Nuclear spin magnetization transfer by doubly selective irradiation

A procedure for doubly selective irradiation of an AX NMR spin system so as to achieve complete magnetization transfer from X to A is described. In combination with suitably gated decoupling this sequence will give substantial savings of time in determinations of T₁ for low-sensitivity spin-½ nuclei, particularly those with negative magnetogyric ratios.     

Nuclear spin selective laser control of rotational and torsional dynamics

We explore the possibility of controlling rotational-torsional dynamics of non-rigid molecules with strong, non-resonant laser pulses and demonstrate that transient, laser-induced torsional alignment depends on the nuclear spin of the molecule. Consequently, nuclear spin isomers can be manipulated selectively by a sequence of time-delayed laser pulses. We show that two pulses with different polarization directions can induce either overall rotation or internal torsion, depending on the nuclear spin. Nuclear spin selective control of the angular momentum distribution may open new ways to separate and explore nuclear spin isomers of polyatomic molecules.     

Tunneling-induced spin alignment at low and zero field

The transfer of rotational to spin angular momentum of CH3 groups according to the Haupt effect is shown to be independent of magnetic field strength, including zero field. Haupt enhanced pulsed nuclear resonance signals of gamma-picoline have been observed at fields below 50 mT with a sensitivity enhancement of more than 3 orders of magnitude over thermally polarized experiments.     

Topology and spin alignment utilizing the excited molecular field in π-conjugated organic spin systems

The design and experimental investigations of photo-induced high-spin organic systems (the photo-excited quartet (S = 3/2) and quintet (S = 2) states) is reviewed with focusing π-conjugated organic spin systems. In order to study the photo-induced spin alignments on the excited states, the photo-excited high-spin states of π-conjugated stable radical systems and their π-topological isomers were studied by the time-resolved ESR experiments. The relationship between the π-topology and spin alignment on the photo-excited states is clarified. The mechanism of the photo-induced intramolecular spin alignment and the role of the spin polarization and spin delocalization are revealed with the help of the molecular orbital calculations. One of the key processes for the photo-control of the organic molecular magnetism is established. The guiding principle designing the photo-excited high-spin system and the role of π-topology are clarified. Potential developments toward the functional materials are also proposed utilizing the π-conjugated organic spin systems with the photo-excited high-spin states.     

Nuclear spin polarization in radical reactions

An extension of the Closs-Kaptein-Oosterhoff theory concerning nuclear spin polarization resulting during free radical reactions is presented. This extension is based on the Merrifield model for the magnetic field dependence of triplet-triplet annihilation.     

Nuclear spin catalysis in living nature

Experiments with cells enriched in stable magnesium isotopes, magnetic ²⁵Mg or nonmagnetic ²⁴Mg and ²⁶Mg, are carried out. It is revealed that adaptation of bacteria E. coli to the growth media enriched in magnetic ²⁵Mg proceeds faster as compared to the growth media enriched in nonmagnetic magnesium isotopes. In experiments with another commonly accepted cell model, S. cerevisiae yeast, it is revealed that the rate constant of postradiation recovery of the cells after UV irradiation is two times higher for cells enriched in ²⁵Mg than for cells enriched in the nonmagnetic isotope. In collaboration with Ukrainian colleagues from the Palladin Institute of Biochemistry, the effects of different isotopes of magnesium on ATPase activity of myosin isolated from myometrium are studied. It is found that the rate of the enzymatic hydrolysis of ATP for ²⁵Mg is 2.0–2.5 times higher as compared to nonmagnetic isotopes ²⁴Mg and ²⁶Mg. Some possible mechanisms of magnetic isotope effects (nuclear spin catalysis) in biological objects are discussed.     

Nuclear spin catalysis in biochemical physics

Magnetic isotope effects have been recently discovered in living nature. They were observed for the first time in experiments on cells enriched with various magnesium isotopes, magnetic ²⁵Mg or non-magnetic ones. A catalytic effect of the magnetic isotope of magnesium was discovered in experiments with myosin, the most important biomolecular motor utilizing the energy of ATP to perform mechanical work. The rate of ATP hydrolysis with the magnetic ²⁵Mg isotope is 2.0–2.5 times higher than that obtained with nonmagnetic ²⁴Mg or ²⁶Mg. A similar effect of the nuclear spin catalysis was experimentally observed for zinc isotopes. The rate of ATP hydrolysis in the case of magnetic ⁶⁷Zn increased by 40–50% as compared to that observed experimentally for nonmagnetic isotopes (⁶⁴Zn or ⁶⁸Zn). Catalytic effects of the magnetic isotope of magnesium were also experimentally found for H⁺-ATPase isolated from yeast mitochondria and ATPase of the plasma membrane of the myometrium. The magnetic isotope effect indicates unambiguously that the chemomechanical processes involve a limiting step catalyzed by biomolecular motors, which depends on the electronic spin state, and that this step is accelerated in the presence of nuclear spin of the magnetic isotope.

     

Development of erbium phosphate doped poly(glycidyl methacrylate/ethylene dimethacrylate) spin columns for selective enrichment of phosphopeptides

In this study, a novel method for the highly selective enrichment of phosphopeptides using erbium phosphate doped poly(glycidyl methacrylate/ethylene dimethacrylate) spin columns is presented. Erbium phosphate was synthesized by precipitation from boiling phosphoric acid and incubated overnight in erbium chloride solutions. The resulting powder was embedded in a monolithic poly(glycidyl methacrylate/ethylene dimethacrylate) polymer. The monolith was synthesized in a spin column by radical polymerization. Erbium phosphate demonstrated a high affinity and selectivity for phosphopeptides due to the strong interaction of trivalent erbium ions with the phosphate groups of phosphopeptides. The high selectivity and performance of the designed spin columns were demonstrated by successfully enriching phosphopeptides from tryptically digested protein mixtures containing the model phosphoproteins α‐ and β‐casein, bovine milk, and human saliva. By the implementation of several washing steps, unspecific components were removed and the enriched phosphopeptides were effectively eluted from the spin columns under alkaline conditions. The selective performance of the presented method was further demonstrated by the enrichment of two synthetic phosphopeptides, which were spiked in tryptically digested and dephosphorylated HeLa cell lysates at low ratios. Finally, the presented approach was compared to conventional phosphopeptide enrichment by titanium oxide and revealed higher recoveries for the erbium phosphate doped monoliths.     

Triradical cation of p-phenylenediamine having two nitroxide radical groups: spin alignment mediated by delocalized spin

The cationic state of a p-phenylenediamine (PDA) molecule having two nitroxide radical groups was prepared and characterized using electrochemical, electron spin resonance (ESR) spectroscopic, and absorption spectroscopic methods. The delocalized intervalence state of the p-phenylendiamine (PDA) moiety was detected in the cationic state. From the pulsed ESR measurements, it was confirmed that the delocalized spin induces parallel spin alignment between the localized two nitroxide groups which are magnetically weakly coupled in the neutral state. It was found that the resulting high-spin alignment does not seriously affect the delocalized intervalence state of the PDA radical cation.     

Nuclear spin state conservation in photodissociation of formaldehyde

Excitation of formaldehyde to the 2¹4¹ band of the S₁ state leads to photodissociation with nuclear spin state conservation of the hydrogen. Dissociation of ortho-formaldehyde gives ortho-hydrogen as a product. Interconversion between ortho- and para-formaldehyde at 1 Torr occurs with a rate constant greater than 0.1 min^?1.     

Nuclear spin–spin coupling and nuclear motion

The vibration and rotation of molecules affects nuclear spin–spin coupling constants. This manifests itself as a temperature dependence of the coupling and also as an isotope effect (after allowing, where necessary, for differing magnetogyric ratios of the two nuclei involved in the isotopic substitution). Within the Born–Oppenheimer approximation, a nuclear spin–spin coupling surface can be defined for each pair of coupled nuclei. This surface is sampled by the nuclei as they undergo the excursions about equilibrium geometry that are governed by the force field. An accurate ab initio carbon–proton spin–spin coupling surface for the methane molecule has been calculated. This was obtained by summing the surfaces for each of the four contributions—Fermi contact, spin–dipolar, orbital paramagnetic, and orbital diamagnetic—expressed as power series in terms of symmetry coordinates. Preliminary calculations for ¹³CH₄ and ¹³CD₄ give a difference of only 6% between the calculated and observed nuclear motion contributions. The observed temperature dependence is also accounted for by the calculations. For these isotopomers, bond stretching plays the dominant role. © 1994 John Wiley & Sons, Inc.     

Nuclear spin conversion of methane in solid parahydrogen

The nuclear spin conversion of CH(4) and CD(4) isolated in solid parahydrogen was investigated by high resolution Fourier transform infrared spectroscopy. From the analysis of the temporal changes of rovibrational absorption spectra, the nuclear spin conversion rates associated with the rotational relaxation from the J=1 state to the J=0 state for both species were determined at temperatures between 1 and 6 K. The conversion rate of CD(4) was found to be 2-100 times faster than that of CH(4) in this temperature range. The faster conversion in CD(4) is attributed to the quadrupole interaction of D atoms in CD(4), while the conversion in CH(4) takes place mainly through the nuclear spin-nuclear spin interaction. The conversion rates depend on crystal temperature strongly above 3.5 K for CH(4) and above 2 K for CD(4), while the rates were almost constant below these temperatures. The temperature dependence indicates that the one-phonon process is dominant at low temperatures, while two-phonon processes become important at higher temperatures as a cause of the nuclear spin conversion.     

Folding-induced selective hydrogenation of helical 9,10-anthraquinone analogues

The first selective catalytic hydrogenation induced by the artificial helix based on oligo(phenanthroline dicarboxamide)s containing a 9,10-anthraquinone subunit is described. Due to the steric hindrance within the helically folded oligomers, the selective reductions of the anthraquinone units were completely different from those of model substrates, which subsequently mimicked the enzyme catalysis for preventing some reactions from occurring.     

Nuclear spin relaxation in biaxial liquid crystal phases

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J_mLm'L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the 'biaxial' spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Nuclear spin relaxation in biaxial liquid crystal phases

Abstract

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J _mLm′L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the ‘biaxial’ spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Nuclear spin relaxation in poly(diethylsiloxane)

Earlier work showed that heating causes poly(diethylsiloxane) to undergo a first-order transition from a semicrystalline solid to a more mobile viscous—crystalline material. The latter is composed of two phases and analogies between polymer and liquid crystal morphology and behavior have been made. The viscous—crystalline phase in PDES appears to be unique since the literature is devoid of other documented examples. In this study, spin—lattice and spin—spin relaxation times were measured over a wide temperature range. They show a glass transition at 138°K, a crystal—crystal transition at 206°K, and a transition around 250°K which results from translational motion of the polymer chains with respect to each other. This motion is observed in the amorphous phase at a lower temperature than in the crystalline phase. Translational motion in the crystalline phase is observed on melting of the crystallites. The spin—spin data permitted monitoring of the molecular motions in each phase and the data suggest that these phases exert some influence on the molecular motions of each other. The viscous—crystalline phase in PDES may represent a unique model for studying and understanding “precrystalline” behavior and structure in amorphous solids.