1. Introduction 3721.1 Residual dipolar couplings as a route to structure and dynamics 3721.2 A brief history of oriented phase high resolution NMR 3742. Theoretical treatment of dipolar interactions 3762.1 Anisotropic interactions as probes of macromolecular structure and dynamics 3762.1.1 The dipolar interaction 3762.1.2 Averaging in the solution state 3772.2 Ordering of a rigid body 3772.2.1 The Saupe order tensor 3782.2.2 Orientational probability distribution function 3802.2.3 The generalized degree of order 3802.3 Molecular structure and internal dynamics 3813. Inducing molecular order in high resolution NMR 3833.1 Tensorial interactions between the magnetic field and anisotropic magnetic susceptibilities 3833.2 Dilute liquid crystal media: a tunable source of order 3843.2.1 Bicelles : from membrane mimics to aligning media 3853.2.2 Filamentous phage 3873.2.3 Transfer of alignment from ordered media to macromolecules 3883.3 Magnetic field alignment 3893.3.1 Paramagnetic assisted alignment 3893.3.2 Advantages of using magnetic alignment 3894. The measurement of residual dipolar couplings 3914.1 Introduction 3914.2 Frequency based methods 3924.2.1 Coupling enhanced pulse schemes 3924.2.2 In phase anti-phase methods (IPAP): 1DNH couplings in proteins 3934.2.3 Exclusive correlated spectroscopy (E-COSY): 1DNH,
1DNC′ and 2DHNC′ 3954.2.4 Extraction of splitting values from the frequency domain 3964.3 Intensity based experiments 3974.3.1 J-Modulated experiments: the measurement of
1DCαHα in proteins 3974.3.2 Phase modulated methods 3994.3.3 Constant time COSY – the measurement of DHH couplings 3994.3.4 Systematic errors in intensity based experiments 4005. Interpretation of residual dipolar coupling data 4015.1 Structure determination protocols utilizing orientational constraints 4015.1.1 The simulated annealing approach 4015.1.2 Order matrix analysis of dipolar couplings 4025.1.3 A discussion of the two approaches 4025.2 Reducing orientational degeneracy 4035.2.1 Multiple alignment media in the simulated annealing approach 4045.2.2 Multiple alignment media in the order matrix approach 4055.3 Simplifying effects arising due to molecular symmetry 4065.4 Database approaches for determining protein structure 4076. Applications to the characterization of macromolecular systems 4086.1 Protein structure refinement 4086.2 Protein domain orientation 4096.3 Oligosaccharides 4136.4 Biomolecular complexes 4156.5 Exchanging systems 4167. Acknowledgements 4188. References 419Within its relatively short history, nuclear magnetic resonance (NMR) spectroscopy has
managed to play an important role in the characterization of biomolecular structure.
However, the methods on which most of this characterization has been based, Nuclear
Overhauser Effect (NOE) measurements for short-range distance constraints and scalar
couplings measurements for torsional constraints, have limitations (Wüthrich, 1986). For
extended structures, such as DNA helices, for example, propagation of errors in the short
distance constraints derived from NOEs leaves the relative orientation of remote parts of the
structures poorly defined. Also, the low density of observable protons in contact regions of
molecules held together by factors other than hydrophobic packing, leads to poorly defined
structures. This is especially true in carbohydrate containing complexes where hydrogen
bonds often mediate contacts, and in multi-domain proteins where the area involved in
domain–domain contact can also be small. Moreover, most NMR based structural applications
are concerned with the characterization of a single, rigid conformer for the final structure.
This can leave out important mechanistic information that depends on dynamic aspects and,
when motion is present, this can lead to incorrect structural representations. This review
focuses on one approach to alleviating some of the existing limitations in NMR based
structure determination: the use of constraints derived from the measurement of residual
dipolar couplings (D).
On the properties of residual dipolar coupling alignment tensors: Simulations illuminate how residual dipolar couplings depend on the relative orientations of the magnetic field, the director of the alignment phase and the bond vectors