Dipolar interactions, long-distance

Apart from their short lifetimes, another important reason for not observing the O species is dipolar interaction. As explained later (Section IV), if the distance between one O species and another paramagnetic center (O or a metal ion) decreases, the linewidth may increase to the extent that the 0 species become EPR invisible. In this category of EPR invisible O species, there are not only the short-lived O produced according to Eq. (8), but also the long-lived O formed by heat pretreatment or adsorption, as discussed in the following section. 

It is well known that 2D NOESY is an effective method to study the three-dimensional (3D) structure of large molecules, such as proteins which have long motional correlation times.70-71 Cross-dipolar interaction peaks in a NOESY spectrum rely on the cross-relaxation of the longitudinal magnetization during the mixing time. One can extract valuable information about intermolecular distances from the intensity of the NOESY cross-peaks. The appearance of... 

It is recognized that the signal of all benzyl protons is clearly removed in deutrated samples of the benzyl group [Glu(OBzl)]n-(/7, but the signal-to-noise (S/N) ratio and the resolution are not sufficient, as shown in Fig. 30(C). In spite of adopting a very long recycle delay time of 200 s in NMR measurements (the routine recycle delay is 10 s), measurement of the appropriate spectrum of a [Glu(OBzl)]n-r/7 sample is not successful. The most likely reason for this may be a heteronuclear dipolar dipolar interaction between proton and deuterium. For instance, if it is assumed that the distance between a proton and deuterium in... 

The energy is transferred through space via dipolar interactions to the acceptor and can occur over rather long distances of up to several nanometers. Consequently, in the Forster mechanism, for a D-A dyad in a given medium, the rate of energy transfer will be dependent on the distance r separating D and A, and is expressed as... 

As mentioned above, the cross peak intensities from NOESY spectra taken at long mixing times caimot be related in a simple and direct way to distances between two protons due to spin diffusion effects that mask the actual proton distances. A possibiUty to extract such information is provided by relaxation matrix analysis that accounts for all dipolar interactions of a given proton and hence takes spin diffusion effects explicitly into consideration. Several computational procedures have been developed which iteratively back-calculate an experimental NOESY spectrum, starting from a certain molecular model that is altered in many cycles of the iteration process to fit best the experimental NOESY data. In each cycle, the calculated structures are refined by restrained molecular dynamics and free energy minimization [42,43]. 

---