Chapter 1. Spectroscopically orthogonal spin labels and distance measurements in biomolecules

Abstract

Essential details of two techniques for distance measurements between non-identical spin labels are summarized. One technique is based on double electron–electron resonance (DEER) between Gd(iii) ions and nitroxide radicals. The other technique is based on indirect measurements of stochastic dipolar interaction between Ln(iii) ions and organic radicals via the change of longitudinal relaxation of the latter species. Combination of these techniques with double electron–electron resonance in pairs of identical spin labels (nitroxide–nitroxide or Gd(iii)–Gd(iii)) allows to suggest a new experimental strategy for multiple distance measurements in orthogonally-labelled samples. General discussion of advantages and disadvantages of the new strategy for studies of biomacromolecules and their complexes is given along with illustrative experimental examples. In particular, performance of Gd(iii)–nitroxide DEER is compared to other possible combinations of non-identical spin label pairs, while relaxation enhancement in pairs Fe(iii)–organic radical is compared to the case of Dy(iii)–nitroxide pairs.