Non-linear, continuous-wave EPR spectroscopy and spin-lattice relaxation: spin-label EPR methods for structure and dynamics

The sensitivity of continuous-wave, non-linear EPR signals to spin-lattice (T-1) relaxation has been investigated. The aim was to identify those spectral displays that are most appropriate to obtain structural and dynamic information from spin-label EPR experiments that involve detection of T-1-relaxation enhancements. This has been achieved by solving the Bloch equations for the various harmonics of the absorption and dispersion components of the spin magnetisation. Of interest are the magnetisation components that are out-of-phase with respect to the static magnetic field modulation, under conditions of partial saturation of the microwave absorption. It is found that both the first-and second-harmonic out-of-phase absorption EPR signals are particularly sensitive to T-1-relaxation. The first-harmonic absorption quadrature-phase signal is favoured for determining T-1-relaxation enhancements because of its superior intensity and relative insensitivity to transverse (T-2) relaxation. The second-harmonic absorption quadrature-phase EPR signal has lower relative intensity and is more sensitive to T-2-relaxation, but has a unique sensitivity to rotational diffusion that is exploited in saturation transfer EPR spectroscopy. The non-linear dispersion signals are less appropriate because of their principal sensitivity to T-2-relaxation and because they saturate less readily. These novel non-linear EPR spectroscopies can be contrasted with the conventional progressive saturation behaviour of the in-phase absorption signals that are determined by the T1T2 relaxation time product.