George C. Levy

The experimental approach to accurate carbon-13 spin-lattice relaxation measurements

Abstract The experimental requirements for pulse FT NMR measurements of spin-lattice relaxation times ( T 1 s) are quite demanding. This paper treats many of the critical experimental parameters in T, experiments; the data and discussions rely on previous literature as well as an extensive set of new measurements. It is shown that variables such as sample geometry and state, transmitter rf pulse power, computer and data processing characteristics, and instrumental gain stability and magnetic field homogeneity must all be optimized or closely controlled in order to obtain accurate data. The choice of a specific pulse sequence [inversion-recovery (IRFT), saturation-recovery (SRFT), progressive saturation (PSFT) or other variations] depends on a number of factors. The SRFT and PSFT sequences are faster and somewhat more convenient than the IRFT method. However, the pulsing requirements in the PSFT method are most stringent and large errors may easily be introduced in these T, measurements. The effect on determined T 1 s of mis-set pulse lengths and variable spectral offsets was studied for the three pulse sequences. The PSFT sequence can give substantially (25% or more) incorrect T 1 s while systematic errors in the IRFT and SRFT experiments are under 10–15% except under extreme conditions. The SRFT sequence is approximately twice as fast as the IRFT scheme for determination of T 1 s longer than a few seconds. Because of their relative insensitivity to instrumental and other experimental deficiencies the IRFT or SRFT pulse sequences will be preferred for T, measurements in the great majority of laboratories. Because homospoil pulses are utilized in the SRFT sequence, it is the preferred method only for measurement of T 1 s longer than about 5 seconds.

The application of maximum entropy processing to the deconvolution of coupling patterns in NMR

Abstract The deconvolution of J -coupling patterns in NMR by iterative maximum entropy processing is demonstrated. Both the in-phase and the antiphase coupling patterns are considered. The deconvolution of the coupling pattern, either for one value of the coupling constant (1D J deconvolution) or for a range of coupling constants (2D J deconvolution) is shown. It is demonstrated that the method can be used for improving the signal-to-noise ratio for known coupling patterns by removing the coupling structure, as well as for extracting coupling constants from an unknown spectrum. Examples are shown both in 1D NMR and in slicewise processing of 2D spectra.

Multivariate data analysis for pattern recognition in two-dimensional NMR

Abstract A new concept for spectral interpretation is presented, relying on multivariate analysis (MVA) of two-dimensional NMR spectra. A data representation of 2D NMR spectra suitable for MVA is introduced, which is shown to have several fortunate properties. Results show that MVA can be used to separate mixtures of spin systems, regardless of coupling approximation, and to classify them. Principal component analysis (PCA) for 2D NMR is described. Two-dimensional simulations in the weak coupling approximation, of the six different coupling topologies that a four-spin system can assume, are generated and PCA is used to reduce each entire spectrum to a simple pattern. These patterns are unique to the spin systems that give rise to them. On the basis of these general models, we correctly classify the aromatic spin system from measured spectra of L-tryptophan. PCA is also used to separate spin systems in an l -tryptophan/salicylic acid mixture. These methods are shown to be fairly insensitive to spectral quality and missing peaks. Extensions of this approach to automated spectral analysis of weakly and strongly coupled systems such as biopolymers are outlined.

High field carbon-13 NMR spectroscopy. Conformational mobility in gramicidin S and frequency dependence of 13C spin-lattice relaxation times

Summary The use of superconducting solenoids in 13C NMR can result in a substantial increase in spectral resolution over that obtained at commonly employed magnetic fields. Results are presented here for the cyclic decapeptide gramicidin S in both CD3OD and DMSO-d6 solutions. It was possible to monitor the complete motional behavior of the side chains of gramicidin S in CD3OD using 13C spin lattice relaxation times. Preliminary data are reported which confirm the frequency dependence of spin-lattice relaxation times for carbons in molecules not satisfying the extreme narrowing condition.

Paramagnetic relaxation reagents as a probe for translational motion of liquids

Use of the paramagnetic relaxation reagent tris‐ (acetylacetonato) chromium(III) [Cr(acac)3] proves capable of probing the solution structure of organic liquids. The theory describing the relaxation effects of this chelate is presented, wherein it is assumed that the nuclear spin–lattice relaxation mechanism is due to the modulation of the electron–nuclear dipole–dipole interaction. The electronic relaxation time Te1 (or τs) and the mutual translational motion of the paramagnetic species and the solvent molecules, in our case carbon tetrachloride, are considered the only causes of this modulation. Since for the paramagnetic molecules it is possible to calculate the electronic relaxation times, such systems are well defined for the description of translational diffusion. It is found that a jump diffusion model for translational motion explains the experimental data with jump distances comparable to the diameter of the solvent molecules.

Mechanisms for interactions between organic molecules and paramagnetic relaxation reagents

Measurements of 13C spin—lattice relaxation times for a number of representatives organic compounds in the presence of paramagnetic β-diketone derivatives of Cr, Mn, Fe, Ni, and Gd are reported and discussed in terms of several possible interaction mechanisms. Hydrogen bonding, metal—ligand coordination, and electrostatic effects modulated by contributing steric effects are indicated as causes for substrate—chelate complexation. When no specific complexation occurs, the observed relaxation results from electron-nuclear dipole—dipole interactions modulated by translational motion and also under some conditions, the electronic relaxation. Specific complexation or orientation of the substrate around the chelate leads to enhanced relaxation rates dependent on the degree of substrate—chelate interaction. This can be modified by other competitive interactions around the chelate or in the bulk of the solution. At practical doping concentrations the limiting relaxation rate is dependent only on the chelate concentration and the substrate-chelate complex equilibrium constant.

Conformation and Dynamics of Short DNA Duplexes: (dC-dG)3 and (dC-dG)4

Natural abundance 13C NMR spectra of duplexed (dC-dG)3 and (dC-dG)4 exhibit resolved resonances for most of the carbons at 0.1M NaCl in aqueous solution. Large transitions in chemical shift for many of the hexamer carbons (up to 1.8 ppm) are observed in variable temperature measurements. Determination of spin-lattice relaxation times and nuclear Overhauser enhancements in 0.1M NaCl indicate that the duplexes tumble almost isotropically, with overall correlation times near 5 nsec; the sugar carbons experience more rapid local motions than do the base carbons. The relaxation data are also consistent with the most rapid local motions occurring at the chain-terminal residues, especially in the Cyd(1) sugar. 4M NaCl causes changes in the 13C chemical shifts of most of the guanine base carbons, and rearrangements in the deoxyribose carbon shifts; this is consistent with changes predicted by a salt-induced B to Z transition, viz. conversion of the guanylates from the anti to syn range about the glycosyl bond, and from the S to N pseudorotational state of the deoxyribose ring.

Aqueous relaxation reagents in nitrogen-15 NMR

Abstract Electron-nuclear relaxation times (T12′s) for 15N and 13C in natural abundance are measured for a series of amines in aqueous solution using Gd(III) complexes of a series of polyaminocarboxylate ligands as paramagnetic relaxation reagents (PARRs). The PARRs are classified by their predominant mode of interaction with the amine substrates (i.e., specific or non specific). The specific PARRs are evaluated qualitatively as NMR spin labels through their selectivity as measured by 15N Te1′s toward subtrates of different Lewis base strength and through the degree of scalar line broadening induced in the substrate resonances. Additionally, the aqueous PARRs are compared with better characterized nonaqueous PARRs, Cr(acac)3 and Gd(dpm)3. It is concluded that the diethylenetriaminepentaacetic acid complex, Gd(DTPA)2−, is a satisfactory specific PARR and NMR spin label. The triethylenetetraaminehexaacetic acid complex, Gd(TTHA)3−, is shown to be a useful non specific PARR although it is a charged complex; the presence of charged groups in substrates results in weak spin label effects. Finally, the behaviour of these PARRs is contrasted with that of hydrated transition metal and lanthanide ions.

Generalized maximum entropy deconvolution of spectral segments

Abstract A maximum entropy Fourier spectral deconvolution (MEFSD) program capable of selecting segments of larger spectra is used to deconvolute overlapping peaks in experimental and synthetic test cases. Low signal-to-noise features in spectra which also have very large features (dynamic ranges 100:1, 1000:1, or more) can be selected for maximum entropy processing by performing calculations on a pseudo-FID constructed from the selected region. Results show efficient and powerful deconvolution with simultaneous noise suppression for spectral segments, even when relatively crowded. Quantitative tests of MEFSD on low signal-to-noise synthetic test spectra, modeled after experimental examples, showed accuracies which were at least as good, or better, in relative peak integrations, over conventional FFT processing followed by Lorentzian curve fitting, for segments from several different test cases with dynamic ranges of 1000:1 to 3:1.

Yttrium-89 NMR. A possible spin relaxation probe for studying metal ion interactions with organic ligands

Abstract The spin-lattice relaxation mechanisms for aqueous and dimethyl sulfoxide solutions of Y(NO3)3 have been found to be mainly spin-rotation and dipolar relaxation with solvent protons, unlike most heavy spin = 1 2 metal ions which are relaxed mainly by spin-rotation and chemical shift anisotropy. The theoretical maximum 89Y{1H} NOEF value of -10.2 was observed when τc for the ion was lengthened by lowering the temperature of the aqueous salt solution to 5°C, or when yttrium was complexed to an organic ligand. Since 89Y has a sensitivity of 0.67 relative to that of 13C, the relative importance of dipolar relaxation and the large theoretical maximum NOEF make 89Y a possible valuable NOE structure probe. Such studies can complement relaxation and shift data obtained from other diamagnetic and paramagnetic lanthanide ions.