Experiments Probing Phase Transition Order

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. 

In this technique, the local dipolar interaction of individual protons in the liquid crystalline state can be obtained via resonances in the isotropic phase [51-53]. A phase transition from a nematic to an isotropic phase is completed rapidly within the spin-lattice relaxation times of nuclei by applying a pulsed microwave. By this method, homonuclear dipolar interactions associated with individual protons can be separately observed without applying a multiple pulse sequence, and hence a detailed information on the microscopic order parameters, geometry of different chemical groups and IH spin networks can be obtained. In particular, recent technical improvements in the microwave temperature jump probe have realized a transition in even less than 10 ms [52, 53], and enable us to obtain simpler dipolar patterns. In this Chapter, SC-2D NMR experiments between the nematic and the isotropic phase of liquid crystalline samples are described, in which well separated dipolar pattern for individual protons are observed as cross sectional spectra. Besides, this technique can also provide spin diffusion pathways among proton spin networks. These pieces of information provide insights into the microscopic order of liquid crystalline materials. 

MD-EPR approach has also been sueeessfully applied to study the dy-namies and ordering of the molecules in the bulk phases of soft matter systems sueh as nematic liquid crystals nCB doped with nitroxide spin probes. MD simulations have been reported at both coarsegrained and fully atomistic levels. Predicted ehanges in molecular order, dynamics and variable temperature EPR line shapes across the nematic (N) to isotropic (I) phase transitions showed excellent agreement with experiment. A combined MD-EPR approach provides a new level of detail to descriptions of molecular motions and order. Figure 7 shows snapshots of isotropie (top) and nematic (bottom) states of 8CB with doped CLS spin probe. It also presents comparison between predicted and measured EPR spectra of 8CB along the N-I phase transition curve... 

The ability to accurately compute kinetic isotope effects (KIEs) for chemical reactions in solution and in enzymes is important because the measured KIEs provide the most direct probe to the nature of the transition state and the computational results can help rationalize experimental findings. This is illustrated by the work of Schramm and co-workers, who have used the experimental KIEs to develop transition state models for the enzymatic process catalyzed by purine nucleoside phosphorylase (PNP), which in turn were used to design picomolar inhibitors. In principle, Schramm s approach can be applied to other enzymes however, in order to establish a useful transition state model for enzymatic reactions, it is often necessary to use sophisticated computational methods to model the structure of the transition state and to match the computed KIEs with experiments. The challenge to theory is the difficulty in accurately determining the small difference in free energy of activation due to isotope replacements, especially for secondary and heavy isotope effects. Furthermore, unlike studies of reactions in the gas phase, one has to consider... 

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