A.J. Shaka

BROADBAND HOMONUCLEAR CROSS POLARIZATION USING FLIP-FLOP SPECTROSCOPY

We have developed a new type of pulse sequence for broadband homonuclear cross polarization (l-10), known as TOCSY or HOHAHA in the literature. Our new sequences, called FLOPSY-8 and FLOPSY16 (flip-flop spectroscopy), offer a noticeable improvement in polarization transfer efficiency for a given expenditure of RF power, and outperform commonly used windowless sequences like WALTZ-16, DIPSI-2, DIPSI-3, and MLEV-16 or MLEV-17. In this Communication we describe how the FLOPSY sequences work by a geometrical analogy with the firmly established area of composite 180” pulses, and we compare FLOPSY with the other sequences. In Fig. 1 we show the familiar rotating-frame geometrical picture for an ensemble of isolated spin-f nuclei, in which the orthogonal axes are labeled with the linear spin operators I,, I,,, and I,. The state of the spins is represented as a vector from the origin to a point on the surface of the unit sphere, and evolution under an RF field is represented as a rotation of the spin vector about a vector representing the effective field. Spin inversion is accomplished on resonance by 180” pulse, mapping 1, onto 1,. Off resonance the effective field has a nonzero z component, and complete inversion is not possible with a single elementary rotation. On the right we show an analogous picture for the zero-quantum (ZQ) manifold of a two-spin system. Here it is the average d@erence in z magnetization II, Izz which labels the z component, and the transverse components in the picture label the x and y components of ZQ coherence. The x component is familiar as the transverse part of the scalar spin-spin coupling; the y component does not normally appear as part of the high-resolution spin Hamiltonian. It is formally related to the x component by a zero-quantum phase shift, that is, a unitary transformation by the longitudinal component in the ZQ frame. The three components in the ZQ frame are easily derived by considering the single-transition (II) or fictitious spin-4 (12) operators for the flip-flop transition I+-) I -+). Provided the dynamics remain within the ZQ manifold, our geometrical picture makes it easy to understand broadband cross polarization. An on-resonance 180” pulse in the ZQ frame will invert the average difference in z magnetization. The total z magnetization remains unaffected by the ZQ rotations, so we can write

Processing DOSY spectra using the regularized resolvent transform

A new method for processing diffusion ordered spectroscopy (DOSY) data is presented. This method, the regularized resolvent transform (iRRT-the i denoting the adaptation of the method to evaluate the inverse Laplace transform), is better than conventional processing techniques for generating 2D DOSY spectra using data that has poor chemical shift resolution. From the same data, it is possible to use the iRRT to generate 1D subspectra corresponding to different components of the sample mixture; these subspectra compare favorably to 1D spectra of the pure substances. Both the 2D spectra and the 1D subspectra offer a vast improvement over results generated using a conventional processing technique (non-linear least-squares fitting). Consequently, we present the iRRT as a stable and reliable tool for solving the inverse Laplace transform problem present in experiments such as DOSY.

Broadband proton decoupling for in vivo brain spectroscopy in humans

A new decoupling sequence, PBAR, is described for broadband heteronuclear decoupling in vivo in humans at 1.5T. The sequence uses non‐adiabatic, frequency‐ and amplitude‐modulated inversion pulses designed to minimize decoupling sidebands at low applied γB2 RF field levels and to cover only the narrow range of resonance offsets encountered in practice. The offset dependence of the decoupling efficiency of PBAR is demonstrated and compared to the conventional WALTZ‐4 sequence. At the same average power levels, PBAR had slightly reduced bandwidth but significantly less intense decoupling sidebands. Applications of PBAR are shown in vivo in the human brain both for 31P and natural abundance 13C spectroscopy using volume decoupling coils. The PBAR sequence allows whole brain {1H}‐13C decoupling to be performed at 1.5T with a standard head coil within FDA guidelines for RF power deposition. Magn Reson Med 45:226–232, 2001.

Highly resolved double absorption 2D NMR spectra from complex severely truncated 2D phase-modulated signals by filter-diagonalization-averaging method

Abstract Application of the filter diagonalization method (FDM) to the spectral estimation of purely phase-modulated 2D-J NMR data is described. It is shown that obtaining an entire 2D line list of spectral parameters is not a prerequisite for constructing a spectral estimate of the data. Common problems with 2D FT analysis, such as phase-twist lineshapes, spectral broadening from the time-frequency uncertainty principle, and vanishing phase-sensitive 45° projections, can be ameliorated using 2D FDM. Averaging over several different FDM calculations improves the spectral presentation. Examples are shown with complex spectra obtained using only a handful of time increments.

Two‐dimensional HSQC NMR spectra obtained using a self‐compensating double pulsed field gradient and processed using the filter diagonalization method

New data acquisition and data processing strategies are combined to give enhanced 2D HSQC spectra. Complete carbon‐13 assignments can be obtained using only two proton spectra in some cases. A composite pulsed field gradient is employed which appears effectively instantaneous, as far as spin evolution is concerned, and which offers superb recovery and does not perturb the field/frequency lock. The spectra are analyzed by a new linear algebraic method called the filter diagonalization method (FDM). FDM can be used to extract 2D spectral parameters directly from 2D time signals without any Fourier transformation and can speed up the spectral throughput.

Three-stranded mixed artificial β-sheets

Abstract This paper describes the design, synthesis, and structural evaluation of a pair of compounds, comprising molecular templates and attached peptide strands, that mimic a small three-stranded mixed β-sheet ( 6a and b ). Each of these artificial β-sheets is composed of two different molecular templates and two dipeptide strands. One of the templates is based on a 5-amino-2-methoxybenzoic hydrazide group that mimics the hydrogen-bonding functionality of a peptide β-strand and serves as the top strand. This template forms a pattern of hydrogen bonds similar to that of an antiparallel β-sheet with the middle peptide strand. The middle peptide strand forms a pattern of hydrogen bonds similar to that of a parallel β-sheet with the bottom peptide strand. The other template holds the three strands next to each other and is based upon a triurea. In one artificial β-sheet ( 6a ), both the upper and middle urea groups and the middle and lower urea groups are linked by dimethylene (CH 2 CH 2 ) chains; in the other ( 6b ), the upper and middle urea groups are linked by a dimethylene chain, while the middle and lower urea groups are linked by a trimethylene (CH 2 CH 2 CH 2 ) chain. 1 H NMR chemical shift and NOE studies establish that both of these compounds fold to adopt a hydrogen-bonded β-sheetlike structure in CDCl 3 solution. Chemical shift studies establish that three-stranded mixed artificial β-sheets 6 are more tightly folded than their smaller two-stranded homologues, artificial parallel β-sheet 1 and artificial antiparallel β-sheet 3 , as well as their three-stranded homologues with truncated β-strand mimics, artificial β-sheets 5 . These studies show that the folding of artificial β-sheets 6 is cooperative, with the interactions between the upper and middle strands and between the middle and lower strands reinforcing each other.

Regularized resolvent transform for direct calculation of 45° projections of 2D J spectra

Abstract The regularized resolvent transform (RRT) has been applied in a novel way to J-resolved spectra. This involves the direct calculation of the 45° projection without constructing the 2D spectrum. The results show a significant resolution enhancement over that obtained by the 45° projection of a 2D Fourier spectrum, even for much larger signals. In particular, RRT is able to resolve peaks that belong to different overlapping multiplets in a very crowded spectral region, where the conventional technique fails for any signal size. The resolving power of this method along with the significantly shorter signals required, make this method a powerful tool in spectral assignment.

STABILIZATION OF A C7 EQUATORIAL GAMMA TURN IN DMSO-D6 BY A DITRYPTOPHAN CROSSLINK

Covalent crosslinks can control local peptide conformation. In tripeptide sequences of the general formula Cys-Xxx-Cys, cysteine disulfides have been previously shown to enforce a C7 equatorial gamma-turn conformation (also referred to as an inverse gamma-turn). Much less is known about the effects of dityrosine and ditryptophan crosslinks on local peptide structure. In a series of tripeptides, ditryptophan crosslinks were formed using the two-step process of acid-promoted Mannich dimerization followed by oxidative aromatization. In these peptides, with the general formula Trp-Xxx-Trp (Xxx not equal to Gly), ditryptophan crosslinks were found to stabilize a C7 equatorial gamma-turn conformation in DMSO-d6. Rigorous support for a C7 equatorial conformation in the crosslinked sequence Trp-Pro-Trp came from a variety of 1H NMR experiments and molecular modelling. Interproton distances were derived from NOE buildups that were determined through a series of double pulsed field gradient spin echo (DPFGSE) experiments. In addition, the small temperature dependence of the i+2 NH chemical shifts (delta delta/delta T < 2 ppm/degree C) provided further support for the intramolecular hydrogen bond which defines a gamma-turn.

STRUCTURAL ASSIGNMENT OF A MOLYBDENUM-CONTAINING SILSESQUIOXANE WHICH CATALYZES THE METATHESIS OF OLEFINS : DPFGSE-NOE AND X-RAY DIFFRACTION STUDIES

The reaction of [(c‐C6H11)7Si7O9(OSiMe3)(OTl)2] with [Mo(CHCMe2Ph)(NAr)(OSO2CF3)2(dme)] (Ar=2,6‐diisopropylphenyl) affords 4a as the predominant (>99%) molybdenum‐containing product. Assignment of 4a as the syn‐isomer with juxtaposed arylimido and Me3Si groups was made on the basis of double pulsed field gradient spin echo (DPFGSE) NOE experiments and confirmed by a single‐crystal x‐ray diffraction study.

Filter diagonalization using a "sensitivity-enhanced basis": improved performance for noisy NMR spectra.

The Filter Diagonalization Method (FDM) has been used to process NMR data in liquids and can be advantageous when the spectrum is sparse enough, the lines are sharp and Lorentzian, raw sensitivity is adequate, and the measured time-domain data is short, so that the Fourier Transform spectrum exhibits distorted line shapes. Noise can adversely impact resolution and/or frequency accuracy in FDM spectral estimates. Paradoxically, more complete data can lead to worse FDM spectra if there is appreciable noise. However, by modifying the numerical method, the FDM noise performance improves significantly, without apparently losing any of the existing advantages. The two key modifications are to adjust the FDM basis functions so that matrix elements computed from them have less noise contribution on average, and to regularize each dimension of a multidimensional spectrum independently. The modifications can be recommended for general-purpose use in the case of somewhat noisy, incomplete data.