Correlation time filtering

In this section, some of the approaches described above for enhancing the sensitivity and information content of protein-ligand NOEs are demonstrated for relatively large protein-inhibitor complexes. In addition, we demonstrate that a medium-quality 3D X-filtered NOESY spectrum can be obtained for a large protein-inhibitor complex by using a stabilized, uniformly 13C/15N-labeled protein sample in conjunction with an elevated experimental temperature to increase the rotational correlation time of the protein-ligand complex. 

Absolute accuracy on r Correlation times available lAr = 0.01 Ar = 0.01 with emission monochromator Ar = 0.004 with emission optical filter... 

An application of the saturation-recovery filter to the suppression of signal from rigid components in bisphenol-apoly(carbonate) is shown in Fig. 7.2.2 [Hanl]. The wideline solid-echo spectrum of the phenyl deuterons exhibits a range of broad and narrow components (a) as a result of a distribution of motional correlation times. The mobile components are characterized by a shorter T than the more rigid components. Consequently the rigid components can be suppressed by partial saturation. After application of the saturation-recovery filter the shape of the wideline spectrum is dominated by the narrow signal in the centre from the mobile ring deuterons (b). 

Fig. 7.2.1 Pulse sequences for T and related magnetization filters, typical evolution curves of filtered magnetization components, and schematic filter transfer functions applicable in the slow motion regime. Note that the axes of correlation times start at Tc = Wo (a) Saturation recovery filter, (b) Inversion recovery filter, (c) Stimulated echo filter.

Heterogeneous soft matter, in particular polymer materials, are often characterized by distributions of correlation times of molecular motion. For relaxation studies of polymers, sophisticated filters have been developed which fit the classification of combination filters because they combine Tj and Tip relaxation [Gotl, Gbt2, G6t3]. These filters can be used... 

In spin-diffusion studies it is possible to detect not only two but three domain sizes. The third domain can be considered the interface (i) between the other two domains, which can be different chemical species in a polymer blend or rigid crystalline (r) and mobile amorphous (m) material in a semicrystalline polymer. To illustrate this point, a mobility timescale is depicted in Fig. 7.2.25(a) and the simplified ID domain structure of PE underneath in (b). Rigid crystalline and mobile amorphous materials exhibit motion of chain segments with different correlation times Tc. The chains at the interface between both domains exhibit intermediate mobility. The exact ranges of correlation times in the individual domains depend on the particular choice of filters. Therefore, the values of domain sizes derived through spin-diffusion NMR also depend on the type of filters used. In particular, the interface is defined solely by the NMR experiment and can only be detected if the filters are properly chosen. 

Fig. 10.3.7 Pulse sequence for spin-diffusion imaging with ID spatial resolution [Wei8] and effect of mobility filters, (a) The magnetization source is selected by the dipolar filter which suppresses the magnetization in the sink. During the spin-diffiision time the magnetization dif ses from the source to the sink, (b) The dipolar filter selects magnetization from chain segments which are highly mobile and intermediately mobile. By use of a lineshape filter the signal loss is analysed only for the mobile components. IP(Tc) is the probability for a particular correlation time to arise in the sample. It is essentially the spectral density of motion.

The time-correlated single photon counting electronics incorporate ORTEC modules including a 454 timing filter amplifier,... 

Of particular importance for detection of chemical or physical change in polymer materials are mobility filters, which are sensitive to differences in the numbers of molecules within a given window of correlation times. Within reasonable approximation such filters are relaxation filters. Here, Tj filters are sensitive to differences in the fast motion regime while T2 and Tip filters are sensitive to the slow motion regime. Which time window is of importance can be seen from Fig. 5.7 [101]. It shows a double-logarithmic plot of the mechanical relaxation strengths Hi(t) for two carbon-black filled styrene-butadiene rubber (SBR) samples as a function of the mechanical relaxation time T. They have been measured by dynamic mechanical relaxation spectroscopy. In terms of NMR, the curves correspond to spectral densities of motion. But the spectral densities relevant to NMR are mainly those referring... 

An important class of magnetization filters are mobility filters which select magnetization based on the time scale of segmental motions ((19), and references therein). The parameters for discrimination are the amplitude and characteristic frequency or the correlation time tc of molecular motions. The effect a filter exerts on a NMR signal can be represented by the filter transfer function. Examples are given in Figure 30 (163,164) with transfer function for filters, which select magnetization based on the time scale of molecular motion. 

Wall M R, Dieckmann T, Feigon J and Neuhauser D 1998 Two-dimensional filter-diagonalization spectral inversion of 2D NMR time-correlation signals including degeneracies Chem. Phys. Lett. 291 465... 

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