Accurate measurement of longitudinal cross-relaxation rates in nuclear magnetic resonance (original) (raw)
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The Journal of Physical Chemistry B, 2016
Transverse relaxation rate measurements in MAS solid-state NMR provide information about molecular motions occurring on nanoseconds-to-milliseconds (ns-ms) time scales. The measurement of heteronuclear (13 C , 15 N) relaxation rate constants in the presence of a spin-lock radio-frequency field (R 1ρ relaxation) provides access to such motions, and an increasing number of studies involving R 1ρ relaxation in proteins has been reported. However, two factors that influence the observed relaxation rate constants have so far been neglected, namely (i) the role of CSA/dipolar cross-correlated relaxation (CCR), and (ii) the impact of fast proton spin flips (i.e. proton spin diffusion and relaxation). We show that CSA/D CCR in R 1ρ experiments is measurable, and that this cross-correlated relaxation rate constant depends on ns-ms motions, and can thus itself provide insight into dynamics. We find that proton spin-diffusion attenuates this cross-correlated relaxation, due to its decoupling effect on the doublet components. For measurements of dynamics, the use of R 1ρ rate constants has practical advantages over the use of CCR rates, and the present manuscript reveals factors that have so far been disregarded and which are important for accurate measurements and interpretation.
An NMR Experiment for the Accurate Measurement of Heteronuclear Spin-Lock Relaxation Rates
Journal of The American Chemical Society, 2002
Rotating-frame relaxation rates, R1F, are often measured in NMR studies of protein dynamics. We show here that large systematic errors can be introduced into measured values of heteronuclear R1F rates using schemes which are usually employed to suppress cross-correlation between dipole-dipole and CSA relaxation mechanisms. For example, in a scalar-coupled two-spin X-H spin system the use of 1 H WALTZ16 decoupling or 1 H pulses applied at regularly spaced intervals leads to a significant overestimation of heteronuclear R1F values. The problem is studied experimentally and theoretically for 15 N-1 H and 13 C-1 H spin pairs, and simple schemes are described which eliminate the artifacts. The approaches suggested are essential replacements of existing methodology if accurate dynamics parameters are to be extracted from spin-lock relaxation data sets.
Journal of Magnetic Resonance, 2000
Chemical shift anisotropy (CSA) and dipolar CODEX (Cenralband Only Detection of EXchange) experiments enable abundant quantitative information on the reorientation of the CSA and dipolar tensors to be obtained on millisecond-second timescales. At the same time, proper performance of the experiments and data analysis can often be a challenge since CODEX is prone to some interfering effects that may lead to incorrect interpretation of the experimental results. One of the most important such effects is RIDER (relaxation-induced dipolar exchange with recoupling). It appears due to the dipolar interaction of the observed X nuclei with some other nuclei, which causes an apparent decay in the mixing time dependence of the signal intensity reflecting not molecular motion, but spin flips of the adjacent nuclei. This may hamper obtaining correct values of the parameters of molecular mobility. In this contribution we consider in detail the reasons why the RIDER distortions remain even under decoupling conditions and propose measures to eliminate them. That is, we suggest (1) using an additional Z filter between the cross-polarization (CP) section and the CODEX recoupling blocks that suppresses the interfering anti-phase coherence responsible for the X-H RIDER and (2) recording only the cosine component of the CODEX signal since it is less prone to the RIDER distortions in comparison to the sine component. The experiments were conducted on rigid model substances as well as microcrystalline 2 H / 15 N-enriched proteins (GB1 and SH3) with a partial back-exchange of labile protons. Standard CSA and dipolar CODEX experiments reveal a fast-decaying component in the mixing time dependence of 15 N nuclei in proteins, which can be misinterpreted as a slow overall protein rocking motion. However, the RIDER-free experimental setup provides flat mixing time dependences, meaning that the studied proteins do not undergo global motions on the millisecond timescale.
NMR relaxation under spin-locking conditions
2010
I: NOESY-AFP English. We have developed a method that allows us to investigate proteinligand binding using cross-relaxation experiments in combination with adiabatic fast passge (AFP) pulses. Cross-relaxation is a widely used mechanism yielding valuable parameters for structural studies since it can be used to probe the surroundings of any NMR-active nucleus (predominantly protons) within a 5°A radius. AFP pulses distinguish themselves by their ease of implementation and general advantages, like negligible offset-dependence and a perfect inversion profile. The combination of cross-relaxation and AFP pulses allows us to gain information about the binding epitope including properties such as internal mobility or spin diffusion, which is a direct measure for the surrounding proton density. Epitope mapping provides relevant information for drug development and optimization. We were able to apply this method successfully to several protein-ligand systems (Q83-Vanillic acid, ADH-AMP, ADH-...
Protein Backbone Dynamics through 13 C‘− 13 C α Cross-Relaxation in NMR Spectroscopy
Journal of the American Chemical Society, 2006
Internal dynamics of proteins are usually characterized by the analysis of 15 N relaxation rates that reflect the motions of NH N vectors. It was suggested a decade ago that additional information on backbone motions can be obtained by measuring cross-relaxation rates associated with intra-residue C′C α vectors. Here we propose a new approach to such measurements, based on the observation of the transfer between two-spin orders and . This amounts to "anchoring" the and operators to the N z term from the amide of the next residue. In combination with symmetrical reconversion, this method greatly reduces various artifacts. The experiment is carried out on human ubiquitin at 284.1 K, where the correlation time is 7.1 ns. The motions of the C′C α vector appear more restricted than those of the NH N vector.
Transverse relaxation optimized triple-resonance NMR experiments for nucleic acids
Journal of Biomolecular Nmr, 2000
Triple resonance HCN and HCNCH experiments are reliable methods of establishing sugar-to-base connectivity in the NMR spectra of isotopicaly labeled oligonucleotides. However, with larger molecules the sensitivity of the experiments is drastically reduced due to relaxation processes. Since the polarization transfer between 13C and 15N nuclei relies on rather small heteronuclear coupling constants (11-12 Hz), the long evolution periods (up to 30-40 ms) in the pulse sequences cannot be avoided. Therefore any effort to enhance sensitivity has to concentrate on manipulating the spin system in such a way that the spin-spin relaxation rates would be minimized. In the present paper we analyze the efficiency of the two known approaches of relaxation rate control, namely the use of multiple-quantum coherence (MQ) and of the relaxation interference between chemical shift anisotropy and dipolar relaxation - TROSY. Both theoretical calculations and experimental results suggest that for the sugar moiety (H1'-C1'-N1/9) the MQ approach is clearly preferable. For the base moiety (H6/8-C6/8-N1/9), however, the TROSY shows results superior to the MQ suppression of the dipole-dipole relaxation at moderate magnetic fields (500 MHz) and the sensitivity improvement becomes dramatically more pronounced at very high fields (800 MHz). The pulse schemes of the triple-resonance HCN experiments with sensitivity optimized performance for unambiguous assignments of intra-residual sugar-to-base connectivities combining both approaches are presented.
Journal of Magnetic Resonance, 2005
The purpose of this communication is to describe a method for rapid and simultaneous determination of longitudinal (T1) and transversel (T2) relaxation times, based on a single continuous wave free precession (CWFP) experiment which employs RF pulses with a π/2 flip angle. We analyze several examples, involving nuclei such as 1H, 31P, and 19F, where good agreement with T1 and T2 measurements obtained by traditional methods is apparent. We also compare with the more time-consuming steady-state free precession (SSFP) method of Kronenbitter and Schwenk where several experiments are needed to determine the optimum flip angle. The role of an inhomogeneous magnetic field on the observed decays and its effect upon the accuracy of relaxation times obtained by these methods is examined by comparing numerical simulations with experimental data. Possible sources of error and conditions to minimize its effects are described.
Journal of biomolecular NMR, 1999
A method is presented that makes it possible to estimate both the orientation and the magnitude of the chemical shift anisotropy (CSA) tensor in molecules with a pair of spin 1/2 nuclei, typically (13)C-(1)H or (15) N-(1)H. The method relies on the fact that the longitudinal cross-correlation rate as well as a linear combination of the autorelaxation rates of longitudinal heterospin magnetization, longitudinal two-spin order and longitudinal proton magnetization are proportional to the spectral density at the Larmor frequency of the heterospin. Therefore the ratio between the cross-correlation rate and the above linear combination is independent of the dynamics. From the field dependence of the ratio both the magnitude and the orientation of the CSA tensor can be estimated. The method is applicable to molecules in all motional regimes and is not limited to molecules in extreme narrowing or slow tumbling, nor is it sensitive to chemical exchange broadening. It is tested on the 22 ami...