The primary aim of the Laboratory for Biomolecular NMR Spectroscopy is to study structure and dynamics for biological macromolecules using a combination of solid- and liquid-state NMR spectroscopy. This research represents an integral part of a large activity at the University of Aarhus using a combination of protein chemistry, gene expression, macromolecular X-ray diffraction, and NMR spectroscopy to study structure/function relationships for proteins. The NMR work covers in particular peptides/proteins immobililized by polymerization, aggregation, or membrane association. To achieve this goal the research covers methodology development, experiment setup/implementation on model samples, and applications involving studies of, e.g., membrane proteins (ion channels), active metal binding sites, biomineralization, colloid formation etc.

The following sections exemplify these aspects by describing some recent research projects of the group. Although described separately, the methodology and application project represent an unity upon which the coming research of the laboratory is based.

NMR Methodology

The solid-state NMR methodology work primarily focus on the design of efficient methods which through appropriate high-order re- and decoupling of anisotropic nuclear spin interactions tailor the Hamiltonian to accomplish optimum coherence transfer in complicated nuclear spin systems. Recently, this has lead to the design of pulse sequences for homonuclear dipolar recoupling and 2Q filtration (e.g., rotary resonance, HORROR, C7, POST-C7), heteronuclear dipolar recoupling (e.g., GATE), combined dipolar and anisotropic shielding recoupling (e.g., MSD-HORROR, rotary resonance type experiments), homonuclear dipolar decoupling (e.g., MSHOT-3, HOT-FSLG), and spectral editing (e.g., CPD and CPDR). A variety of these experiments represents important building blocks in pulse sequences for solid-state NMR structure determination of peptides/proteins in rotating and uniaxially oriented samples. The most recent methods have been designed using new concepts for high-order average Hamiltonian theory (the semi-continuous Baker-Campbell-Hausdorff (scBCH) expansion) and an advanced computer simulation package (SIMulation Package for SOlid-state Nmr spectroscopy: SIMPSON) developed in this laboratory. For integer quadrupolar nuclei we have recently demonstrated new variants of 2D MAS satellite transition NMR experiments (2H) developed using an advanced Lie algebra formalism. For half-integer quadrupolar nuclei with large quadrupole couplings we have recently introduced a series of new pulse sequences exploiting QCPMG sampling of the signal to improve the sensitivity of quadrupolar-echo experiments by more than an order of magnitude. These experiments (QCPMG, QCPMG-MAS, MQ-QCPMG-MAS) open up new possibilities to study the structure and dynamics for, e.g., metalloproteins.

Development of Liquid-State NMR Pulse Sequences

A considerable part of our research activities focus on systematic development of optimum pulse sequences for liquid-state NMR studies of proteins. The methodology development is based on unitary bounds on spin dynamics (which provides information about the maximum transfer efficiency for given coherence transfer processes and the unitary propagator, i.e., the experiment accomplishing this) as well as numerical and analytical approaches for systematic experiment design. The latter involves software which using a gradient based approach determines the unitary bound for the relevant coherence transfer process and software which based on this information in an iterative fashion designs the optimum experiment. Using this setup (analytical or numerical) it is straightforward to test whether existing (if any) experimental procedures are optimal and if not design experiments providing the largest possible coherence transfer with the desired selectivity concerning initial and target spin operators. That this method is generally applicable has recently been demonstrated, for example, by the development of INADEQUATE CR (which double the sensitivity relative to the popular INADEQUATE experiment) and in- and antiphase COS HSQC experiments for sensitivity optimized Coherence Order Selective (COS) heteronuclear coherence transfer in IS, I2S, and I3S spin systems. As another example, we have recently developed Coherence-Order- and Spin-State-Selective (COS3) HSQC experiments providing the theoretical maximum transfer efficiency as well as improved spectral resolution by transferring all coherence into one of four spectral lines.

Software Development

The construction of efficient numerical procedures for efficient simulation of solid- and liquid-state NMR pulse experiments and spectra represents a fundamental part of our research. This applies both to the methodology projects described above but also to the more application-oriented projects as a tool for extraction of structural parameters from solid-state NMR spectra and for the relation of these to molecular structure and dynamics. Besides a large number of specialized simulation programs and procedures for fast calculation of solid-state NMR powder spectra, this has recently led to the software package SIMPSON (SIMulation Package for SOlid-state Nmr spectroscopy) which represents a flexible "computer spectrometer" enabling simulation of essentially all types of multiple-spin, multiple-pulse, multiple-dimension solid-state NMR experiments and their associated spectra.

Development of Quantum Mechanical Tools

Another important element in our strategy for systematic design of optimum NMR methods is the development of quantum mechanical tools which provide analytical information about details of the spin dynamics. This information can be the effective (i.e., time independent "average") Hamiltonian for a given pulse sequence (or pulse sequence element) or information about the theoretical maximum efficiency for a given coherence transfer process (or for the scaling factor for the effective Hamiltonian). In this context we have have recently introduced the "the semi-continuous Baker-Campbell-Hausdorff (scBCH) expansion" to calculate the effective Hamiltonian for multiple-pulse experiments to high order and had a number of projects concerning "unitary bounds on quantum dynamics" which determine the theoretical upper limit for any relevant coherence or polarization transfer process.

Hardware Development

The group has been involved in several hardware development projects enabling specific solid-state NMR experiments to be performed under optimum conditions. This involves, for example, design of solid-state NMR probes with resonator coils to improve rf inhomogeneity and flat-coil NMR probes with effective hydration control for studies of peptides/proteins uniaxially oriented in phospholipid bilayer membranes.

Application Projects