Showing papers on "Nuclear quadrupole resonance published in 1981"

Rotational jumps of the tyrosine side chain in crystalline enkephalin. Hydrogen-2 NMR line shapes for aromatic ring motions in solids

Abstract: Deuterium NMR spectra of polycrystalline (tyrosine-3,5-*H2) (Leus)enkephalin show that the aromatic tyrosyl ring of this pentapeptide is executing 180' flips about the Cfl-C' axis in the solid state. Specifically, the axially symmetric powder pattern observed at low temperature collapses to an axially asymmetric pattern with TJ 0.6 at high temperature. Computer simulations of the NMR line shapes, which account for spectral distortions induced by the quadrupole echo technique, indicate that at room temperature the flipping rate is approximately 5 X IO4 s-* and that it increases to about lo6 s-I at 101 "C. In the past few years it has become apparent that solid-state NMR spectra can be employed to investigate the rate and mechanism of molecular reorientation processes in solids, and there are now several examples in the literature which demonstrate this point."" To date these studies have focussed on small crystalline molecules, but the techniques are equally applicable to large molecules such as peptides and proteins, where they can be used to obtain information on phenomena such as the motion of amino acid side chains. As part of a study of side-chain reorientation in peptides and proteins by solid-state NMR, we report here an investigation of the motion of the aromatic tyrosine ring in polycrystalline (Le~~jenkephalin. Specifically, the 2H NMR powder line shapes of ( tyrosine-3 ,S2H2) ( LeuS) enkephalin show that the tyrosyl group is not rigid, but instead is executing jumps of 180' about the Co-Cc axis of the aromatic ring. At room temperature the jump rate is approximately lo4 s-l and inceases to about IO6 s-' at 101 OC. (Le~~lenkephalin is a natural pentapeptide with the sequence Tyr-Gly-Gly-Phe-Leu which has been chemically labeled with (3,5-ZH2)tyrosine for this study as is depicted in Figure 1. The peptide is of considerable interest because of its structural similarity to opiate agonists. Theoretical and experimental evidence from solution measurementsl2 reveal that enkephalin may exist in a relatively rigid conformation in which the phenyl and tyrosyl rings are separated by a distance similar to that of opiates, so that these groups may participate in binding to the receptor. (Leus)enke- phalin has been crystallized, and its structure determined;l3-l5 the X-ray studies have suggested that the orientation of the tyrosyl side chain is not unique, the ring experiencing disorder of either a static or dynamic nature. Because of the possible existence of dynamic disorder, and because NMR can detect this type of disorder and reveal the details of its nature, a solid-state NMR investigation of (Leus)enkephalin appeared promising. Experimental Section ~~-(3,5-2H,)Tyrosine was purchased from Merck, Sharpe, and Dohme (St. Louis, Mo.). The DL form was resolved by treatment of the N-trifluoroacetyl derivative with carboxypeptidase A to yield the L isomer of the free amino acid. The latter was used to synthesize (Leus)enke- phalin by the procedure reported previously.12 The 2H labeled peptide was recrystallized from ethanol. Samples for the NMR experiments typically consisted of approxi- mately 50 mg of the 2H-labeled peptide and were placed in vacuum- sealed glass tubes. The ZH NMR spectra were obtained on a home-built spectrometer operating at 45.1 MHz for 2H, using a quadrupole echo pulse seq~encc.'~~'~ The r/2 pulse width was 2-2.5 ps and the T value in the echo experiment was 30 ps. In order to ensure that the line shapes we obtain are accurate, we have found it necessary to employ quadrature, rather than single sided, phase detection. With the latter method gross

A continuum theory for quadrupole relaxation of ions in solution

Abstract: A simple continuum theory for quadrupole relaxation rates of ions in polar solvents is presented. The predicted rates agree fairly well with experimental rates determined for various ions in several solvents. The surprising success of the relatively crude continuum model is discussed and the implications for molecular theories of quadrupole relaxation are indicated.

17O NMR–NQR study of molecular motion in the liquid, and α and β solid phases of CO

Abstract: A comprehensive NMR−NQR study of carbon monoxide enriched 24.5% in 17O (I = 5/2) is reported, which provides interesting information on the motions of the CO dipoles in the liquid and in the a and b solid phases. In the liquid phase the 17O NMR spin‐lattice relaxation due to the tumbling rotational motions of the molecular axis, combined with the experimental evaluation of the rigid electric field gradient (EFG) at the 17O site has been utilized to achieve a direct and precise measurement of the reorientational correlation time of the dipole and its temperature dependence. In the b phase, the quadrupole perturbed NMR spectra and the spin‐lattice relaxation indicate the occurrence of a precessional motion of the dipole around the c axis. An effective EFG is consequently present, whose order of magnitude is about 103 smaller than the static one. The precessional correlation time is derived, which is surprisingly very close to the reorientational correlation time of the liquid phase, suggesting a quasifree rotation character for the precession. 17O NQR signals in the a phase have been detected and resonance frequencies and linewidths have been measured. From the temperature behavior of the NQR frequency, information on the local order parameter for the librational motion of the dipoles is obtained. The linewidth exhibits a peculiar temperature dependence, with a maximum around 37 K and a dramatic divergence for T→29 K. While an orientational static disorder could be expected in view of the residual entropy, it is shown how the linewidth, as well as some pure quadrupole relaxation measurements, seem to indicate the occurrence in the a phase of a dynamical disorder, with sudden head‐to‐tail reorientations of the dipole, at least for T≳29 K. In the framework of such a dynamical model, a theoretical picture for the motional contribution to the NQR allows one to obtain insights on the features of the orientational disordering processes. The hypothesis of a phase transition of antiferroelectric type below T?37 cannot be ruled out. Finally, some evidence of softening of the libronic frequencies on approaching the a–b phase transition is obtained.

Deuteron quadrupole coupling in hydrogen bonded systems. IV. Deuteron quadrupole coupling in substituted phenols

Abstract: The pure nuclear quadrupole spectrum of deuterium at oxygen in chlorinated phenols has been observed by level crossing double resonance with the ring protons. Observed coupling constants, ranging from 200 to 250 kHz, exhibit the effects of strong hydrogen bonding through their correlation with the intermolecular O–H⋅⋅⋅O bond lengths in those cases where structure data are available. This is interpreted below to indicate that a primary effect of chlorine substitution is the influence it exerts on intermolecular hydrogen bonding through modification of the barrier to hydroxyl group rotation. From an extrapolation of the observed distance dependence, one may obtain a coupling constant of ∼283 kHz for deuterium at the oxygen position in non‐hydrogen‐bonded phenol. An interpretation of these data is made using model calculations on phenol, vinyl alcohol, and chlorovinyl alcohol. Barriers to internal rotation, as well as the dependence of the deuterium quadrupole coupling constant on the orientation of the –OD bond with respect to the ring plane are reported using a standard STO‐4/31G basis. A value of 299 kHz is obtained for the deuterium coupling constant in the gas phase by interpolation from STO‐4/31G calculations and gas phase data for water, methanol, and formic acid. Calculation demonstrates that the coupling constant of deuterium is essentially independent of –OD twist angle in phenol, and that vinyl alcohol is an excellent model fragment for phenol insofar as deuterium field gradients are concerned. Chlorovinyl alcohol exhibits a substantially larger barrier to internal –OD rotation than phenol, and also shows a weak dependence of field gradient on twist angle. Since solid phenol and substituted phenols show substantial deviations of the aromatic ring and –OD group from coplanarity, one may rationalize trends in the coupling constants on the basis of a modification of the barrier to internal hydroxyl group rotation by chlorine substitution. Attempts were made to obtain spectra in methyl‐ and bromo‐substituted phenols, but with the exception of 2,4‐dimethylphenol, low field relaxation times were too short to permit their observation.

Structure of the incommensurate phase of Rb 2 Zn Cl 4 as determined by Cl 35 nuclear quadrupole resonance

Abstract: $^{35}\mathrm{Cl}$ nuclear quadrupole resonance "lines" in the incommensurate phase of ${\mathrm{Rb}}_{2}$Zn${\mathrm{Cl}}_{4}$ were assigned as singularities in the quasicontinuous spectrum due to the frozen-in displacements. The plane-wave model describes suitably the microscopic structure down to ${T}_{c}+10$ K, when narrow solitons become dominant.

Determination of the 14N electric field gradient tensor in the nitro group: A single crystal nuclear double resonance study of p‐nitrotoluene

Abstract: The 14N electric field gradient tensor in single crystalline p‐nitrotoluene has been determined at room temperature via proton–nitrogen double resonance in the rotating frame. The major principal (Z) axis of the tensor lies along the C–N bond axis. The quadrupole coupling constant (1340 kHz) and asymmetry parameter (0.408) determined from analysis of the rotation patterns are in excellent agreement with the values measured in a zero field double resonance experiment on powdered p‐nitrotoluene.

14N nuclear quadrupole resonance in nicotinic acid derivatives

Abstract: Nitrogen-14 nuclear quadrupole resonance (NQR) spectra of nicotinic acid derivatives, known as antitumor agents, are reported and analyzed in the framework of the Townes and Dailey theory. A strong correlation exists between the charge difference σNC − π at the pyridinic nitrogen and the inhibition of acetylcholinesterase by nicotinic acid derivatives, in which the inhibition potency (pI50) increases as the charge difference σNC − π increases. There are significant differences between 14N NQR spectra for the pyridinic nitrogen sites of nicotinic acid and nicotinamide; these can be explained in terms of the differences in hydrogen bond lengths.

Correlation of nitrogen‐14 nuclear quadrupole resonance data in sulfanilamides with the in vitro activities

Abstract: Several sulfanilamide-type compounds in their solid state have been investigated by the 14N nuclear quadrupole resonance (NQR) technique. The electron distributions at the sites of the nitrogen atoms, calculated by the Townes and Dailey theory, indicate a significant correlation between the quadrupole coupling constant for the primary amino nitrogen (N4), which is a measure of the (π-σ) electron density at N4, and the in vitro bacteriostatic activity of the sulfanilamides.

14N nuclear quadrupole resonance in N-acetyl amino acids†

Abstract: 14N Nuclear quadrupole resonance (NQR) spectra of several N-acetyl amino acids and related compounds are reported and analyzed within the framework of the Townes and Dailey theory. The inductive effect of the chloroacetyl group on the nitrogen is discussed. A positive correlation between the π-σNC electron density at the nitrogen and the Taft inductive parameter σ* is observed, suggesting that the nitrogen π-charge density in the N-acetyl amino acids does not vary appreciably. NQR data for hippuric acid are compared to the 14N quadrupole parameters extracted from ENDOR data reported for X-irradiated hippuric acid.

Zeeman study of a boron trichloride single crystal by pulsed FT NQR spectroscopy

Abstract: A 35Cl Zeeman NQR study of a single crystal of boron trichloride yields the value of the field‐gradient asymmetry parameter of 0.557±0.011, in agreement with previous values of 0.54 and 0.50 obtained from studies of polycrystalline samples. Apart from the well known doublet due to an isotope effect, further fine structure whose details depend on the orientation of the Zeeman field is observed. It is shown that this fine structure arises from 11B–35Cl dipole–dipole interaction.

Pressure and temperature dependence of the nuclear quadrupole resonance of 75As in the ferroelectric phase of PbHAsO4

Abstract: The NQR frequency of 75As nuclei in PbHAsO4 was studied vs. temperature 77 to 255 K and hydrostatic pressure (up to 300 MPa). An increase in temperature led to a decrease in the NQR frequency. The latter decreased linearly with increasing hydrostatic pressure, and the absolute value of the pressure coefficient increased with increasing temperature. The results are not suitable for an interpretation based on an effect of lattice vibrations on the EFG, but express the proton dynamics in hydrogen bonds O-H…O between the neighboring AsO4 groups. The analysis of the results is based on the pseudospin model of Blinc.

14N nuclear quadrupole interaction in salicylaldoxime and in Cu(II) doped bis(salicylaldoxime) Ni(II)

Abstract: 14N NQR in pure salicylaldoxime is studied at 77 °K. ENDOR measurements on single crystals of CU(II) doped bis (salicylaldoxime) also provide quadrupole data. The two data are compared.

Nitrogen and deuterium nuclear quadrupole resonance in lanthanum nicotinate dihydrate

Abstract: Using nuclear double quadrupole resonance techniques the nuclear quadrupole resonance (NQR) spectra of the three inequivalent 14N sites and the four water 2D sites have been obtained at 77 K for lanthanum nicotinate dihydrate. The quadrupole coupling constants e2qQ/h in kHz and the anisotropy constant η are for nitrogen 4418 and 0.362, 4222 and 0.335, and 4220 and 0.327. For the deuterons they are 233.73 and 0.080, 177.47 and 0.153 for one water molecule and 219.73 and 0.098, 189.07 and 0.148 for the other. An attempt is made to extract structural data from these NQR results.