Robert G. Griffin

Heteronuclear decoupling in rotating solids

A simple two pulse phase modulation (TPPM) scheme greatly reduces the residual linewidths arising from insufficient proton decoupling power in double resonance magic angle spinning (MAS) experiments. Optimization of pulse lengths and phases in the sequence produces substantial improvements in both the resolution and sensitivity of dilute spins (e.g., 13C) over a broad range of spinning speeds at high magnetic field. The theoretical complications introduced by large homo‐ and heteronuclear interactions among the spins, as well as the amplitude modulation imposed by MAS, are explored analytically and numerically. To our knowledge, this method is the first phase‐switched sequence to exhibit improvement over continuous‐wave (cw) decoupling in a strongly coupled homogeneous spin system undergoing sample spinning.

Dynamic nuclear polarization at high magnetic fields

Dynamic nuclear polarization (DNP) is a method that permits NMR signal intensities of solids and liquids to be enhanced significantly, and is therefore potentially an important tool in structural and mechanistic studies of biologically relevant molecules. During a DNP experiment, the large polarization of an exogeneous or endogeneous unpaired electron is transferred to the nuclei of interest (I) by microwave (microw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (gamma(e)gamma(l)), being approximately 660 for protons. In the early 1950s, the DNP phenomenon was demonstrated experimentally, and intensively investigated in the following four decades, primarily at low magnetic fields. This review focuses on recent developments in the field of DNP with a special emphasis on work done at high magnetic fields (> or =5 T), the regime where contemporary NMR experiments are performed. After a brief historical survey, we present a review of the classical continuous wave (cw) DNP mechanisms-the Overhauser effect, the solid effect, the cross effect, and thermal mixing. A special section is devoted to the theory of coherent polarization transfer mechanisms, since they are potentially more efficient at high fields than classical polarization schemes. The implementation of DNP at high magnetic fields has required the development and improvement of new and existing instrumentation. Therefore, we also review some recent developments in microw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.

Chemical shift correlation spectroscopy in rotating solids: Radio frequency‐driven dipolar recoupling and longitudinal exchange

We present a new method of performing chemical shift correlation spectroscopy in solids with magic angle spinning (MAS). Its key feature is a longitudinal mixing period of π pulses that recouples the dipolar interaction. We discuss experimental results for triply‐13C‐labeled alanine and a theory combining MAS and π pulses.

Rotational resonance in solid state NMR

Abstract In magic angle spinning experiments, on samples containing dilute homonuclear dipolar-coupled spin pairs, rotational resonance occurs when the spinning speed is adjusted so that the condition ωδiso=nωr is satisfied. Here ωδiso is the difference isotropic chemical shifts, ωr is the spinning speed, and n is an integer. Under these conditions the normally sharp resonance lines broaden and split. In addition, a rapid oscillatory exchange of Zeeman order between the dipolar-coupled spins is observed. The time dependence of the exchange and the spectral lineshapes agree with numerical simulations. The method is potentially useful for estimating through-space dipolar couplings, and therefore internuclear distances in polycrystalline solids.

Cross polarization in the tilted frame: assignment and spectral simplification in heteronuclear spin systems

A frequency selective heteronuclear polarization transfer technique is introduced for rotating solids. In this method, radiofrequency fields comparable with the frequency offsets are applied to establish Hartmann-Hahn cross polarization that therefore depends explicitly on the resonance offset of the nuclei involved. Under these conditions, spectrally induced filtering in combination with cross polarization (SPECIFIC CP) can be achieved and is demonstrated to be useful for spectral simplification or assignment in heteronuclear spin pairs. The design principles are outlined and demonstrated experimentally on 13C, 15N labelled amino acids.

Theory and simulations of homonuclear spin pair systems in rotating solids

The theory of nuclear magnetic resonance (NMR) on a solid sample containing pairs of coupled homonuclear spins‐1/2, rotating in a large magnetic field, is presented. The time dependence introduced by the sample rotation, in conjunction with the spin–spin coupling, makes it appear that each of the central two levels in the four‐level system split into a pair of ‘‘virtual states.’’ Each of the eight possible single‐quantum coherences between the virtual states and the two outer levels in general contribute to the spectrum, although four of these contributions are forbidden unless a rotational resonance occurs (matching of an integer multiple of the spinning speed with the difference in isotropic shifts). Analytical line shapes for the case of vanishing shift anisotropy are given and techniques for numerical simulation in the general case demonstrated. The theory of Zeeman magnetization exchange in the presence of zero‐quantum dephasing is presented.

Rotary resonance recoupling of dipolar interactions in solid‐state nuclear magnetic resonance spectroscopy

A new resonance effect in solid‐state nuclear magnetic resonance (NMR) is described. The effect involves a combination of magic‐angle sample rotation with irradiation of a heteronuclear spin system at the Larmor frequency of one of the spin species. If the irradiation intensity is such as to establish a match between spin nutation and sample rotation, it is shown that the heteronuclear dipolar spin interaction is selectively reintroduced into the spectrum. This allows small dipolar coupling constants to be measured in the presence of large shielding anisotropies. Applications are anticipated for determination of internuclear distances in materials lacking long‐range order, such as polycrystalline materials, polymers, and surfaces.

Atomic structure and hierarchical assembly of a cross-β amyloid fibril.

The cross-β amyloid form of peptides and proteins represents an archetypal and widely accessible structure consisting of ordered arrays of β-sheet filaments. These complex aggregates have remarkable chemical and physical properties, and the conversion of normally soluble functional forms of proteins into amyloid structures is linked to many debilitating human diseases, including several common forms of age-related dementia. Despite their importance, however, cross-β amyloid fibrils have proved to be recalcitrant to detailed structural analysis. By combining structural constraints from a series of experimental techniques spanning five orders of magnitude in length scale—including magic angle spinning nuclear magnetic resonance spectroscopy, X-ray fiber diffraction, cryoelectron microscopy, scanning transmission electron microscopy, and atomic force microscopy—we report the atomic-resolution (0.5 Å) structures of three amyloid polymorphs formed by an 11-residue peptide. These structures reveal the details of the packing interactions by which the constituent β-strands are assembled hierarchically into protofilaments, filaments, and mature fibrils.

Dipolar recoupling in MAS spectra of biological solids

In combination with magic angle spinning, dipolar recoupling yields solid state NMR spectral assignments and provides constraints on internuclear distances and torsion angles. The method offers a fresh approach to structural studies of a variety of systems that cannot be examined with conventional tools available to structural biology.

Functional and shunt states of bacteriorhodopsin resolved by 250 GHz dynamic nuclear polarization–enhanced solid-state NMR

Observation and structural studies of reaction intermediates of proteins are challenging because of the mixtures of states usually present at low concentrations. Here, we use a 250 GHz gyrotron (cyclotron resonance maser) and cryogenic temperatures to perform high-frequency dynamic nuclear polarization (DNP) NMR experiments that enhance sensitivity in magic-angle spinning NMR spectra of cryo-trapped photocycle intermediates of bacteriorhodopsin (bR) by a factor of ≈90. Multidimensional spectroscopy of U-13C,15N-labeled samples resolved coexisting states and allowed chemical shift assignments in the retinylidene chromophore for several intermediates not observed previously. The correlation spectra reveal unexpected heterogeneity in dark-adapted bR, distortion in the K state, and, most importantly, 4 discrete L substates. Thermal relaxation of the mixture of Ls showed that 3 of these substates revert to bR568 and that only the 1 substate with both the strongest counterion and a fully relaxed 13-cis bond is functional. These definitive observations of functional and shunt states in the bR photocycle provide a preview of the mechanistic insights that will be accessible in membrane proteins via sensitivity-enhanced DNP NMR. These observations would have not been possible absent the signal enhancement available from DNP.