Pulsed electron-electron double resonance (PELDOR, also known as DEER) has become a method of choice to measure distances in biomolecules. In this work we show how the performance of the method can be improved at high EPR frequencies (94 GHz) using variable dual frequency irradiation in a dual mode cavity in order to obtain enhanced resolution toward orientation selection. Dipolar evolution traces of a representative RNA duplex and an α-helical peptide were analysed in terms of possible bi-radical structures by considering the inherent ambiguity of symmetry-related solutions.
A new tool: The amino acid 4‐(3,3,5,5‐tetramethyl‐2,6‐dioxo‐4‐oxylpiperazin‐1‐yl)‐L‐phenylglycine (TOPP), which has a rigid nitroxide spin label, can be used for EPR‐based distance measurements in peptides. The key feature of the design is the defined orientation of the nitroxide in space with respect to the peptide backbone. EPR measurements provide evidence for the low conformational flexibility of the TOPP label.
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