The function of biomacromolecules is controlled by their structure and conformational flexibility. Investigating the structure of biologically important macromolecules can, therefore, yield information that could explain their complex biological function. In addition to X ray crystallography and nuclear magnetic resonance (NMR) methods, pulsed electron paramagnetic resonance (EPR) methods, in particular the pulsed electron electron double resonance (PELDOR) technique has, during the last decade, become a valuable tool for structural determination of macromolecules. Long range distance constraints obtained from pulsed EPR measurements, make it possible to carry out structural refinements on structures from NMR and X ray methods. In addition, EPR yields distance distributions that give information about structural flexibility. The use of EPR for structural studies of biomacromolecules requires in most cases site specific incorporation of paramagnetic centres known as spin labelling. To date, spin labelling nucleic acids has required complex spin labelling chemistry. The first application of a site directed and noncovalent spin labelling method for distance measurements on DNA is described. It is demonstrated that noncovalent spin labelling with a rigid spin label can afford detailed information on internal DNA dynamics using PELDOR. Furthermore, it is shown that noncovalent spin labelling can be used to study DNA protein complexes. PELDOR can also yield information about spin label orientation. Therefore, spin labels with limited flexibility can be used to measure the relative orientation of the spin labelled sites. Although information on orientation can be obtained from 9.7 GHz PELDOR measurements in selected applications, measurements at 97 GHz or higher, increases orientation selection. It is shown that PELDOR measurements on semi rigid and rigid nitroxide biradicals using a home built high power 97 GHz EPR spectrometer (Hiper) and model based simulations yield quantitative information on spin label orientations and dynamics. The most widely used spin labels for EPR studies on biomacromolecules are the aminoxyl (nitroxide) radicals. The major drawbacks of nitroxide spin labels include low sensitivity for distance measurements, fast spin spin relaxation in solution and limited stability in reducing environments. Carbon centered triarylmethyl (trityl) radicals have properties that could eliminate some of the limitations of nitroxide spin labels. To evaluate the use of trityl spin labels for nanometer distance measurements, models systems with trityl and nitroxide spin labels were measured using PELDOR and Double Quantum Coherence (DQC). This study shows that trityl spin labels yield reliable information on interlabel distances and dynamics, establishing the trityl radical as a viable spin label for structural studies on biomacromolecules.

The function of G protein-coupled receptors is intrinsically linked to their conformational dynamics. In conjugation with site-directed spin labeling, electron paramagnetic resonance (EPR) spectroscopy provides powerful tools to study the highly dynamic conformational states of these proteins. Here, we explored positions for nitroxide spin labeling coupled to single cysteines, introduced at transmembrane, intra- and extra-cellular sites of the human neuropeptide Y2 receptor. Receptor mutants were functionally analyzed in cell culture system, expressed in Escherichia coli fermentation with yields of up to 10 mg of purified protein per liter expression medium and functionally reconstituted into a lipid bicelle environment. Successful spin labeling was confirmed by a fluorescence assay and continuous wave EPR measurements. EPR spectra revealed mobile and immobile populations, indicating multiple dynamic conformational states of the receptor. We found that the singly mutated positions by MTSL ((1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl) methyl methanesulfonothioate) have a water exposed immobilized conformation as their main conformation, while in case of the IDSL (bis(1-oxyl-2,2,5,5-tetramethyl-3-imidazolin-4-yl) disulfide) labeled positions, the main conformation are mainly of hydrophobic nature. Further, double cysteine mutants were generated and examined for potential applications of distance measurements by double electron–electron resonance (DEER) pulsed EPR technique on the receptor.