Dynamics of nitroxide probes linked to flexible chains

The motional processes of nitroxide spin labels, linked by alkyl chains to macromolecules in nonviscous solvents, are analyzed by considering the conformational transitions of the chain superimposed on the overall reorientation of the bulky moiety. The transition rates for the configurational jumps are evaluated according to the rotational isomeric state (RIS) approximation, after a suitable discretization of the multidimensional diffusion equation defined in terms of the internal degrees of freedom. The effect of the overall motion is taken into account by the inclusion of random jumps among different orientations of the macromolecules. The calculations are performed with reference to spin-labeled polypeptides undergoing helix-coil transitions. The results show that, in the case of relatively short chains, the ESR spectral profiles are particularly affected by the slow overall motions of the macromolecules, which cause random fluctuations of the residual anisotropies, incompletely averaged by the fast conformational motions. In order to check the validity of the RIS procedure, the average values of angular functions, obtained by the RIS model, have been compared with those calculated with the full multidimensional distribution function, by using a Monte Carlo approach.