Gd^{3 +} Spin Labeling for Measuring Distances in Biomacromolecules: Why and How?

Applications of distance measurements by pulse dipolar electron-paramagnetic resonance (PD-EPR) spectroscopy to structural biology are based on introducing spin labels (SLs) at well-defined locations in the biomacromolecule. The most commonly used SLs are nitroxyl radicals, but recently SLs based on high-spin Gd^{3 +} (S = 7/2) complexes have been shown to be an attractive alternative for PD-EPR, particularly double electron-electron resonance (DEER), at spectrometer frequencies higher than 30 GHz. In this chapter, we describe the advantage of using this new family of SLs in terms of sensitivity, stability, and chemical diversity. We present current labeling strategies for proteins, discuss the approximations under which DEER data analysis of a pair of Gd^{3 +} SLs (GdSLs) is equivalent to that of a pair of S = 1/2 SLs, and discuss the reduction in multispin effects in a cluster of GdSLs, as opposed to a cluster of nitroxide labels, which can be found in oligomeric systems. In addition, we provide a brief overview of the current, rather limited, knowledge of Gd^{3 +} phase relaxation behavior and describe experimental strategies in terms of optimizing sensitivity. The possibility of using several types of SLs in a system allows one to isolate effects due to the chemical nature of the SL itself; several such examples are presented, focusing on comparing nitroxide and GdSLs. Finally, we will discuss the initial results on in-cell DEER with GdSLs.