Spin labeling orientational ordering

Spin-labelling of free, or cell-surface, sialic acids has been used in order to obtain information about the rate of rotational orientation of the label after attachment to macromolecules this knowledge is important in the investigation of the orientation and mobility of sialogly-coproteins in, for example, cell membranes. In a first approach, the label was introduced into the carboxyl groups by a carbodiimicle-me-diated, amidation procedure.177 This method is, however, not specific... 

A spin label such as l(m,n), as indicated in Fig. 8.14 is used for the determination of the degree of orientation. The so called order parameter can be estimated by the anisotropic ESR spectra of spin label 1(13,2) in an oriented sample of smectic liquid crystal, as shown in Fig. 8.15 [19]. There is a dramatic difference in the hyperfine splitting and position of the spectrum, when the axis of the long chains is oriented parallel and perpendicular to the direction of the applied magnetic field. 

Chemical-shift anisotropy is very sensitive to molecular structure and dynamics. Each nucleus can be pictured as being surrounded by an ellipsoidal chemical-shift field, A, arising from the influences of neighboring spins, as described by Eq. (4). If the molecules in the sample have no preferred orientational order, these tensors will be randomly distributed, and the line-shape is predictable. If the shielding is equivalent in all directions = (5yy = zi, A is spherical), a symmetric peak, like shown that in Fig. 29a, will be observed at qjso, which is defined in Eq. (5). Axial symmetry = Gyy A is, more or less, football-shaped) results in a powder pattern like that shown in Fig. 29b. In this case, the tensor elements may be labeled CTy ) and (g x and Gj ). If there is no symmetry in the chemical-shift field (gxx is a flattened football), then the... 

Fig. 15. Population distribution of the instant orientation of the effective nitroxide rotational tensor symmetry axis with respect to the director as characterized by the ordering potential parameters c, C, and determined from the ESR spectra of spin-labeled PHEMA in methanol at 50 wt% concentration measured at (a) 191 K, (b) 221 K, and (c) 249 K. The magnitude of the ordering potential decreases with increasing temperature as seen in Fig. 14. (From Ref. 44, with permission.)...

Analysis of the shape of the orienting potential, which constrains the preferred orientation of the effective axis of internal rotation of the tethered nitroxide, revealed the presence of two different conformations of the spin-label moiety. In order to explain these results, the quantum chemical procedure was utilized. The spin-label moiety in the polymer system was reduced to 4-acetamido-2,2,6,6-tetramethylpiperidine-l-yloxyl (II) (spin label plus tether). The molecule was subjected to a search for stable confonn-ers, which resulted in the two minima corresponding to the structures Da and nb." ... 

A 2D MAS experiment that provides information about orientational order based on spin-1/2 chemical shift anisotropies was introduced by Harbison and other workers [26,47]. Compared to NMR spectroscopy of selectively deuterated liquid crystals, no isotopic labeling is required for this experiment. Spinning side-band spectra of oriented samples show variations in the phases and intensities of the spinning sidebands, which depend on the degree of order present and the position of the rotor at the start of acquisition of the NMR signal. These variations can be translated into a 2D side-band pattern by making the initial rotor position a function of the evolution time ti of a rotor-synchronized 2D NMR experiment. From the side-band intensities of the 2D spectrum, the moments of the orientational distribution function can be extracted. 

An example of typical ESR spectra, measured in the first derivative mode, is shown in Fig. 12. Just like NMR, ESR can be used to detect phase transitions and to study the orientation and dynamics of liquid crystals. The spectra shown in Fig. 12, for example, are from a study comparing the dynamics of the spin label at the end of the polymer chain and the freely dissolved spin probe in a liquid-crystalline polyether by continuous wave ESR (Fig. 12) and 2D Fourier transform ESR experiments [137]. The end label showed smaller ordering and larger reorientational rates than the dissolved spin probe. Furthermore, it was demonstrated that the advanced 2D FT ESR experiments (see below) on the end-labeled polymer chain could not be explained by the conventional Brownian model of reorientation, although this model could explain the ID spectra. This led to the development of a new motional model of a slowly relaxing local structure, which enabled differentiation between the local internal modes experienced by the end label and the collective reorganization of the polymer molecules around the label. The latter was shown to be slower by two orders of magnitude. 

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