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Biomedical subjects

W E Palke

Publications and source records attributed to W E Palke.

6 recordsLinked to original sources
PubMed2000

Spin relaxation and chemical exchange in NMR simulations.

Theory for describing the density matrix of a spin system experiencing chemical exchange and relaxation during the steps of an NMR experiment is presented in a form suitable for computation. Features in the theory that arise from exchange are discussed in detail, and comparisons to the exchange-free situation are made. A general computer program to carry out simulations of NMR experiments is described, and several examples of its performance are presented.

Computer Simulation↗
PubMed1999

Pulsed field gradients in simulations of one- and two-dimensional NMR spectra.

A method for the inclusion of the effects of z-axis pulsed field gradients in computer simulations of an arbitrary pulsed NMR experiment with spin (1/2) nuclei is described. Recognizing that the phase acquired by a coherence following the application of a z-axis pulsed field gradient bears a fixed relation to its order and the spatial position of the spins in the sample tube, the sample is regarded as a collection of volume elements, each phase-encoded by a characteristic, spatially dependent precession frequency. The evolution of the sample's density matrix is thus obtained by computing the evolution of the density matrix for each volume element. Following the last gradient pulse, these density matrices are combined to form a composite density matrix which evolves through the rest of the experiment to yield the observable signal. This approach is implemented in a program which includes capabilities for rigorous inclusion of spin relaxation by dipole-dipole, chemical shift anisotropy, and random field mechanisms, plus the effects of arbitrary RF fields. Mathematical procedures for accelerating these calculations are described. The approach is illustrated by simulations of representative one- and two-dimensional NMR experiments.

Anisotropy↗
PubMed1998

Symmetry and phase-selected NMR spectra of liquid crystalline samples.

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin-12 nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180 degrees. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.

Magnetic Resonance Spectroscopy↗
PubMed1996

Cross-correlation effects on NMR lineshapes and peptide conformation.

Information about molecular structure and dynamics can potentially be obtained by studying dipole-dipole and chemical-shift anisotropy (CSA) auto-correlation and dipole-CSA cross-correlation effects in high-resolution NMR spectra. Equations for the lineshapes of the HN multiplet in the fragment- 15NH-CH- as a function of NH-CH dihedral angle are derived by including these effects within the framework of the Redfield treatment of relaxation. To test the utility of the theoretical results, 1H[15N] HSQC proton lineshape data for a variant of the enzyme staphylococcal nuclease in which all valine residues are labeled with 15N have been analyzed to obtain the conformational angle (phi) between the N-H and adjacent C-H bonds. The results are generally in good agreement with values of phi obtained from crystal structure data. Considerations in the further development of the analysis of the lineshape of the HN multiplet for experimental determinations of phi are discussed.

Algorithms↗
PubMed1980

Electron density functions for simple molecules.

Trial electron density functions have some conceptual and computational advantages over wave functions. The properties of some simple density functions for H(+) (2) and H(2) are examined. It appears that for a diatomic molecule a good density function would be given by rho = N(A(2) + B(2)), in which A and B are short sums of s, p, d, etc. orbitals centered on each nucleus. Some examples are also given for electron densities that are appropriate for excited states.

Journal Article↗