EPR users often face the problem of extracting information from frequently low-resolution and complex EPR spectra. Simulation programs that provide a series of parameters, characteristic of the investigated system, have been used to achieve this goal. This work describes the general aspects of one of those programs, the NLSL program, used to fit EPR spectra applying a nonlinear least squares method. Several motion regimes of the probes are included in this computational tool, covering a broad range of spectral changes. The meanings of the different parameters and rotational diffusion models are discussed. The anisotropic case is also treated by including an orienting potential and order parameters. Some examples are presented in order to show its applicability in different systems.

Electron paramagnetic resonance (EPR) has been used to monitor the interaction of bovine serum albumin (BSA) with cationic cethyltrimethylammonium chloride (CTAC) at pH 7.0. EPR results using 5-DSA and 16-DSA nitroxide spin labels show that in the presence of BSA the EPR spectra are composed of two label populations, one contacting the protein and a second one due to label localization in the micelles. Evidence is also obtained for a competition of the surfactants with the spin labels for the high affinity binding sites of the stearic acid spin labels as monitored by changes in the fraction of the two label populations as the surfactant concentration is increased. The effect of sodium dodecylsulfate (SDS) reported previously seems to be stronger in the sense that increase in SDS concentration leads to a complete transfer of spin label from close protein contact sites to the micelles while for CTAC, apparently, a significant immobilization of probe remains even at higher surfactant concentrations. EPR gives information on the dynamics inside the protein-surfactant aggregates and associated to label localization and motion. The dynamics of the nitroxide spin-labels bound to the protein correlate to the stronger binding of SDS to BSA as compared to CTAC binding. Simulation of EPR spectra for spin labels in pure CTAC micelles, in pure protein or in protein-bound micelles show rotational correlation times similar to those obtained from the simple evaluation based on the intensities of nitrogen hyperfine coupling components. Rotational correlation times obtained for 5-DSA bound to protein are larger as compared to 16-DSA values suggesting greater mobility for the later even when bound to the protein.