carbons to be calculated. Proton-carbon distances calculated on the basis of the experimental results for methyl salicylate were in excellent agreement with the theoretical canonical distance ‘.
’ This data is available
on request.
x 110-” x 110-” x lo-” x lo-”
(iii) To obtain the efficiency of the dipolar relaxation contribution to the carbon spin-lattice relaxation rate by analysis of the ratio RyS/Ry
= Fexp.
The fractional termined as
dipolar contribution
(11) xnn
is then de-
(l-2) F theor range from 1.5y,/ yc, in the extreme narrowing limit, to 1 in slow motion conditions. From analysis of the ratio Ryz/RSE we can calculate the efficiency of any dipolar contribution to ‘the carbon relaxation rate. Table 1 shows the non-selective and selective carbon spin-lattice relaxation rates of methyl salicilate, the F_, values, the fractional dilpolar term x,,n and the effective correlation time calculated using the Allerhand equation [ll. As shown in Table dipolar interaction is modulated by I the C13,-%s the longer correlation time, since the bond direction is parallel to the internal reorientational axis. This is also evident in Fig. 5, where the C,,, carbon has the most efficient biselective relaxation rate of all the aromatic protonated carbons. In our opinion the use of biselective carbon relaxation experiments in scalar coupled systems (where the effects of ‘J modulation appears) together with the measurements could be important for understanding carbon relaxation processes and for application in structural and dynamical analysis. The method is also useful for determinating the Shimizu-Fujiwara-Mackor-Maclean
102
C. Rossi et al. /Chemical
Physics Letters 241 (1995) 97-102
multiplet asymmetry relaxation rate on the basis of selective perturbation of a single ‘satellite peak’ proton spin population.
Acknowledgements
We gratefully acknowledge the financial support for this project by the Italian National Research Council (CNR) under grant CU 93.03555.CT.
References [1] A. Allerhand and R.A. Komoroski, J. Am. Chem. Sot. 95 (1973) 8228. [2] C. Rossi, Chem. Phys. Letters 193 (1992) 553. [3] C.L. Mayne, D.M. Grant and D.W. Alderman, J. Chem. Phys. 65 (197.5) 1684.
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El N. Niccolai,