Ligand relaxation

Effect of Ligand Relaxation Times and Conformation on STD Intensities... 

This so-called ligand relaxation process is detected by the positive shift of the po-larographic reduction wave of Fe(acac)3. If the cation of the supporting electrolyte is R4N+, which is a very weak Lewis acid, this process does not occur. 

Internal ligand relaxation allows the removal of strain possibly imposed on the ligands by the receptor during correlation-coupled refinement but usually yields suboptimal models. Therefore, correlation-coupled receptor minimization followed by unconstrained ligand relaxation is repeated several times until a highly correlated pseudoreceptor model is obtained in the relaxed state (designated ligand equilibration). 

Mapping Pharmacophores in Ligands. Relaxed MCS approach. A rigid alignment, based on clique detection and genetic algorithm. (141)... 

Scheme VIII has the form of Scheme II, so the relaxation time is given by Eq. (4-15)—appjirently. However, there is a difference between these two schemes in that L in Scheme VIII is also a participant in an acid-base equilibrium. The proton transfer is much more rapid than is the complex formation, so the acid-base system is considered to be at equilibrium throughout the complex formation. The experiment can be carried out by setting the total ligand concentration comparable to the total metal ion concentration, so that the solution is not buffered. As the base form L of the ligand undergoes coordination, the acid-base equilibrium shifts, thus changing the pH. This pH shift is detected by incorporating an acid-base indicator in the solution.

In aqueous solution, the water molecule (L) can be replaced by other ligands including a range of biomolecules. The nature of the molecule bound affects the position and relaxation time of the dihydrogen resonance (and also JHD when D20 is present). Both the dihydrogen and the water can... 

Proteins, 109,110, 116.Seealso Enzymes Macromolecules average thermal amplitudes, MD simulations, 119 binding of ligands to, 120 dielectric relaxation time of, 122 electrostatic energies in, 122, 123-125 flexibility of, 209,221,226-227, 227 folding, 109,227... 

The possibility of experimental investigation is, of course, restricted to systems with particular ligand partition, where the number of isomers is not too big. The aim of such investigations is to obtain information about the relative probabilities of the five mechanisms which have been defined, till now, on symmetry grounds only. We think that a comparison of the experimental spectra of relaxation times with the various possible theoretical ones (see Table 3) should furnish answers to that question, provided the assumptions of the model are a good description of the physical system. 

On Table 3, it is clear that the spectra of relaxation times differ from one process to the other. But, if we progressively make the ligands equal to each other going from the partition MABCDE of Fig. 1 to the partition MA5, we expect that the differences between the relaxations due to the various processes will vanish progressively (for MA5... 

To conclude, we think that valuable information can ce obtained from such relaxation experiments. They could provide a direct, kinetic proof of the conjecture that the Berry mechanism is the most probable one, as is indicated by some recent experimental and theoretical work. The applicability of this model is however restricted to situations where the energy of the molecule does not depend on the distribution of the ligands on the skeleton and where, as a consequence, there is one rate constant for each process. If this is not true, the present description could be the first-order approximation of a perturbation calculation. Such a work will be undertaken soon. 

In the oxidized form, the weak coupling of the high-spin Fe(III) ion to its surroundings amd the very large ligand-field energy of about 10,000 cm (12) are not liable to give rise to very efficient relaxation processes (see Appendix). However, the S = 5/2 manifold provides a set of transitions for multiple direct processes that may be efficient... 

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