Susumu Shibata

Deuterium substitution effect on relaxation times (DESERT) and its application to the study of conformation of some purine nucleoside derivatives

Abstract Substitution of a proton with a deuterium at a specific position in a molecule reduces spin-lattice relaxation rates of other protons in a manner that depends critically on the conformation taken by the molecule in solution. This phenomenon occurs as a result of dipolar interaction between protons in the molecule that is quenched when a proton is replaced by a deuterium. Differential relaxation rates of protons before and after specific deuterium substitution can be combined with the rotational correlation time of the molecule obtained from 13C spin-lattice relaxation measurements to give reciprocal sixth-power-averaged interproton distances. Range of interproton distances that can be measured by this method is limited by the reproducibility in T1 measurements as well as by the deviation of proton relaxation from a single time-exponential function in a regular FT experiment which utilizes the nonselective 180°-t-90° pulse sequence. The measurable region for the interproton distance is shown graphically in the general three-proton case by taking these factors into account. The interproton distance (13) from proton 3 (to be replaced by a deuterium) to proton 1 is measurable within ±10% error up to approximately 1.6 × r12 except in a region where proton 3 is close to proton 2. This method, termed “DESERT” (as an abbreviation of Deuterium Substitution Effect on Relaxation Times), is tested successfully with 2′,3′-iso-propylidene-3,5′-cycloguanosine whose conformation is nearly fixed. Then, the method is applied to the study of syn-anti conformational equilibrium in some adenosine derivatives, i.e., 2′,3′-isopropylidene adenosine (IPA), adenosine, and 5′-adenosine monophosphate (5′-AMP) in DMSO-d6 or D2O. Averaged interproton distances are determined from the base proton H(8) to the ribose protons, H(1′), H(2′), and in some cases H(3′), in these compounds. The distances obtained indicate that all these compounds exist in mixtures of conformational isomers with respect to the rotation about the glycosidic bond. The average conformation of 5′-AMP is found to be distinctively different from those of IPA and adenosine in that H(8) becomes far from H(1′) but close to H(2′) in 5′-AMP.

Proton spin relaxation in biopolymers at high magnetic fields

Abstract High magnetic fields realized by superconducting magnets not only improved resolution and sensitivity in the NMR spectroscopy dramatically, but also extended and modified the use of spin relaxation in studying the dynamics of a macromolecule. In particular, the mechanism of proton longitudinal relaxation in solution is strongly modified for a macromolecule at high magnetic field by the presence of cross relaxation and spin diffusion. This paper describes the basis for interpreting proton longitudinal relaxation of proteins in a solution subject to a variety of experimental conditions based on experimental data as well as computer generated relaxation curves for individual protons by solving simultaneous differential equations for a dipolar-coupled multi-proton system. The extent of spin diffusion is examined as a function of the molecular weight of the protein and of the magnetic field strength. Emphasis is placed on the relationship of spin diffusion with the macromolecular dynamics in solution. Finally, the practical aspect of spin diffusion in spectral analysis and macromolecular dynamics is presented.

General features of proton longitudinal relaxation in proteins in solution

Abstract Time courses of the recovery upon nonselective inversion of all individual proton magnetizations in several globular proteins in aqueous ( 2 H 2 O) solution were calculated for varying degrees of rotational correlation time of the molecule (10 −9 s ∼ ∞) and compared with the experimental data on various proteins at 400 MHz. In the calculation, the spinrelaxation mechanism was assumed to be solely the dipolar interaction between protons, and the three-site random jumps of the methyl groups, along with the rotation of the whole molecule, were taken into account. The following conclusions were drawn. ( 1 ) For proteins whose molecular weights are below ∼ 10,000, whole-molecule rotation is a dominant source of relaxation, and the longitudinal relaxation times may vary considerably from proton to proton. (2) For proteins whose molecular weights are above ∼20,000, methyl group rotations assisted by spin diffusion are common and major sources of relaxation, producing T 1 values close to 1 s. In the intermediate region (molecular weight 10,000 ∼ 20,000), both whole-molecule rotation and methyl group rotations contribute significantly to relaxation. (3) In some proteins, segmental motions are as important as methyl group rotations in determining relaxation rate.

Vortices in Bi2212 single crystal under magnetic fields tilted to c-axis

Abstract The vortex structure of a Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) single crystal was investigated using a vibrating reed technique. Below about 30 K, the reed touched to a electrode above the magnetic field B s , which was caused by the large magnetic torques originating from the intrinsic pinning and/or the flux pinning perpendicular to the CuO 2 plane. The value of B s depends on the angle between the applied magnetic field and the CuO 2 plane. The temperature dependences of B s were measured at the various conditions.

Simulation of proton spin-lattice relaxation times of yeast transfer RNAPhe

Abstract Spin-lattice relaxation times ( T 1 ) have been measured at 100 MHz with the pulse Fourier transform NMR method for nonexchangeable protons of yeast tRNA Phe in the neutral D 2 O solution as a function of temperature. The proton relaxation times were simulated by the use of X-ray-structure coordinates and the rotational correlation times deduced from 31 P magnetic relaxation measurements. For the ribose protons (2′-5′) the simulated relaxation times closely agree with the experimental ones, indicating that the molecular motion as viewed through the ribose protons is the same as that observed through phosphorus, and therefore that all sections of ribose-phosphate part are characterized by common correlation times. For the ribose 1′ and the base protons, experimentally obtained T 1 values show much less temperature dependence than those calculated, suggesting that there is a wider distribution in the rotational correlation times in the base part than in the ribose-phosphate part.

Montgomery Type Analysis for the Anisotropic Resistivity of Bi2Sr2CaCu2Ox Below and Above Tc

Multi-terminal measurements were performed for a Bi2Sr2CaCu2O single crystal, and the Montgomery type analysis was applied. It edgave the consistent results in the normal state, but did not below T c . However, the Montgomery type analysis seems to be applicable again at lower temperatures in low magnetic fields. One possible model of the nonlocal conductivity to explain these experimental facts is tested.

Nonlocal Electrodynamics in Bi2Sr2CaCu2O8+δ Single Crystal in a Corbino Disk Geometry

Electrical transport measurements in Bi2Sr2CaCu2O8+δ single crystal have been made using a Corbino disk geometry with current applied between center of disk and its perimeter and voltage response measured across two pairs of equally spaced potential contacts located along sample radius. In the normal state radial potential distribution is well discribed by a local conductivity model. Below Tc at 0.02T≤B≤0.2T contrary to 1/r current density distribution voltage response measured near the sample perimeter exceeds that taken closer to the center. This nonlocal behavior was observed above vortex melting transition indicating strong transverse vortex-vortex correlation in the vortex liquid state.