Biomedical subjects

P K Madhu

Publications and source records attributed to P K Madhu.

13 recordsLinked to original sources

PubMed2006

Proton line narrowing in solid-state nuclear magnetic resonance: new insights from windowed phase-modulated Lee-Goldburg sequence.

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the (1)H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional (1)H-(1)H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.

Journal Article↗

PubMed2006

Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in static NMR of half-integer spin quadrupolar nuclei.

In solid-state NMR of quadrupolar nuclei with half-integer spin I, fast amplitude-modulated (FAM) pulse trains have been utilised to enhance the intensity of the central-transition signal, by transferring spin population from the satellite transitions. In this paper, the signal-enhancement performance of the recently introduced SW-FAM pulse train with swept modulation frequency [T. Bräuniger, K. Ramaswamy, P.K. Madhu, Enhancement of the central-transition signal in static and magic-angle-spinning NMR of quadrupolar nuclei by frequency-swept fast amplitude-modulated pulses, Chem. Phys. Lett. 383 (2004) 403-410] is explored in more detail for static spectra. It is shown that by sweeping the modulation frequencies linearly over the pulse pairs (SW1/tau-FAM), the shape of the frequency distribution is improved in comparison to the original pulse scheme (SWtau-FAM). For static spectra of 27Al (I=5/2), better signal-enhancement performance is found for the SW1/tau-FAM sequence, as demonstrated both by experiments and numerical simulations.

Journal Article↗

PubMed2004

Improvement of homonuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance utilising radiofrequency imperfections.

The often annoying imperfections in the phases and pulses of typical radiofrequency multiple-pulse irradiation schemes for homonuclear dipolar decoupling are revisited and analysed here. The analysis is with respect to one such multiple-pulse sequence, namely, the windowed phase-modulated Lee-Goldburg sequence. The error terms in the Hamiltonian due to pulse imperfections may lead to effective rotation of the spins around the z-axis giving rise to image free and high-resolution 1H spectra. Certain precautions to be taken with regard to scale factor estimation are also detailed. The analysis also points out the range of off-set values where the best homonuclear dipolar decoupling performance of a particular pulse scheme may be obtained.

Algorithms↗

PubMed2003

Efficient 5QMAS NMR of spin-5/2 nuclei: use of fast amplitude-modulated radio-frequency pulses and cogwheel phase cycling.

We report here an efficient multiple-quantum magic-angle spinning (MQMAS) pulse sequence involving fast amplitude-modulated (FAM) radio-frequency pulses for excitation and conversion of five-quantum (5Q) coherences of spin-5/2 nuclei. The use of a FAM-I type pulse train for the conversion of 5Q into 1Q coherences proves to be easier to implement experimentally than the earlier suggested use of a FAM-II type sequence [J. Magn. Reson. 154 (2002) 280], while delivering at least equal signal enhancement. Results of numerical simulations and experimental 27Al 5QMAS spectra of aluminium acetylacetonate for different excitation and conversion schemes are compared to substantiate these claims. We also demonstrate the feasibility of acquiring 5QMAS spectra of spin-5/2 systems using cogwheel phase cycling [J. Magn. Reson. 155 (2002) 300] to select the desired coherence pathways. A cogwheel phase cycle of only 57 steps is shown to be as effective as the minimum conventional nested 77-step phase cycle.

Journal Article↗

PubMed2003

Application of amplitude-modulated radiofrequency fields to the magic-angle spinning NMR of spin-7/2 nuclei.

We report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin-72 systems. Sensitivity enhancement is obtained with the use of fast amplitude-modulated (FAM) radiofrequency pulses. In one-dimensional magic-angle spinning experiments, signal enhancement of 3 is obtained by a FAM pulse followed by a soft 90 degrees pulse. In two-dimensional multiple-quantum magic-angle spinning experiments, FAM pulses are used for both the excitation of multiple-quantum coherences and for their conversion into observable single-quantum coherences. The observed signal enhancements are 2.2 in 3Q experiments, 3.1 in 5Q experiments, and 4.1 in 7Q experiments, compared to the conventional two-pulse scheme. The pulse schemes are demonstrated on the 45Sc NMR of Sc2(SO4)3 x 5H2O and the 139La NMR of LaAlO3. We also demonstrate the generation of FAM pulses by double-frequency irradiation.

Journal Article↗

PubMed2002

Cross-correlation effects involving curie spin relaxation in methyl groups.

Cross-correlation effects arising in methyl protons due to the simultaneous presence of dipole-dipole, chemical shift anisotropy, and Curie spin relaxation mechanisms in paramagnetic systems are analyzed. We assess the potential of obtaining structural constraints from the cross-correlation of Curie spin relaxation with dipolar relaxation mechanisms among methyl proton spins. By theoretical analysis and numerical simulations we characterize the transfer functions describing the interconversion processes of different ranks of multispin order. The time dependence of these processes contains a new type of structural information, the orientation of the methyl C(3)-axis with respect to the electron center. Experimental confirmation is found for selected methyl groups in low spin Fe(3+) sperm whale myoglobin.

Animals↗

PubMed2002

Signal enhancement in the triple-quantum magic-angle spinning NMR of spins-3/2 in solids: the FAM-RIACT-FAM sequence.

We achieve a significant signal enhancement for the triple-quantum magic-angle spinning NMR of a spin-3/2 system, by using an amplitude-modulated radiofrequency field, followed by a selective 90 degrees pulse and a phase-shifted strong rf field, for the triple-quantum excitation, and an amplitude-modulated radiofrequency field for the conversion of triple-quantum coherence to observable single-quantum coherence. The experiment is demonstrated on the (87)Rb NMR of polycrystalline rubidium nitrate.

Journal Article↗

PubMed2002

Cogwheel phase cycling.

A new method for constructing phase cycles is described. The new schemes apply to experiments involving several consecutive coherence transfer steps. The radiofrequency phases of two or more irradiation blocks are incremented simultaneously, as opposed to the traditional "nested" scheme, in which the block phases are incremented independently. In many cases, the "cogwheel" phase cycles achieve the same selectivity as traditional phase cycles, using fewer steps. Significant time savings are achievable in a wide range of NMR experiments.

Magnetic Resonance Spectroscopy↗

PubMed2001

Geometry dependent two-dimensional heteronuclear multiplet effects in paramagnetic proteins.

We report experimental observation and numerical simulation of a two-dimensional multiplet effect in the heteronuclear correlation spectrum of a paramagnetic protein that depends on molecular geometry. This effect arises as a consequence of cross-correlated relaxation involving the Curie spin relaxation and internuclear dipolar relaxation mechanisms. It also manifests itself in resolution and sensitivity improvement in transverse relaxation optimised spectroscopy (TROSY) kind of experiments. Characteristic multiplet patterns in heteronuclear coupled two-dimensional NMR spectra encode directional information for the heteronuclear bond with respect to the paramagnetic center. These patterns, which are simulated here using Redfield's relaxation theory, can be used to obtain a new type of geometry restriction for structure determination and refinement of paramagnetic macromolecular systems.

Animals↗

PubMed2000

The influence of the radiofrequency excitation and conversion pulses on the lineshapes and intensities of the triple-quantum MAS NMR spectra of I = 3/2 nuclei.

A rigorous examination of the various multiple-quantum magic angle spinning sequences is carried out with reference to sensitivity enhancement in the isotropic dimension and the lineshapes of the corresponding MAS peaks in the anisotropic dimension. An echo efficiency parameter is defined here, which is shown to be an indicator of the performance aspects of the various sequences. This can be used in the design of further new experiments in this field. A consequence of such a systematic analysis has been the combination of a spin-lock pulse for excitation of multiple-quantum coherences and an amplitude-modulated pulse for their conversion to observable single-quantum coherences. This approach has resulted in an improved performance over other sequences with respect to both the anisotropic lineshapes and the isotropic intensities.

Anisotropy↗

PubMed1999

Native-state hydrogen-exchange studies of a fragment complex can provide structural information about the isolated fragments.

Ordered protein complexes are often formed from partially ordered fragments that are difficult to structurally characterize by conventional NMR and crystallographic techniques. We show that concentration-dependent hydrogen exchange studies of a fragment complex can provide structural information about the solution structures of the isolated fragments. This general methodology can be applied to any bimolecular or multimeric system. The experimental system used here consists of Ribonuclease S, a complex of two fragments of Ribonuclease A. Ribonuclease S and Ribonuclease A have identical three-dimensional structures but exhibit significant differences in their dynamics and stability. We show that the apparent large dynamic differences between Ribonuclease A and Ribonuclease S are caused by small amounts of free fragments in equilibrium with the folded complex, and that amide exchange rates in Ribonuclease S can be used to determine corresponding rates in the isolated fragments. The studies suggest that folded RNase A and the RNase S complex exhibit very similar dynamic behavior. Thus cleavage of a protein chain at a single site need not be accompanied by a large increase in flexibility of the complex relative to that of the uncleaved protein.

Amino Acid Sequence↗

PubMed1997

Analysis of spin diffusion and cross correlation on the net nuclear Overhauser effect in NMR.

The effect of dipole-dipole cross correlations on the net nuclear Overhauser effect (NOE) has been analyzed here for realistic systems by extending the three-spin calculations to four and five spins in order to account for additional cross correlations and spin diffusion. These have been compared with the addition of leakage terms to the three-spin system. The additional spins enhance cross-correlation effects on one hand but on the other act as supplementary relaxation pathways for the magnetization to diffuse. This analysis shows that for a linear array of spins in the long-correlation limit, dipole-dipole cross correlations increase net NOE, while spin diffusion decreases it, and that the cumulative effect is a reduced effect of cross correlations. In other geometries and correlation limits, the effect of cross correlations on net NOE is generally small.

Magnetic Resonance Spectroscopy↗

PubMedDate not supplied

Application of fast amplitude-modulated pulse trains for signal enhancement in static and magic-angle-spinning 47,49Ti-NMR spectra.

It is demonstrated that the use of fast amplitude-modulated RF pulse trains with constant (FAM-I) and incremented pulse durations (SW-FAM) leads to considerable sensitivity enhancement for the central-transition signal (via spin population transfer from the satellite transitions) for solid-state NMR spectra of titanium, 47Ti (I = [Formula: see text] and 49Ti (I = [Formula: see text]. For the magic-angle spinning spectra of TiO2 and BaTiO3, the intensity of the 49Ti central-transition line was more than doubled compared to simple Hahn-echo acquisition, while for the static case, enhancement factors of 1.6 (TiO2) and 1.8 (BaTiO3) were obtained. No lineshape distortions are observed in either MAS or static spectra of both compounds. Employment of the FAM and SW-FAM sequences should be useful in the routine acquisition of 47,49Ti spectra, as the NMR signal can be detected much faster.

Journal Article↗