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1.
Geminal two‐bond couplings (2J) in proteins were analyzed in terms of correlation with protein secondary structure. NMR coupling constants measured and evaluated for a total six proteins comprise 3999 values of 2JCαN′, 2JC′HN, 2JHNCα, 2JC′Cα, 2JHαC′, 2JHαCα, 2JCβC′, 2JN′Hα, 2JN′Cβ, and 2JN′C′, encompassing an aggregate 969 amino‐acid residues. A seamless chain of pattern comparisons across the spectrum datasets recorded allowed the absolute signs of all 2J coupling constants studied to be retrieved. Grouped by their mediating nucleus, C′, N′ or Cα, 2J couplings related to C′ and N′ depend significantly on ?,ψ torsion‐angle combinations. β turn types I, I′, II and II′, especially, can be distinguished on the basis of relative‐value patterns of 2JCαN′, 2JHNCα, 2JC′HN, and 2JHαC′. These coupling types also depend on planar or tetrahedral bond angles, whereas such dependences seem insignificant for other types. 2JHαCβ appears to depend on amino‐acid type only, showing negligible correlation with torsion‐angle geometry. Owing to its unusual properties, 2JCαN′ can be considered a “one‐bond” rather than two‐bond interaction, the allylic analog of 1JN′Cα, as it were. Of all protein J coupling types, 2JCαN′ exhibits the strongest dependence on molecular conformation, and among the 2J types, 2JHNCα comes second in terms of significance, yet was hitherto barely attended to in protein structure work. Proteins 2010. © 2009 Wiley‐Liss, Inc. 相似文献
2.
Pervushin K 《Journal of biomolecular NMR》2001,20(3):275-285
The interference between conformational exchange-induced time-dependent variations of chemical shifts in a pair of scalar coupled 1H and 15N spins is used to construct novel TROSY-type NMR experiments to suppress NMR signal loss in [15N,1H]-correlation spectra of a 14-mer DNA duplex free in solution and complexed with the Antp homeodomain. An analysis of double- and zero-quantum relaxation rates of base 1H–15N moieties showed that for certain residues the contribution of conformational exchange-induced transverse relaxation might represent a dominant relaxation mechanism, which, in turn, can be effectively suppressed by TROSY. The use of the new TROSY method for exchange-induced transverse relaxation optimization is illustrated with two new experiments, 2D h1
J
HN,h2
J
NN-quantitative [15N,1H]-TROSY to measure h1
J
HN and h2
J
NN scalar coupling constants across hydrogen bonds in nucleic acids, and 2D (h2
J
NN+h1
J
NH)-correlation-[15N,1H]-TROSY to correlate 1HN chemical shifts of bases with the chemical shifts of the tertiary 15N spins across hydrogen bonds using the sum of the trans-hydrogen bond coupling constants in nucleic acids. 相似文献
3.
Precise vicinal coupling constants3JHNα in proteins from nonlinear fits of J-modulated [15N,1H]-COSY experiments 总被引:6,自引:0,他引:6
Martin Billeter Dario Neri Gottfried Otting Yan Qiu Qian Kurt Wüthrich 《Journal of biomolecular NMR》1992,2(3):257-274
Summary Improved experimental schemes for the recently introduced J-modulated [15N,1H]-correlation experiment for measurements of the homonuclear amide proton-C proton vicinal coupling constants.3JHN, in uniformly15N-labeled proteins are described, and a nonlinear fit procedure is presented for quantitative evaluation of3JHN. The method was first tested with the N-terminal DNA-binding domain of the 434 repressor (M=7.3 kDa), where at 13 C precise values of3JHN in the range 2.0–9.5 Hz were obtained for all residues with resolved15N-1H cross peaks. It was then applied to theAntennapedia homeodomain complexed to a synthetic 14-base pair DNA fragment (molecular weight of the complex 18 kDa). The3JHN values measured were found to be in excellent agreement with those predicted from the secondary structure of this protein in the complex.Abbreviations and symbols NOE
nuclear Overhauser effect
- COSY
two-dimensional correlated spectroscopy
-
3JHN or J
homonuclear vicinal amide proton-C proton coupling constant
- 434 repressor(1–69)
N-terminal DNA-binding domain of the 434 repressor comprising 69 residues 相似文献
4.
Pervushin K Fernández C Riek R Ono A Kainosho M Wüthrich K 《Journal of biomolecular NMR》2000,16(1):39-46
This paper describes NMR measurements of 15N–15N and 1H–15N scalar couplings across hydrogen bonds in Watson–Crick base pairs, h2
J
NN and h1
J
HN, in a 17 kDa Antennapedia homeodomain–DNA complex. A new NMR experiment is introduced which relies on zero-quantum coherence-based transverse relaxation-optimized spectroscopy (ZQ-TROSY) and enables measurements of h1
J
HN couplings in larger molecules. The h2
J
NN and h1
J
HN couplings open a new avenue for comparative studies of DNA duplexes and other forms of nucleic acids free in solution and in complexes with proteins, drugs or possibly other classes of compounds. 相似文献
5.
Markus Blümel Jürgen M. Schmidt Frank Löhr H. Rüterjans 《European biophysics journal : EBJ》1998,27(4):321-334
Quantitative φ-dihedral angle determinations of non-glycine and non-proline residues in Desulfovibrio vulgaris flavodoxin are carried out on the exclusive basis of 3 J coupling constants. In total 124 3 JHNH α , 123 3 JHNC ′i , 118 3 JHNC β , 117 3 JC′ i–1Hα , 109 3 JC′ i–1C′i , and 103 3 JC′ i–1Hβ values form the experimental basis for translating J coupling data into geometry information using various combinations of Karplus parameters given in the literature. In addition, each backbone torsional angle φ is adjusted assuming different models of local geometry, either a rigid torsion, a Gaussian distribution centered at a distinct angle, or a two-site jump model. Numerical optimization is followed by a statistical significance evaluation to assess the results. It is found that experimental coupling constants of most of the residues involved in secondary structure elements agree best with those predicted from rigid local conformations. For dihedral angles in loop regions, mobility effects are not negligible, and a single torsion (Glu 42) is likely to adopt two distinct adjustments. However, α-helix conformations with –60° < φ < –45° give rise to an alternate solution with φ≈+170° with similar statistical significance when using the four traditionally determined proton-involved 3 J couplings. This ambiguity is efficiently avoided only when taking advantage of the complete data set comprising six available experimental 3 J coupling constants and of the degeneracy intrinsic to the Karplus relation. The optimized φ conformations are compared with reference values from the crystal structure of flavodoxin. 相似文献
6.
Dermenkephalin, H-Tyr-(D ) Met-Phe-His-Leu-Met-Asp-NH2, a highly potent and selective δ-opioid peptide isolated from frog skin, was studied in DMSO-d6 solution by two-dimensional nmr spectroscopy, including the determination of NH temperature coefficients, the evaluation of 3J coupling constants from phase-sensitive correlated spectroscopy (COSY) and the volumes of nuclear Overhauser effect (NOE) correlations. The two-dimensional NOE spectroscopy (NOESY) spectrum of dermenkephalin revealed sequential, medium-, and long-range effects. To put this information on a quantitative basis, special attention was devoted to J cross-peak suppression, quantification of the NOE volumes and analysis of the overlaps, normalization of the NOEs against diagonal peaks and Hββ′ geminal interactions. Although most of the dihedral angles deduced from the 3JNα coupling constants together with several Niαi and αiNi + 1 NOEs pointed to a partially extended peptide backbone, several Ni Ni + 1 NOEs and βi Ni + 1 interactions argued in favor of a folded structure. Moreover, several long-range correlations of strong intensities were found that supported a close spatial proximity between the side chains of D -Met2 and Met6, Tyr1 and His4, Tyr1 and Asp7, and His4 and the C-terminal amide group. In Phe, the g? rotamer in the side chain is deduced from the 3Jαβ coupling constants and αβ and Nβ NOE correlations. Whereas the amide proton dependency was not indicative of stable hydrogen bonds, the nonuniform values of the temperature coefficient may reflect an equilibrium mixture of folded and extended conformers. The overall data should provide realistic starting models for energy minimization and modelization studies. © 1993 John Wiley & Sons, Inc. 相似文献
7.
P Gupta-Bhaya 《Biopolymers》1975,14(6):1143-1160
The electron-mediated spin–spin coupling constant J between the amide NH and the α-CH protons in the dipeptide fragment Cα? CO(NH? CαH)R? C′ONH? Cα is dependent on the dihedral angle of rotation (Φ) around the N? C bond. Measurement of J in a series of zwitterionic dipeptides H3N+? CHR1? CONH? CHR2? CO2? (which is conformationally similar to the dipeptide fragment) in TFA solution shows that J is independent of R1, but dependent on the steric bulk of R2. The data are interpreted in terms of a model that assumes that what we measure is an average value of J? a thermal average over all the possible rotamers. The groups R1 and R2 are, in most cases, sterically kept apart by the trans and planar amide bonds, and hence the independence of J of R1. This model is consistent with the theoretical calculations done on the dipeptide fragment. The effect of the structural characteristics of the side chains (e.g., the effect of lengthening and branching the side chains) on the J values in dipeptides is discussed in the light of the existing results of theoretical calculations. Study of 〈J〉 values in tripeptides (C6H5CH2OCONH? CHR1? CONH? CHR2? CO2CH3, essentially three linked peptide units) shows that electrostatic interaction between the two amide bonds modifies the potential energy surface and the 〈J〉 value of a dipeptide subunit in the tripeptides. Also in some cases, direct steric interaction between the two side chains in the two adjacent dipeptide subunits in the tripeptide affects the potential energy surfaces of the individual dipeptide subunits and hence the 〈J〉 values. The influence of the structural characteristics of the side chains of individual amino acids on structure formation at or beyond the dipeptide level is discussed at various points. The J(NH? αCH) values of CH3CONH? CHR? CONH2 and CH3CONH? CHR? CO2CH3 with the same R are quite different for R = valine, leucine, phenylalanine, methionine, but equal for R = glycine. This, coupled with the fact that one of the carboxamide NH resonances has a chemical shift different from its counterpart in simple amides like CH3CONH2 and the other carboxamide NH has the same chemical shift as its counterpart in CH3CONH2, suggest the presence of a hydrogen bond in dipeptide CH3CONH? CHR? CONH2 with carboxamide NH as the donor. Theoretical evidence for two seven-membered hydrogen-bonded rings with the carboxamide NH as donor and the acetyl oxygen as acceptor is summarized. Our data cannot suggest the number of such hydrogen-bonded rings, nor can they conclude the relative proportion of these rings in a particular dipeptide. A discussion of the difficulty of interpretation is presented and the data are discussed under certain simplifying assumptions. 相似文献
8.
Abstract An efficient alternative which makes use of the reliable 3J1′2′. value to derive the endocyclic torsion angle constraints is proposed in this study. Based on the information embedded in the two plots, (i) the vicinal proton-proton J-couplings, 3J1′2′., 3J1′2″., 3J2′3′., 3J2”3′ and 3J3′4′ against the pseudorotation phase angle, and (ii) 3J1′2″, 3J2′3′., 3J2″3′ and 3J3′4′ against 3J1′2′; using the calculated J-couplings obtained for a range of sugar geometries of deoxyribose ring in nucleosides and nucleotides encountered along the pseudorotation itinerary [J. van Wijk, B.D. Huckriede, J.H. Ippel and C. Altona, Methods Enzymol. 211, 286–306 (1992)], it is suggested that the vicinal 3J1′2′ possesses structural information other than the vicinal torsion angle φ1′2′. This study is divided into two parts. In Part I, a correlation diagram between the endocyclic torsion angles vi (i=0,1,2,3,4) and the restrained vicinal torsion angle φ1′2′ is obtained through the use of the J-coupling restrained molecular mechanics (JrMM) protocol. The established φ1′2′.-vi correlation shows vi can be deduced from the reliable 3J1′2′. value and it forms the basis for developing an alternative protocol to derive endocyclic torsion angle constraints. In Part II of this series, extensive testing demonstrating the validity of the JrMM protocol to derive Vi for defining the sugar geometry of solution DNA molecules is presented. 相似文献
9.
Glenn L. Millhauser Chris J. Stenland Kimberly A. Bolin Frank J. M. van de Ven 《Journal of biomolecular NMR》1996,7(4):331-334
Summary Alanine-rich peptides serve as models for exploring the factors that control helix structure in peptides and proteins. Scalar CH-NH couplings (3JHN) are an extremely useful measure of local helix content; however, the large alanine content in these peptides leads to significant signal overlap in the CH region of 1H 2D NMR spectra. Quantitative determination of all possible 3JHN values is, therefore, very challenging. Szyperski and co-workers [(1992) J. Magn. Reson., 99, 552–560] have recently developed a method for determining 3JHN from NOESY spectra. Because 3JHN may be determined from 2D peaks outside of the CH region, there is a much greater likelihood of identifying resolved resonances and measuring the associated coupling constants. It is demonstrated here that 3JHN can be obtained for every residue in the helical peptide Ac-(AAAAK)3A-NH2. The resulting 3JHN profile clearly identifies a helical structure in the middle of the peptide and further suggests that the respective helix termini unfold via distinct pathways.Abbreviations
3JHN
three-bond CH-NH scalar coupling constant
- NOE
nuclear Overhauser enhancement
- NOESY
two-dimensional nuclear Overhauser spectroscopy
- COSY
two-dimensional correlated spectroscopy
- DQF-COSY
two-dimensional double-quantum-filtered correlated spectroscopy
- TOCSY
two-dimensional total correlation spectroscopy
To whom correspondence should be addressed.Deceased March 5, 1996. 相似文献
10.
Theoretical calculations of the heteronuclear vicinal coupling constant 3J(13C′NCαH) in peptides have been carried out using the Dirac vector model. The results showed an angular dependence for this coupling constant, which can be expressed in the form 3J(13C′NCαH) = A cos2 θ + B cos θ + C, where A, B, and C are constants and θ is related to the torsional angle ? of the peptide backbone. The results of the present calculations are in very good agreement with those obtained using finite perturbation theory at the INDO level of approximation. 相似文献
11.
Hao Huang Hiroaki Ishida Hans J. Vogel 《Protein science : a publication of the Protein Society》2010,19(3):475-485
Soybean calmodulin isoform 4 (sCaM4) is a plant calcium‐binding protein, regulating cellular responses to the second messenger Ca2+. We have found that the metal ion free (apo‐) form of sCaM4 possesses a half unfolded structure, with the N‐terminal domain unfolded and the C‐terminal domain folded. This result was unexpected as the apo‐forms of both soybean calmodulin isoform 1 (sCaM1) and mammalian CaM (mCaM) are fully folded. Because of the fact that free Mg2+ ions are always present at high concentrations in cells (0.5–2 mM), we suggest that Mg2+ should be bound to sCaM4 in nonactivated cells. CD studies revealed that in the presence of Mg2+ the initially unfolded N‐terminal domain of sCaM4 folds into an α‐helix‐rich structure, similar to the Ca2+ form. We have used the NMR backbone residual dipolar coupling restraints 1DNH, 1DCαHα, and 1DC′Cα to determine the solution structure of the N‐terminal domain of Mg2+‐sCaM4 (Mg2+‐sCaM4‐NT). Compared with the known structure of Ca2+‐sCaM4, the structure of the Mg2+‐sCaM4‐NT does not fully open the hydrophobic pocket, which was further confirmed by the use of the fluorescent probe ANS. Tryptophan fluorescence experiments were used to study the interactions between Mg2+‐sCaM4 and CaM‐binding peptides derived from smooth muscle myosin light chain kinase and plant glutamate decarboxylase. These results suggest that Mg2+‐sCaM4 does not bind to Ca2+‐CaM target peptides and therefore is functionally similar to apo‐mCaM. The Mg2+‐ and apo‐structures of the sCaM4‐NT provide unique insights into the structure and function of some plant calmodulins in resting cells. 相似文献
12.
We use the nmr data concerning the CαH? CβH fragment in eight peptides with rigid side chains to parametrize a Karplus correlation between the vicinal proton Jαβ coupling constant and the dihedral angle θ. When considering molecules containing the fragment CαHα? CβHβHβ′, the three-dimensional structure of the model peptides does not need to be known with accurate precision, since each set of Jαβ and Jαβ′ coupling constants is then related to the coefficients of the Karplus equation. A good correlation is observed with the Karplus equation, which is in substantial agreement with the Jαβ coupling constants reported for rigid as well as rotating Cα? Cβ bonds in peptides. 相似文献
13.
Wienk HL Martínez MM Yalloway GN Schmidt JM Pérez C Rüterjans H Löhr F 《Journal of biomolecular NMR》2003,25(2):133-145
A quantitative J-correlation pulse sequence is described that allows simultaneous determination of one-bond and two-bond nitrogen-carbon coupling constants for protonated or deuterated proteins. Coupling constants are calculated from volume ratios between cross peaks and reference axial peaks observed in a single 3D spectrum. Accurate backbone 1
J
NC, 1
J
NC, and 2
J
NC coupling constants are obtained for the two [15N;13C]-labeled, medium-sized proteins flavodoxin and xylanase and for the [2H;15N;13C]-labeled, large protein DFPase. A dependence of one-bond and two-bond J
NC values on protein backbone torsion angles is readily apparent, in agreement with previously found correlations. In addition, the experiment is performed on isotropic as well as aligned protein to measure associated 15N-13C residual dipolar couplings. 相似文献
14.
James M. Aramini Anwer Mujeeb Nikolai B. Ulyanov Markus W. Germann 《Journal of biomolecular NMR》2000,18(2):287-302
Nucleic acid duplexes featuring a single alpha-anomeric thymidine inserted into each DNA strand via 3′-3′ and 5′-5′ phosphodiester linkages exhibit local conformational dynamics that are not adequately depicted by conventional restrained molecular dynamics (rMD) methods. We have used molecular dynamics with time-averaged NMR restraints (MDtar) to explore its applicability to describing the conformational dynamics of two α-containing duplexes – d(GCGAAT-3′-3′-αT-5′-5′-CGC)2 and d(ATGG-3′-3′-αT-5′-5′-GCTC)?r(gagcaccau). In contrast to rMD, enforcing NOE-based distance restraints over a period of time in MDtar rather than instantaneously results in better agreement with the experimental NOE and J-data. This conclusion is based on the dramatic decreases in average distance and coupling constant violations (Δd av, J rms, and ΔJ av) and improvements in sixth-root R-factors (R x). In both duplexes, the deoxyribose ring puckering behavior predicted independently by pseudorotation analysis is portrayed remarkably well using this approach compared to rMD. This indicates that the local dynamic behavior is encoded within the NOE data, although this is not obvious from the local R x values. In both systems, the backbone torsion angles comprising the 3′-3′ linkage as well as the (high S-) sugars of the α-nucleotide and preceding residue (α?1) are relatively static, while the conformations of the 5′-5′ linkage and the sugar in the neighboring β-nucleotide (α+1) show enhanced flexibility. To reduce the large ensembles generated by MDtar to more manageable clusters we utilized the PDQPRO program. The resulting PDQPRO clusters (in both cases, 13 structures and associated probabilities extracted from a pool of 300 structures) adequately represent the structural and dynamic characteristics predicted by the experimental data. 相似文献
15.
Gérard Lancelot Alain Gervais Jean Claude Maurizot 《Journal of biomolecular structure & dynamics》2013,31(5):921-933
Abstract 15N uniformly labeled lac repressor and lac repressor headpiece were prepared. 15N NMR spectra of lac repressor were shown resolution inadequate for detailed study while the data showed that the 15N labeled N-terminal part of the protein is quite suitable for this type of study allowing future investigation of the specific interaction of the lac repressor headpiece with the lac operator. We report here the total assignment of proton 1H and nitrogen 15NH backbone resonances of this headpiece in the free state. Assignments of the 15N resonances of the protein were obtained in a sequential manner using heteronuclear multiple quantum coherence (HMQC), relayed HMQC nuclear Overhauser and relayed HMQC-HOHAHA spectroscopy. More than 80 per cent of residues were assigned by their 15NH(i)-N1H(i+1) and 15NH(i)-N1(i-1) connectivities. Values of the 3JNHα splitting for 39 of the 51 residues of the headpiece were extracted from HMQC and HMQC-J. The observed 15NH(i)-CβH cross peaks and the 3JNHα coupling constants values are in agreement with the three α-helices previously described [Zuiderweg, E.R.P., Scheek, R.M., Boelens, R., van Gunsteren, W.F. and Kaptein, R., Biochimie 67, 707 (1985)]. The 3JNHα coupling constants can be now used for a more confident determination of the lac repressor headpiece. From these values it is shown that the geometry of the ends of the second and third α-helices exhibit deviation from the canonical α-helix structure. On the basis of NOEs and 3JNHα values, the geometry of the turn of the helix-turn-helix motif is discussed. 相似文献
16.
Direct NMR observation and DFT calculations of a hydrogen bond at the active site of a 44 kDa enzyme
Eletsky A Heinz T Moreira O Kienhöfer A Hilvert D Pervushi K 《Journal of biomolecular NMR》2002,24(1):31-39
A hydrogen bond between the amide backbone of Arg7 and the remote imidazole side chain of His106 has been directly observed by improved TROSY-NMR techniques in the 44 kDa trimeric enzyme chorismate mutase from Bacillus subtilis. The presence of this hydrogen bond in the free enzyme and its complexes with a transition state analog and the reaction product was demonstrated by measurement of 15N-15N and 1H-15N trans-hydrogen bond scalar couplings, 2h
J
NN and 1h
J
HN, and by transfer of nuclear polarization across the hydrogen bond. The conformational dependences of these coupling constants were analyzed using sum-over-states density functional perturbation theory (SOS-DFPT). The observed hydrogen bond might stabilize the scaffold at the active site of BsCM. Because the Arg7-His106 hydrogen bond has not been observed in any of the high resolution crystal structures of BsCM, the measured coupling constants provide unique information about the enzyme and its complexes that should prove useful for structural refinement of atomic models. 相似文献
17.
Novel α/β-half-filter elements are proposed for the separation of the high-field and low-field component of 1 J HC and 1 J HN splittings into different subspectra. The α/β-half-filter elements are of the same duration as the S3CT pulse sequence element and, like this, are less sensitive to cross talk between different subspectra than the original shorter α/β-half-filters. The filter elements are demonstrated with the measurement of 1 J HC coupling constants of CαH groups in 2D and 3D experiments and the subspectral editing of the four different multiplet components observed in two-dimensional α/β-HSQC-α/β spectra recorded without heteronuclear decoupling in either dimension. 相似文献
18.
Calcineurin (CaN, PP2B, PPP3), a heterodimeric Ca2+‐calmodulin‐dependent Ser/Thr phosphatase, regulates swimming in Paramecia, stress responses in yeast, and T‐cell activation and cardiac hypertrophy in humans. Calcium binding to CaNB (the regulatory subunit) triggers conformational change in CaNA (the catalytic subunit). Two isoforms of CaNA (α, β) are both abundant in brain and heart and activated by calcium‐saturated calmodulin (CaM). The individual contribution of each domain of CaM to regulation of calcineurin is not known. Hydrodynamic analyses of (Ca2+)4‐CaM1–148 bound to βCaNp, a peptide representing its CaM‐binding domain, indicated a 1:1 stoichiometry. βCaNp binding to CaM increased the affinity of calcium for the N‐ and C‐domains equally, thus preserving intrinsic domain differences, and the preference of calcium for sites III and IV. The equilibrium constants for individual calcium‐saturated CaM domains dissociating from βCaNp were ~1 μM. A limiting Kd ≤ 1 nM was measured directly for full‐length CaM, while thermodynamic linkage analysis indicated that it was approximately 1 pM. βCaNp binding to 15N‐(Ca2+)4‐CaM1–148 monitored by 15N/1HN HSQC NMR showed that association perturbed the N‐domain of CaM more than its C‐domain. NMR resonance assignments of CaM and βCaNp, and interpretation of intermolecular NOEs observed in the 13C‐edited and 12C‐14N‐filtered 3D NOESY spectrum indicated anti‐parallel binding. The sole aromatic residue (Phe) located near the βCaNp C‐terminus was in close contact with several residues of the N‐domain of CaM outside the hydrophobic cleft. These structural and thermodynamic properties would permit the domains of CaM to have distinct physiological roles in regulating activation of βCaN. Proteins 2011. © 2010 Wiley‐Liss, Inc. 相似文献
19.
The observation of 2h
J
iso(N, N) coupling has prompted considerable interest in this phenomenon from experimentalists and theoreticians due to the potential these couplings hold for the determination of secondary and tertiary structure in biologically important molecules. Here, we present an ab initio (MCSCF) study of the complete 2h
J(N, N) tensor for a model methyleneimine dimer system as a function of (i) the N-N separation, r
NN, and (ii) the hydrogen bond angle, . This simple system models the 2h
J(N, N) tensor of nucleic acid base pairs. Results indicate that although the Fermi-contact mechanism dominates 2h
J
iso(N, N), the coupling tensor is anisotropic due to contributions from the Fermi-contact spin-dipolar cross term. The variation in 2h
J
iso(N, N) as a function of r
NN is fit to an exponential decay. The influence of on the coupling constant is less pronounced but must be considered if experimental coupling constants are to be used for quantitative structure determination. Our results for this simple model system demonstrate that 2h
J
iso(N, N) is a valuable probe of hydrogen bonding in nucleic acid base pairs. 相似文献
20.
Kees-Jan Van Groenigen Johan Six David Harris‡ Herbert Blum§ Chris Van Kessel 《Global Change Biology》2003,9(12):1751-1762
Reduced soil N availability under elevated CO2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N2‐fixing system. We studied the effects of Trifolium repens (an N2‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO2 under FACE conditions. 15N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha?1 yr?1 and the CO2 concentration was increased to 60 Pa pCO2 using 13C‐depleted CO2. The total soil C content was unaffected by elevated CO2, species and rate of 15N fertilization. However, under elevated CO2, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (fN) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO2, had a significant effect on fN values of the total soil N pool. The fractions of newly sequestered C (fC) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO2, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more 15N fertilizer than T. repens: 179 vs. 101 kg N ha?1 for the low rate of N fertilization and 393 vs. 319 kg N ha?1 for the high N‐fertilization rate. As the loss of fertilizer‐15N contributed to the 15N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N2 fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N2‐fixing L. perenne system. This suggests that N2 fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system. 相似文献