Ole Winneche Sørensen

A general enhancement scheme in heteronuclear multidimensional NMR employing pulsed field gradients.

SummaryGeneral pulse sequence elements that achieve sensitivity-enhanced coherence transfer from a heteronucleus to protons of arbitrary multiplicity are introduced. The building blocks are derived from the sensitivity-enhancement scheme introduced by Cavanagh et al. ((1991) J. Magn. Reson., 91, 429–436), which was used in conjunction with gradient coherence selection by Kay et al. ((1992) J. Am. Chem. Soc., 114, 10663–10665), as well as from a multiple-pulse sequence effecting a heteronuclear planar coupling Hamiltonian. The building blocks are incorporated into heteronuclear correlation experiments, in conjunction with coherence selection by the formation of a heteronuclear gradient echo. This allows for efficient water suppression without the need for water presaturation. The methods are demonstrated in HSQC-type experiments on a sample of a decapeptide in H2O. The novel pulse sequence elements can be incorporated into multidimensional experiments.

Practical aspects of the E.COSY technique. Measurement of scalar spin-spin coupling constants in peptides

Abstract Practical aspects of the E.COSY technique for measurement of coupling constants are discussed. Guidelines are presented for the experimental setup and for spectral assignments. The features of E.COSY cross-peak multiplet patterns are illustrated by experimental spectra of valine, phenylalanine, and proline residues in the decapeptide antamanide.

Elimination of spectral distortion in polarization transfer experiments. Improvements and comparison of techniques

Abstract Three modified polarization transfer techniques, INEPT+, DEPT+, and DEPT++, are introduced for the elimination of distortions in proton-coupled spectra of low-gyromagnetic-ratio spins. The existing polarization transfer techniques are analyzed in view of their susceptibility to intensity, multiplet, and phase anomalies.

Low-pass J filters. Suppression of neighbor peaks in heteronuclear relayed correlation spectra

A procedure is described for the selective removal of NMR signals assocd. with coupling consts. (J) exceeding a lower limit Jmin. The procedure is called qlow-pass J filteringq since it leaves signals assocd. with small J couplings essentially unaffected. The procedure is explained for the suppression of neighbor peaks in heteronuclear relayed correlation spectra. The efficiency of the technique is demonstrated by using the relayed spectra of glucose in an equilibrated mixt. of a and b anomers. [on SciFinder (R)]

Three-dimensional Fourier spectroscopy. Application to high-resolution NMR

Abstract The principles of three-dimensional Fourier spectroscopy, applied to high-resolution NMR, are developed. The main emphasis is placed on the design of experimental techniques, the appearance of 3D spectra in terms of multiplet structures and peak shapes, information content, and assignment procedures. The sensitivity of 3D experiments is analyzed. The potential of 3D experiments is discussed in view of applications to biomolecules.

Two-dimensional NMR spectroscopy with soft pulses

Two-dimensional spectroscopy is today routinely used in order to facilitate the assignment of resonance lines in NMR spectra and to determine molecular structure (1, 2). Homonuclear correlation spectroscopy (COSY) and NOE spectroscopy (NOESY), for example, are the two pivotal methods in the structural analysis of biological macromolecules in solution (3). Whenever a detailed analysis of the cross-peak multiplet structure is desired, e.g., for the quantitative determination of coupling constants, high-resolution 2D spectra are required. High resolution may also favor sensitivity in cases where the cross peaks are antiphase as in COSY spectra. High-resolution 2D spectroscopy is, however, often faced with practical limitations of data storage and excessive measurement time caused by the large number of required t, experiments. We propose in this communication to employ frequency-selective soft radiofrequency pulses in order to circumvent the mentioned limitations. When excitation and detection are focused to a small area of the 2D spectrum, high resolution can be achieved by concentrating all available data points to this area. O ther selective experiments derived from 2D spectroscopy have recently been proposed by Kessler et al. (4). These experiments result, however, in one-dimensional spectra corresponding to cross sections through 2D spectra. The experiment described in this communication, on the other hand, delivers true 2D spectra of restricted frequency ranges. Two types of frequency selectivity in 2D spectra are illustrated in Fig. 1. In Fig. 1 a a restricted frequency range A& is excited in o1 by selective preparation pulses resulting in a band parallel to the w2 axis. If it is desirable to restrict also the frequency range in w2, selective pulses can be. incorporated into the mixing sequence in order to allow coherence transfer only between the ranges AQ, and Aa2 (Fig. lb). Alternatively, w2 selectivity can be achieved by means of audiofrequency filters; however, in some cases it is achieved at the expense of sensitivity as will become clear in the following. Frequency selection with soft pulses throughout the entire pulse sequence offers in addition the possibility to interleave experiments for selection of different nonoverlapping spectral regions thus avoiding relaxation delays between experiments. It is evident that the principles above are applicable to all types of 2D experiments.

Conformation of alamethicin in oriented phospholipid bilayers determined by (15)N solid-state nuclear magnetic resonance.

The conformation of the 20-residue antibiotic ionophore alamethicin in macroscopically oriented phospholipid bilayers has been studied using (15)N solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with molecular modeling and molecular dynamics simulations. Differently (15)N-labeled variants of alamethicin and an analog with three of the alpha-amino-isobutyric acid residues replaced by alanines have been investigated to establish experimental structural constraints and determine the orientation of alamethicin in hydrated phospholipid (dimyristoylphosphatidylcholine) bilayers and to investigate the potential for a major kink in the region of the central Pro(14) residue. From the anisotropic (15)N chemical shifts and (1)H-(15)N dipolar couplings determined for alamethicin with (15)N-labeling on the Ala(6), Val(9), and Val(15) residues and incorporated into phospholipid bilayer with a peptide:lipid molar ratio of 1:8, we deduce that alamethicin has a largely linear alpha-helical structure spanning the membrane with the molecular axis tilted by 10-20 degrees relative to the bilayer normal. In particular, we find compatibility with a straight alpha-helix tilted by 17 degrees and a slightly kinked molecular dynamics structure tilted by 11 degrees relative to the bilayer normal. In contrast, the structural constraints derived by solid-state NMR appear not to be compatible with any of several model structures crossing the membrane with vanishing tilt angle or the earlier reported x-ray diffraction structure (Fox and Richards, Nature. 300:325-330, 1982). The solid-state NMR-compatible structures may support the formation of a left-handed and parallel multimeric ion channel.

Spin-state-selective coherence transfer via intermediate states of two-spin coherence in IS spin systems: Application to E.COSY-type measurement of J coupling constants

It is demonstrated that a new pulse sequence element, Spin-State-SelectiveCoherence Transfer (S3CT), via an intermediate state ofheteronuclear IS zero- or double-quantum coherence can transfer the twosingle-quantum coherences on one of the spins exclusively to any one of thetwo single-quantum coherences on the other spin. This fact is used for editinginto two subspectra that are most suitable for extraction of homo- orheteronuclear J coupling constants when S3CT is combined withhomonuclear coherence transfer during a mixing period. Experimentalconfirmation is obtained using a 15N-labeled 58-residue protein,the C-terminal Kunitz domain from human type VI collagen. The J coupling con-stants determined include 3JHN-Hα and3JN-Hβ related to the φ andχ1 angles, respectively.

Uniform excitation of multiple-quantum coherence: Application to multiple quantum filtering

Abstract A technique for the uniform excitation of multiple-quantum coherence is described. It is based on a symmetrical excitation/detection sequence and permits coaddition of results for varied preparation time τ. It can be applied to multiple-quantum spectroscopy to obtain spectra with interpretable intensities. Its usage to improve the performance of multiple-quantum filter; is demonstrated experimentally. Modulation of the multiple-quantum time period allows even more selective filtering.

Application of ω1-decoupled 2D correlation spectra to the study of proteins

Abstract The utility of ω 1 decoupling for the elucidation of two-dimensional proton magnetic resonance spectra of proteins is demonstrated. The technique previously proposed for correlation spectroscopy (COSY) is analyzed and extensions to other types of two-dimensional spectroscopy such as spin-echo correlated spectroscopy (SECSY) and relayed correlation spectroscopy are proposed. It is further shown that by incorporating a z filter into the ω 1 -decoupled SECSY experiment, a two-dimensional spectrum is obtained in which all peaks are in phase and have absorption lineshapes in both dimensions.