Paramagnetic centers

High thermostabilizing efficiency of polyamine disulphides relative to chemically cross-linked polyethylene is conditioned by the ability to accept macroradicals at the disulphide bridge and imine group. Besides, the presence of paramagnetic centers causes the adherence of macroradicals providing for an extra stabilizing effect [49]. 

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

A resonant Orbach process occurs when the energy of the coupled vibrational modes is equal to the energy A of the first excited level of the paramagnetic center. This leads to the temperature dependence 1/Ti oc (exp(A/ BT) 1) expi- /ksT) when ksT < A. 

Generally speaking, the spin-lattice relaxation properties of a given paramagnetic center depend on several factors ... 

The strength of the coupling of the paramagnetic center to its surroundings, which is determined by the orbital part of the... 

The presence of low-lying excited levels can greatly increase the efficiency of the relaxation processes, especially in the case of paramagnetic centers with half-integer spins. 

The intrinsic relaxation rate of a paramagnetic center can he enhanced hy spin—spin coupling with a nearhy fast-relaxing species. 

The failures of CeOj alone or RhjOj alone to yield RO or 01-type paramagnetic centers, when subjected to the same experimental strategy as that which did yield such centers over RhOj/CeOj after T > 573 K, pointed to synergism between the RhO, and CeO components of the latter in producing at 300 K the dioxygen species required for 01 and for RO-type signals, possibly through the intermediacy of Rh-O-o-Ce and Rh-O-O-Ce interfacial sites. 

As mentioned in Section 9.2.3.3, paramagnetic centers can also be engineered into proteins and nucleic acids by chemical or biochemical means. The method therefore is in principle applicable to a wide range of problems. 

The trace of D vanishes when dipole coupling between paramagnetic centers determines the ZFS, since dipole interaction is traceless. A typical example is the ZFS of triplets arising from coupled radical pairs, for which SOC is negligible. For transition metal ions in contrast, SOC is the leading contribution to ZFS and the trace of Zl in general has finite values. 

Often the electronic spin states are not stationary with respect to the Mossbauer time scale but fluctuate and show transitions due to coupling to the vibrational states of the chemical environment (the lattice vibrations or phonons). The rate l/Tj of this spin-lattice relaxation depends among other variables on temperature and energy splitting (see also Appendix H). Alternatively, spin transitions can be caused by spin-spin interactions with rates 1/T2 that depend on the distance between the paramagnetic centers. In densely packed solids of inorganic compounds or concentrated solutions, the spin-spin relaxation may dominate the total spin relaxation 1/r = l/Ti + 1/+2 [104]. Whenever the relaxation time is comparable to the nuclear Larmor frequency S)A/h) or the rate of the nuclear decay ( 10 s ), the stationary solutions above do not apply and a dynamic model has to be invoked... 

Structural applications range from organic, inorganic and organometallic radicals to coordination complexes and biological macromolecules containing a paramagnetic center. 

Irradiation of high surface area silica has produced several well-defined paramagnetic centers, one of which appears to be an intrinsic defect in the... 

From Equations 11.1 and 11.2 we have seen that the strength of the dipole-dipole interaction decreases rapidly with increasing distance between two paramagnetic centers, and still we choose to call this a long-range interaction. The justification... 

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