Biomedical subjects
Publications and source records attributed to Marina Carravetta.
The existence of long-lived nuclear spin states in four-spin systems is explored by solution-state NMR experiments. Long-lived states are proved to exist in three different natural product molecules, each containing either a AA'BB' or a AA'XX' proton spin system. The measured state lifetimes are between four and eight times the spin-lattice relaxation time constants.
We present the theoretical principles of supercycled symmetry-based recoupling sequences in solid-state magic-angle-spinning NMR. We discuss the construction procedure of the SR26 pulse sequence, which is a particularly robust sequence for double-quantum homonuclear dipole-dipole recoupling. The supercycle removes destructive higher-order average Hamiltonian terms and renders the sequence robust over long time intervals. We demonstrate applications of the SR26 sequence to double-quantum spectroscopy, homonuclear spin counting, and determination of the relative orientations of chemical shift anisotropy tensors.
A new double-quantum solid-state NMR pulse sequence is presented and used to measure one-bond 13C-13C J-couplings in a set of 13C2-labeled rhodopsin isotopomers. The measured J-couplings reveal a perturbation of the electronic structure at the terminus of the conjugated chain but show no evidence for protein-induced electronic perturbation near the C11-C12 isomerization site. This work establishes NMR methodology for measuring accurate 1JCC values in noncrystalline macromolecules and shows that the measured J-couplings may reveal local electronic perturbations of mechanistic significance.
In a nuclear magnetic-resonance (NMR) experiment, the spin density operator may be regarded as a superposition of irreducible spherical tensor operators. Each of these spin operators evolves during the NMR experiment and may give rise to an NMR signal at a later time. The NMR signal at the end of a pulse sequence may, therefore, be regarded as a superposition of spherical components, each derived from a different spherical tensor operator. We describe an experimental method, called spherical tensor analysis (STA), which allows the complete resolution of the NMR signal into its individual spherical components. The method is demonstrated on a powder of a (13)C-labeled amino acid, exposed to a pulse sequence generating a double-quantum effective Hamiltonian. The propagation of spin order through the space of spherical tensor operators is revealed by the STA procedure, both in static and rotating solids. Possible applications of STA to the NMR of liquids, liquid crystals, and solids are discussed.
We have recently demonstrated the existence of exceptionally long-lived nuclear spin states in solution-state nuclear magnetic resonance. The lifetime of nuclear spin singlet states in systems containing coupled pairs of spins-12 may exceed the conventional relaxation time constant T1 by more than an order of magnitude. These long lifetimes may be observed if the long-lived singlet states are prevented from mixing with rapidly relaxing triplet states. In this paper we provide the detailed theory of an experiment which uses magnetic field cycling to observe slow singlet relaxation. An approximate expression is given for the magnetic field dependence of the singlet relaxation rate constant, using a model of intramolecular dipole-dipole couplings and fluctuating external random fields.
In magic-angle-spinning solid-state NMR, the homonuclear J-couplings between pairs of spin-1/2 nuclei may be determined by studying the modulation of the spin echo induced by a pi-pulse, as a function of the echo duration. We present the theory of J-induced spin-echo modulation in magic-angle-spinning solids, and derive a set of modulation regimes which apply under different experimental conditions. In most cases, the dominant spin-echo modulation frequency is exactly equal to the J-coupling. Somewhat surprisingly, the chemical shift anisotropies and dipole-dipole couplings tend to stabilise--rather than abscure--the J-modulation. The theoretical conclusions are supported by numerical simulations and experimental results obtained for three representative samples containing 13C spin pairs.
Nuclear spin order may be stored in a liquid for a much longer time than the longitudinal relaxation time T1, by using rf fields to isolate states of different symmetry. The method is demonstrated on a sample containing AX spin systems.
Low-field nuclear spin singlet states may be used to store nuclear spin order in a room temperature liquid for a time much longer than the spin-lattice relaxation time constant T1. The low-field nuclear spin singlets are unaffected by intramolecular dipole-dipole relaxation, which is generally the predominant relaxation mechanism. We demonstrate storage of nuclear spin order for more than 10 times longer than the measured value of T1. This phenomenon may facilitate the development of nuclear spin hyperpolarization methods and may allow the study of motional processes which occur too slowly for existing NMR techniques. This is the first time that the memory of nuclear spins has been extended well beyond the T1 limit in a system lacking intrinsic magnetic equivalence.
Solid-state 1H experiments were performed an open-cage fullerene hosting molecular hydrogen. The anisotropy of the molecular hydrogen rotation was studied by double-quantum magic-angle-spinning NMR. The time scale of the molecular hydrogen rotation was estimated by spin-lattice relaxation measurements as a function of temperature.
We have obtained carbon-carbon bond length data for the functional retinylidene chromophore of rhodopsin, with a spatial resolution of 3 pm. The very high resolution was obtained by performing double-quantum solid-state NMR on a set of noncrystalline isotopically labelled bovine rhodopsin samples. We detected localized perturbations of the carbon-carbon bond lengths of the retinylidene chromophore. The observations are consistent with a model in which the positive charge of the protonated Schiff base penetrates into the polyene chain and partially concentrates around the C13 position. This coincides with the proximity of a water molecule located between the glutamate-181 and serine-186 residues of the second extracellular loop, which is folded back into the transmembrane region. These measurements support the hypothesis that the polar residues of the second extracellular loop and the associated water molecule assist the rapid selective photoisomerization of the retinylidene chromophore by stabilizing a partial positive charge in the center of the polyene chain.
Cogwheel phase cycles are often significantly shorter than traditional nested phase cycles. However, optimal solutions for cogwheel cycles are often difficult to find. This paper presents techniques and conjectures which the authors have found useful for generating cogwheel phase cycles either without the need for computer searches or with significantly smaller searches than would otherwise be necessary. The conjectures presently lack proofs but have been tested successfully for a large number of cases.
We describe a new method for exciting triple-quantum coherences in 13C-labelled powder samples under MAS. The proposed method combines selective double-quantum excitation with rotational resonance and frequency-selective composite pulses. The spin dynamics of this new method are described theoretically. Numerical calculations of the spin dynamics are compared to experimental results on fully 13C-labelled L-alanine. The observed triple-quantum filtering efficiency is around 10% for the most intense spectral peak. The method is also demonstrated on other fully 13C-labelled compounds, including a uniformly 13C-labelled amino acid.