Detailed analysis of artefacts in the Cu(ii)–nitroxide RIDME experiments, related to orientation averaging, echo-crossing, ESEEM and background-correction is presented.

We describe a broadband light scattering setup for the characterization of size and refractive index of single submicron-to-micron sized aerosol particles. Individual particles are isolated in air by a quadruple Bessel beam optical trap or a counter-propagating optical tweezer. The use of very broadband radiation in the wavelength range from 320 to 700 nm covering the ultraviolet region allows to size submicron particles. We show that a broad wavelength range is required to determine the particle radius and the refractive index with an uncertainty of several nanometers and ∼ 0.01, respectively. The smallest particle radius that can be accurately determined lies around 300 nm. Wavelength-dependent refractive index data over a broad range are obtained, including the ultraviolet region where corresponding data are rare. Four different applications are discussed: (1) the sizing of submicron polystyrene latex spheres, (2) the evaporation of binary glycerol water droplets, (3) hydration/dehydration cycling of aqueous potassium carbonate droplets, and (4) photochemical reactions of oleic acid droplets.

Theoretical and experimental studies of the RIDME background reveal electron and nuclear spectral diffusion contributions.

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Fo ¨rster resonance energy transfer (FRET) and electron paramagnetic resonance (EPR) spectroscopy are complementary techniques for quantifying distances in the nanometer range. Both approaches are commonly employed for probing the conformations and conformational changes of biological macromolecules based on site-directed fluorescent or paramagnetic labeling. FRET can be applied in solution at ambient temperature and thus provides direct access to dynamics, especially if used at the single-molecule level, whereas EPR requires immobilization or work at cryogenic temperatures but provides data that can be more reliably used to extract distance distributions. However, a combined analysis of the complementary data from the two techniques has been complicated by the lack of a common modeling framework. Here, we demonstrate a systematic analysis approach based on rotamer libraries for both FRET and EPR labels to predict distance distributions between two labels from a structural model. Dynamics of the fluorophores within these distance distributions are taken into account by diffusional averaging, which improves the agreement with experiment. Benchmarking this methodology with a series of surface-exposed pairs of sites in a structured protein domain reveals that the lowest resolved distance differences can be as small as $0.25 nm for both techniques, with quantitative agreement between experimental and simulated transfer efficiencies within a range of 50.045. Rotamer library analysis thus establishes a coherent way of treating experimental data from EPR and FRET and provides a basis for integrative structural modeling, including studies of conformational distributions and dynamics of biological macromolecules using both techniques.