Filtering according to coupling network connectivity 438 8.3.4. Relayed coherence transfer 440 8.3.5. Coherence transfer by an average Hamiltonian in total correlation spectroscopy 444 8.4. Homonuclear two-dimensional multiple-quantum spectroscopy 8.4.1. Excitation and detection of multiple-quantum coherence 8.4.2. Double-quantum spectra of two-spin systems 451 8.4.3. Multiple-quantum spectra of scalar-coupled networks in isotropic phase 8.4.4. Multiple-quantum spectra of dipole-coupled nuclei in anisotropic phase 463 8.4.5. Double-quantum spectra of quadrupolar nuclei with S = 1 in anisotropic phase 465 8.5. Heteronuclear coherence transfer 8.5.1. Sensitivity considerations 8.5.2. Coherence transfer pathways 8.5.3. Heteronuclear two-dimensional correlation spectroscopy in isotropic phase 471 8.5.3.1. Transfer of in-phase magnetization 8.5.3.2. Broadband decoupling 8.5.3.3. Decoupling by refocusing pulses 475 8.5.3.4. Bilinear rotation decoupling 475 8.5.3.5. Editing of heteronuclear correlation spectra 477 8.5.4. Relayed heteronuclear correlation spectroscopy 479 8.5.5. Heteronuclear correlation experiments involving double transfer 482 8.5.6. Heteronuclear correlation in solids 485 9. DYNAMIC PROCESSES STUDIED BY TWO-DIMENSIONAL EXCHANGE SPECTROSCOPY 9.1. Polarization transfer in one-and two-dimensional methods 9.2. Selection of coherence transfer pathways 10.5. Comparison of sensitivity and performance time of various imaging techniques 10.

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Nuclear spin-lattice relaxation times of Al in pure Al and Cu6' in annealed pure Cu have been measured with a nuclear induction spectrometer, by the method of saturation, The experimental values of Ti are 4.1~0.8 milliseconds for A127 and 3.0~0.6. milliseconds for Cu", in reasonable agreement with theory. The dispersion mode of the nuclear resonance was also observed, and it was found that y' (the real part of the rf susceptibility) does not saturate at the same level as the absorption, x", but remains roughly constant out to a radio-frequency 6eld intensity of about 2 gauss. Both x' and p" become narrower and nearly Lorentzian in shape above saturation. When the dc magnetic Geld modulation is increased from 14 to 41 cps the phase of the dispersion signal lags behind the modulation, presumably because the modulation period is then comparable to Ti. Large dispersion signals above saturation have also been observed for the Na" resonance in NaC1. This behavior of the dispersion mode is in conQict with the predictions of Bloembergen, Purcell, and Pound and of the Bloch equations. The validity of these theories is reexamined , and it is concluded that although they are applicable to nuclear resonance in liquids and gases, and to solids at small rf intensities, they contain incorrect assumptions as applied to solids at high rf power levels. The theory of Bloembergen, Purcell, and Pound is based on an assumption equivalent to that of a spin temperature. It is '

31 P relaxation of the diester phosphate of phospholipids in unilamellar vesicles has been studied from 0.004 to 11.7 T. Relaxation at very low fields, below 0.1 T, shows a rate increase that reflects a residual dipolar interaction with neighboring protons, probably dominated by the glycerol C3 protons. This interaction is not fully averaged by faster motion such as rotational diffusion perpendicular to the membrane surface. The remaining dipolar interaction, modulated by overall rotational diffusion of the vesicle and lateral diffusion of the lipid molecules, is responsible for the very low-field relaxation. These measurements yield a good estimate of the time-average angle between the membrane surface and the vector connecting the phosphorus to the glycerol C3 dynamics ͉ membranes ͉ phosphorus M uch remains to be known about the details of the configuration and dynamics of the phosphodiester region of phospholipid bilayer membranes, despite a large body of work (1, 2). Yet such knowledge is likely to be very useful in understanding how proteins and assemblies interact with this interfacial region. For example, there is indirect evidence that the peripheral membrane protein phosphatidylinositol-specific phospholipase inserts a tryptophan residue into a phosphatidylcholine membrane surface, which in turn activates the enzyme toward its substrates, but how this happens in detail is unknown (3, 4).The lack of structural information for phospholipid polar and interfacial moieties results from the difficulty in obtaining crystals and making other ordered structures that are necessary for most structural techniques. Even when available, the resemblance of such ordered structures to membranes functioning in vivo can be questioned. Modern methods of NMR spectroscopy are generally difficult to apply to this problem, mainly because of the slow rate of tumbling of molecules in reasonable analogs of biological membranes such as vesicles. Most NMR studies ( 1 H, 13 C, and 2 H) of membranes have focused on acyl chain dynamics (5-7). Phosphorus-31 NMR, which would appear ideally suited for obtaining information on the phosphodiester linkage conformation and dynamics, has seen limited use, because 31 P resonances in phospholipid aggregates exhibit a large linewidth due to their chemical-shift anisotropy (CSA) [although this property had made this nucleus very useful in characterizing the phase behavior of phospholipid bilayers (5)]. 31 P chemical shifts in phosphodiesters embedded in membranes do reflect orientation of chemical bonds relative to the membrane surface but not in a very usefully specific way. Structures deduced from other NMR data suffer from unknown dynamic averaging effects. Computer simulations show great promise to eventually give completely detailed information (8, 9) but still need to be validated with quantitative experiments.As a contribution to this problem, we have estimated the angle PH between the vector connecting the phosphorus to its nearest protons and the vector perpendicular to the membrane sur...

The proton has been the nucleus of choice for NMR studies of macromolecules because it is ubiquitous; it provides the highest sensitivity; its resonances can be identified with types of amino and nucleic acids by means of experiments utilizing proton spin-spin interaction and chemical shift; and, most important, proton NMR yields distance information via the nuclear Overhauser effect (NOE). Many of these advantages are lost for larger biopolymers (molecular weight more than 15 kDa) for which the line width is considerably greater than the proton-proton spin-spin interaction. The spin-spin interaction is then useless or difficult to use for assignment; and furthermore the proton line width and the number of proton resonances both increase in proportion to the molecular weight, thereby increasing the problem of resonance overlap to an intolerable degree.

We have used high-resolution field-cycling 31P NMR spectroscopy to measure spin-lattice relaxation rates (R1 = 1/T1) of multicomponent phospholipid vesicle and micelle samples over a large field range, from 0.1 to 11.7 T. The shape of the curve for R1 as a function of field and a model-free analysis were used to extract tauc, a correlation time for each type of phospholipid molecule in the bilayer that is likely to reflect rotation of the molecule about the axis perpendicular to the membrane surface; Sc2, a chemical shift anisotropy (CSA) order parameter; and tauhf, a time constant reflecting faster internal motion. This 31P technique was also used to monitor association of a peripheral membrane protein, Bacillus thuringiensis phosphatidylinositol-specific phospholipase C, with both phosphatidylcholine and phosphatidylmethanol bilayers. Differences in phospholipid dynamics induced by the protein shed light on how zwitterionic phosphatidylcholine, and not the anionic phosphatidylmethanol, activates the enzyme toward its substrate.