Gd3+ spin labeling for distance measurements by pulse EPR spectroscopy

Methods for measuring nanometer scale distances between specific sites in biomolecules (proteins and nucleic acids) and their complexes are essential for describing and analyzing their structure and function. In the last decade pulse EPR techniques were proven very effective for measuring distances between two spin labels attached to a biomolecule. The most commonly used spin labels for such measurements are nitroxide stable radicals. Recently, a new family of spin labels, based on Gd3+ chelates, has been introduced to overcome some of the limitations of using nitroxides, particularly at high magnetic fields, which are attractive due to the increased sensitivity they offer. The benefits that such S = 7/2 spin labels offer for frequencies of 30 GHz and higher, particularly at 95 GHz, include (1) high sensitivity, only similar to 0.15 nmol of doubly labeled biomolecule is needed, (2) the lack of orientation selection, which allows straightforward data analysis. Gd3+-Gd3+ DEER (double electron-electron resonance) distance measurements on labeled peptides, proteins and DNA have already been demonstrated and the results show that they are very promising in terms of sensitivity. In this Perspective we review these new developments. We briefly introduce the characteristics of the DEER experiment on a pair of S = 1/2 spins and characterize the EPR spectroscopic properties of Gd3+ ions. We then introduce some of the tags employed to attach Gd3+ to biomolecules and provide a few experimental examples of Gd3+-Gd3+ DEER measurements. This is followed by a discussion of the parameters that affect the sensitivity of such DEER measurements. Since an important term in the spin Hamiltonian of Gd3+ is the zero-field splitting (ZFS), its effect on the DEER modulation frequencies must be considered and this is discussed next. Finally, another recently reported approach for using Gd3+ in distance measurements will be presented: the use of Gd3+-nitroxide pairs.