ScienceWeek

CHEMISTRY: ON NUCLEAR QUADRUPOLE RESONANCE

The following points are made by J.B. Miller and G.A. Barrall (American Scientist 2005 93:50):

1) Nuclear quadrupole resonance (NQR) has much in common with nuclear magnetic resonance (NMR), the fundamental physical process that makes magnetic resonance imaging possible. Nuclear magnetic resonance, first demonstrated in 1946, takes advantage of the fact that certain atomic nuclei possess magnetic dipole moments -- that is, these nuclei act like tiny bar magnets, each with a north magnetic pole at one end and a south magnetic pole at the other. The laws of quantum mechanics dictate that when such nuclei are subjected to an externally applied magnetic field, they must align themselves along it. But the magnetic moments of these nuclei, usually depicted as arrows, are allowed only two possible orientations: in the same direction as the applied magnetic field or opposite to it.

2) Although alignment with the applied field is favored (this being the lower-energy condition), the energy difference between the two orientations is such that thermal agitation is usually sufficient to ensure that only slightly more than half the nuclei are in the lower-energy state. The key is that the nuclei can occupy two distinct states separated by a well-defined increment in energy. (It will be well defined as long as the applied magnetic field is uniform.) In that sense, the situation is much like that of an electron in an atom, which can be in the "ground" state or in a higher-energy "excited" state.

3) A ground-state electron shifts to an excited state when the atom receives a dollop of electromagnetic radiation of just the right energy to put it there -- that is, when it absorbs a photon of just the right frequency. Conversely, if this excited-state electron falls back to the ground state, the atom will emit a photon of the exact same frequency to carry away the difference in energy. In NMR, the energy difference between states is much less than for the electronic states of an atom, so the relevant frequencies are much lower. Rather than dealing with optical frequencies, NMR typically involves oscillations of just a few tens to hundreds of megahertz, which includes the band where broadcast FM radio stations operate.

4) Nuclear quadrupole resonance is similar in concept, but unlike nuclear magnetic resonance it does not rely on nuclei aligning themselves in an externally applied magnetic field. Instead, NQR exploits the fact that some nuclei possess an electric quadrupole moment, which can be thought of as arising from two back-to-back electric dipoles (positive and negative charges separated by a short distance). Why do some atomic nuclei have an electric quadrupole moment? Physicists would say because they have a spin quantum number greater than 1/2. A more intuitive explanation is because the positive electric charge these nuclei carry is not distributed with perfect spherical symmetry.

5) Consider for a moment a spherical nucleus with its positive charge distributed uniformly throughout. Now squeeze that nucleus in your mind's eye so that what was originally shaped like a basketball is flattened into a pumpkin. A pumpkin of positive charge can be thought of, to a rough approximation, as being the sum of a sphere of positive charge and two oppositely directed electric dipoles, one at the top and one at the bottom. That is, the only requirement for an electric quadrupole moment is that the nucleus be squashed (or stretched) along one axis.

6) When a nucleus possessing such an electric quadrupole moment is subjected to an electric field that varies from place to place, interesting things happen. The intrinsic electric quadrupole moment of the nucleus and the electric-field gradient imposed from outside together create distinct energy states. This result is analogous to the multiple energy states in NMR, where the critical ingredients were the intrinsic magnetic dipole moment of the nucleus and a magnetic field imposed from the outside.

7) The key difference between NMR and NQR is the definition of "outside." In NMR, the outside magnetic field arises because the experimenter has invested considerable effort in setting it up, perhaps using a superconducting electromagnet. In NQR, the required electric field (or, more precisely, the required electric-field gradient) comes free: It reflects the local arrangement of electrons around the nucleus under study. That arrangement, in turn, depends not only on the nature of the atom but also on its chemical environment. This feature accounts for one of the chief benefits of NQR -- the method is exquisitely sensitive to chemistry.[1-4]

References (abridged):

1. Garroway, A. N., M. L. Buess, J. B. Miller, B. H. Suits, A. D. Hibbs, G. A. Barrall, R. Matthews and L. J. Burnett. 2001. Remote sensing by nuclear quadrupole resonance. IEEE Transactions on Geoscience and Remote Sensing 39:1108-1118

2. Miller, J. B. 1998. NMR imaging of materials. Progress in Nuclear Magnetic Resonance Spectroscopy 33:273-308

3. Vierkötter, S. A., C. R. Ward, D. M. Gregory, S. M. Menon and D. P. Roach. 2003. NDE of composites via quadrupole resonance spectroscopy. Proceedings of SPIE 5046:176-184

4. Barrall, G. A., L. J. Burnett, K. A. Derby, A. J. Drew, K. V. Ermolaev, S. Huo, D. K. Lathrop, T. R. Petrov, M. J. Steiger, S. H. Stewart and P. J. Turner. 2005. Nuclear quadrupole resonance for landmine detection. Proceedings of the Sixth Joint International Military Sensing Symposium (in press)

American Scientist http://www.americanscientist.org

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Related Material:

ENZYME DYNAMICS DURING CATALYSIS

The following points are made by E. Eisenmesser et al (Science 2002 295:1520):

1) Although classical enzymology together with structural biology have provided profound insights into the chemical mechanisms of many enzymes (1), enzyme dynamics and their relation to catalytic function remain poorly characterized. Because many enzymatic reactions occur on time scales of micro- to milliseconds, it is anticipated that the conformational dynamics of the enzyme on these time scales might be linked to its catalytic action (2). Classically, enzyme reactions are studied by detecting substrate turnover.

2) Dynamics of enzymes during catalysis have been detected with methods such as fluorescent resonance energy transfer, atomic force microscopy, and stopped-flow fluorescence, which report on global motions of the enzyme or dynamics of particular molecular sites. In contrast, nuclear magnetic resonance (NMR) spectroscopy enables investigations of motions at many atomic sites simultaneously (3,4). NMR studies reporting on the time scales, amplitudes, and energetics of motions in proteins have provided information on the relation between protein mobility and function (5).

3) The authors report an examination of enzyme catalysis in a nonclassical way by characterizing motions in the enzyme during substrate turnover. The authors have used NMR relaxation experiments to advance these efforts by characterizing conformational exchange in an enzyme, human cyclophilin A (CypA), during catalysis. CypA is a member of the highly conserved family of cyclophilins that are found in high concentrations in many tissues. Cyclophilins are peptidyl-prolyl cis/trans isomerases that catalyze the interconversion between cis and trans conformations of X-Pro peptide bonds, where "X" denotes any amino acid. CypA operates in numerous biological processes. It is the receptor for the immunosuppressive drug cyclosporin A, is essential for HIV infectivity, and accelerates protein folding in vitro by catalyzing the rate-limiting cis/trans isomerization of prolyl peptide bonds. However, its function in vivo and its molecular mechanism are still in dispute.

4) In summary: Internal protein dynamics are intimately connected to enzymatic catalysis. However, enzyme motions linked to substrate turnover remain largely unknown. The authors have studied dynamics of an enzyme during catalysis at atomic resolution using nuclear magnetic resonance relaxation methods. During catalytic action of the enzyme cyclophilin A, the authors detect conformational fluctuations of the active site that occur on a time scale of hundreds of microseconds. The rates of conformational dynamics of the enzyme strongly correlate with the microscopic rates of substrate turnover. The authors suggest that the present results, together with available structural data, allow a prediction of the reaction trajectory.

References (abridged):

1. T.C. Bruice and S.J. Benkovic, Biochemistry 39, 6267 (2000)

2. A. Fersht, Structure and Mechanism in Protein Science. A Guide to Enzyme Catalysis and Protein Folding (Freeman, New York, ed. 1, 1999) pp. 44-51

3. M.W. Fischer, A. Majumdar, E. R. P. Zuiderweg, Progr. Nucl. Magn. Reson. Spectrosc. 33, 207 (1998)

4. A.G. Palmer, 3rd, Curr. Opin. Struct. Biol. 7, 732 (1997)

5. R. Ishima and D.A. Torchia, Nature Struct. Biol. 7, 740 (2000)

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