Morphology of three lyotropic liquid crystalline biological NMR media studied by translational diffusion anisotropy.

The morphologies of three dilute liquid crystalline phases, which are widely used for biological NMR spectroscopy, are investigated by the study of tracer self-diffusion. The aqueous liquid crystalline media investigated include the common phospholipid bicelle medium, a phase consisting of a mixture of pentaethyleneglycol mono dodecyl ether and hexanol, and a medium containing cetylpyridinium bromide and hexanol. Threonine and water were used as tracer molecules for probing the aqueous environment, and tetramethylsilane (TMS) was for probing the lipophilic environment. Pulsed field gradient NMR was used to measure tracer self-diffusion rates in three orthogonal directions. Although results for the water-soluble tracers in bicelle media do not contradict the widely accepted disk-shaped bicelle model, the high TMS diffusion rate observed in the bilayer plane requires extensive transient edge-to-edge contacts of such disks. This morphology is essentially that of a heavily perforated lamellar bilayer phase and explains why this medium remains liquid crystalline well below the Onsager limit for disk-shaped nematogens. Below 25 degrees C, a bicelle mixture consisting of dimyristoyl phosphatidyl choline and dihexanoyl phosphatidyl choline remains isotropic, but tracer diffusion obstruction indicates that the particles are significantly oblate. The diffusion anisotropy in the penta(ethyleneglycol) mono dodecyl ether liquid crystals confirms the previously proposed alpha-lamellar phase. However, weak inhibition of aqueous-phase self-diffusion in the z direction points to the presence of bridge- or caplike obstructions, and the bilayers appear slightly permeable to water. If the previously proposed concentric cylinder superstructure of bilayers applies, the diffusion data indicate that the most outer cylinder must have a diameter greater than 50 microm. The tracer self-diffusion data for the cetylpyridinium bromide/hexanol medium is only compatible with a planar alpha-lamellar phase, with its local director orthogonal to the magnetic field, and a very large domain size over which the director remains parallel.

A simple apparatus for generating stretched polyacrylamide gels, yielding uniform alignment of proteins and detergent micelles.

Compressed and stretched polyacrylamide hydrogels previously have been shown to offer a robust method for aligning proteins. A simple, funnel-like apparatus is described for generating uniformly stretched hydrogels. For prolate-shaped proteins, gels stretched in the direction of the magnetic field yield two-fold larger alignment than gels compressed to the same aspect ratio in this direction. Empirically, protein alignment is found to be proportional to (c-2.3)2 [(d(o/dN)3-1], where do and dN are the diameters of the cylindrical gels before and after stretching, respectively, and c is the polyacrylamide weight fraction in percent. Low gel densities, in the 4-7% range, are found to have minimal effects on macromolecular rotational correlation times, tauc, and no effect of the compression ratio on tauc could be discerned over the range studied (do/dN < or = 1.4). Application is demonstrated for a sample containing the first Ig-binding domain of protein G, and for a detergent-solubilized peptide.

Density-functional computation of 99Ru NMR parameters

Gradient-corrected and hybrid variants of density-functional theory are used to compute the geometries and 99Ru chemical shifts of RuO4, [RuCp2], [K4Ru(CN)6], [Ru3(CO)12], [Ru(CO)3X3]- (X=Cl, I), [Ru(CO)2Cl4]2-, [Ru(bipy)3]2+, and [Ru(CO)2(iPr-DAB)(X)(Y)] [XY= Cl2, I2, MeCl, MeI, or (SnMe3)2]. For this set of compounds, substituent effects on delta(99Ru) are somewhat underestimated with the BPW91 pure density functional but are described well by the B3LYP hybrid functional, which can also be used to reproduce empirical trends in electric field gradients (EFGs) at the Ru nucleus qualitatively. In the [Ru(CO)2(iPr-DAB)XY] series, trends in the computed EFGs parallel those in the observed 99Ru NMR linewidths, in accordance with the quadrupolar relaxation mechanism expected for this nucleus. For this series of compounds, the use of X-ray-derived geometries affords a worse correlation between calculated EFGs and experimental linewidths than does the use of optimized geometries.