2,876 research outputs found
Theory of long-lived nuclear spin states in solution nuclear magnetic resonance. II. Singlet spin locking
No full textIn a previous paper [M. Carravetta and M. H. Levitt, J. Chem. Phys. 122, 214505 (2005)], we presented the theory of long-lived nuclear spin singlet states in low magnetic field. In this paper, we consider the spin locking of long-lived singlet states in high magnetic field by the application of resonant radio frequency irradiation. We present theoretical results for unmodulated irradiation, including approximate analytical expressions for the singlet decay rate constants. We show the results of numerical simulations, which indicate that modulated radio frequency fields may be used to achieve broadband spin locking of singlet states but only in the case of a small difference in Larmor frequencies between the members of the spin pair.<br/
Infrared spectroscopy of endohedral HD and D2 in C60
No full textWe report on the dynamics of two hydrogen isotopomers, D2 and HD, trapped in the molecular cages of a fullerene C60 molecule. We measured the infrared spectra and analyzed them using a spherical potential for a vibrating rotor. The potential, vibration-rotation Hamiltonian, and dipole moment parameters are compared with previously studied H2@C 60 parameters [M. Ge, U. Nagel, D. Hvonen, T. Rm, S. Mamone, M. H. Levitt, M. Carravetta, Y. Murata, K. Komatsu, J. Y.-C. Chen, and N. J. Turro, J. Chem. Phys. 134, 054507 (2011)10.1063/1.3535598]. The isotropic part of the potential is similar for all three isotopomers. In HD@C60, we observe mixing of the rotational states and an interference effect of the dipole moment terms due to the displacement of the HD rotation center from the fullerene cage center. © 2011 American Institute of Physics
Solid-state nuclear magnetic resonance of rhodopsin and its photointermediates
No full textPhotoisomerization of the membrane-bound light receptor protein rhodopsin leadsto a highly energetic species called bathorhodopsin, which is stable at temperaturesbelow 125 K. Bathorhodopsin stores about 2/3 of the absorbed photon energy butthe mechanisms with which this energy is stored is not completely understood. Anew insight into these mechanisms by means of low-temperature solid-state NMRis both subject and aim of this Ph.D. thesis. The issue of the energy storage hasbeen investigated by a solid state magic angle spinning technique which combinesmodern symmetry-based recoupling techniques with in situ cooling of the sample.Production of bathorhodopsin is also done in situ in a customized NMR probe.Three kind of experiments are discussed: chemical shift, distance and torsionalangle measurements. The first kind of experiments led to carbon chemical shiftsvalues for almost all the carbons along the retinylidene chain of the retinal chromophoreof bathorhodopsin. Our measurements show a significant perturbations ofthe 13C chemical shifts in bathorhodopsin which is interpreted in terms of chargedelocalization along the chain and therefore indicates a participation of an electrostaticmechanism to the energy storage. This is at variance with an earlier solidstate NMR study where only minor perturbations of the electronic structure in theisomerized retinylidene chain were observed. We believe that these data incorrectlyrefer to bathorhodopsin because of the incorrect conditions of temperature and illuminationapplied. To sample for other local mechanisms that may contribute to theenergy storage, the C-C distance of the last two carbons of the retinylidene chain,at the link with the protein opsin, was also measured but no significant differenceswith rhodopsin have been found. Finally, the H-C=C-H torsional angle at the doublebound where the isomerization takes place was measured in a double-quantumheteronuclear local field spectroscopy (2Q-HLF) experiment. Results indicate adeviation from planarity of at least 40? about this double bond in bathorhodopsinsuggesting an unquantified amount of torsional strain acting as a further energystorage mechanism. In addition to these very interesting results, this thesis reportsmethods, equipment and procedures ready to be used for the study of other similarlight-triggered processes
Infrared spectroscopy of endohedral HD and D2 in C60
No full textWe report on the dynamics of two hydrogen isotopomers, D-2 and HD, trapped in the molecular cages of a fullerene C-60 molecule. We measured the infrared spectra and analyzed them using a spherical potential for a vibrating rotor. The potential, vibration-rotation Hamiltonian, and dipole moment parameters are compared with previously studied H-2@C-60 parameters [M. Ge, U. Nagel, D. Hiivonen, T. Room, S. Mamone, M. H. Levitt, M. Carravetta, Y. Murata, K. Komatsu, J. Y.-C. Chen, and N. J. Turro, J. Chem. Phys. 134, 054507 (2011)]. The isotropic part of the potential is similar for all three isotopomers. In HD@C-60, we observe mixing of the rotational states and an interference effect of the dipole moment terms due to the displacement of the HD rotation center from the fullerene cage cente
Orientational Sampling Schemes Based on Four Dimensional Polytopes
No full textThe vertices of regular four-dimensional polytopes are used to generate sets of uniformly distributed three-dimensional rotations, which are provided as tables of Euler angles. The spherical moments of these orientational sampling schemes are treated using group theory. The orientational sampling sets may be used in the numerical computation of solid-state nuclear magnetic resonance spectra, and in spherical tensor analysis procedures
Hyperpolarization and the physical boundary of Liouville space
No full textThe quantum state of a spin ensemble is described by a density operator, which corresponds to a point in the Liouville space of orthogonal spin operators. Valid density operators are confined to a particular region of Liouville space, which we call the physical region and which is bounded by multidimensional figures called simplexes. Each vertex of a simplex corresponds to a pure-state density operator. We provide examples for spins I=1/2, I=1, I=3/2 and for coupled pairs of spins-1/2. We use the von Neumann entropy as a criterion for hyperpolarization. It is shown that the inhomogeneous master equation for spin dynamics leads to non-physical results in some cases, a problem that may be avoided by using the Lindbladian master equation.</p
Steady-state in magnetic resonance pulse experiments
No full textRelaxation during multiple-pulse magnetic resonance experiments is treated by an average Liouvillian technique. The finite lattice temperature is taken into account by means of phenomenological correction terms. Both the transient and long-term response of the spin system are readily treated. We predict and verify a novel steady state of correlated spin polarizations in a spin-pair system under a periodic sequence of strong pi pulses
A large geometric distortion in the first photointermediate of rhodopsin, determined by double-quantum solid-state NMR
No full textDouble-quantum magic-angle-spinning NMR experiments were performed on 11,12-C-13(2)-retinylidene-rhodopsin under illumination at low temperature, in order to characterize torsional angle changes at the C11-C12 photoisomerization site. The sample was illuminated in the NMR rotor at low temperature (similar to 120 K) in order to trap the primary photointermediate, bathorhodopsin. The NMR data are consistent with a strong torsional twist of the HCCH moiety at the isomerization site. Although the HCCH torsional twist was determined to be at least 40A degrees, it was not possible to quantify it more closely. The presence of a strong twist is in agreement with previous Raman observations. The energetic implications of this geometric distortion are discussed
- …
