Conformational distributions powder patterns

This chapter considers the distribution of spin Hamiltonian parameters and their relation to conformational distribution of biomolecular structure. Distribution of a g-value or g-strain leads to an inhomogeneous broadening of the resonance line. Just like the g-value, also the linewidth, W, in general, turns out to be anisotropic, and this has important consequences for powder patterns, that is, for the shape of EPR spectra from randomly oriented molecules. A statistical theory of g-strain is developed, and it is subsequently found that a special case of this theory (the case of full correlation between strain parameters) turns out to properly describe broadening in bioEPR. The possible cause and nature of strain in paramagnetic proteins is discussed. 

Owing to the orientation dependence that it imparts to the NMR frequency, the chemical-shift anisotropy (CSA) has proven useful not only in studies of slow dynamics but also for characterizing segmental orientation distributions and fast segmental reorientations. While static powder patterns provide this CSA information in the most accessible form, site resolution by MAS is indispensable in all but the simplest unlabeled systems. The two requirements can be combined in two-dimensional (2D) separation experiments. Recently, a robust sequence, termed separation of undistorted powder-patterns by effortless recoupling (SUPER), was introduced that makes CSA measurements under standard MAS conditions routine.28 It enables identification of functional groups and measurements of orientation distributions, segmental dynamics, and conformations. 

It is known that polyoxymethylene in the crystalline phase takes the all gauche conformation with a 9/5 helix [21]. However, the amorphous phase has a distribution of gauche and frans-conformations. Figures 7.10 and 7.11 show the CPMAS NMR spectra and powder pattern spectra of polyoxymethylene, respectively [22]. Sample A is a polyoxymethylene single crystal. To produce Sample B, Sample A is heated to 200°C and then quenched in ice water. Sample C is a melt-quenched sample of bulk polyoxymethylene and sample D is a bulk polyoxymethylene heated and cooled at a rate of... 

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