A four-pulse version of the pulse double electron–electron resonance (DEER) experiment is presented, which is designed for the determination of interradical distances on a nanoscopic lengthscale. With the new pulse sequence electron–electron couplings can be studied without dead-time artifacts, so that even broad distributions of electron–electron distances can be characterized. A version of the experiment that uses a pulse train in the detection period exhibits improved signal-to-noise ratio. Tests on two nitroxide biradicals with known length indicate that the accessible range of distances extends from about 1.5 to 8 nm. The four-pulse DEER spectra of an ionic spin probe in an ionomer exhibit features due to probe molecules situated both on the same and on different ion clusters. The former feature provides information on the cluster size and is inaccessible with previous methods.

A novel class of metal organic frameworks (MOFs) has been synthesized from Cu-acetate and dicarboxylic acids using liquid phase epitaxy. The SURMOF-2 isoreticular series exhibits P4 symmetry, for the longest linker a channel-size of 3 × 3 nm² is obtained, one of the largest values reported for any MOF so far. High quality, ab-initio electronic structure calculations confirm the stability of a regular packing of (Cu⁺⁺)₂- carboxylate paddle-wheel planes with P4 symmetry and reveal, that the SURMOF-2 structures are in fact metastable, with a fairly large activation barrier for the transition to the bulk MOF-2 structures exhibiting a lower, twofold (P2 or C2) symmetry. The theoretical calculations also allow identifying the mechanism for the low-temperature epitaxial growth process and to explain, why a synthesis of this highly interesting, new class of high-symmetry, metastable MOFs is not possible using the conventional solvothermal process.

Measurement of distances with the Double Electron–Electron Resonance (DEER) experiment at X-band frequencies using a pair of nitroxides as spin labels is a popular biophysical tool for studying function-related conformational dynamics of proteins. The technique is intrinsically highly precise and can potentially access the range from 1.5 to 6–10 nm. However, DEER performance drops strongly when relaxation rates of the nitroxide spin labels are high and available material quantities are low, which is usually the case for membrane proteins reconstituted into liposomes. This leads to elevated noise levels, very long measurement times, reduced precision, and a decrease of the longest accessible distances. Here we quantify the performance improvement that can be achieved at Q-band frequencies (34.5 GHz) using a high-power spectrometer. More than an order of magnitude gain in sensitivity is obtained with a homebuilt setup equipped with a 150 W TWT amplifier by using oversized samples. The broadband excitation enabled by the high power ensures that orientation selection can be suppressed in most cases, which facilitates extraction of distance distributions. By varying pulse lengths, Q-band DEER can be switched between orientationally non-selective and selective regimes. Because of suppression of nuclear modulations from matrix protons and deuterons, analysis of the Q-band data is greatly simplified, particularly in cases of very small DEER modulation depth due to low binding affinity between proteins forming a complex or low labelling efficiency. Finally, we demonstrate that a commercial Q-band spectrometer can be readily adjusted to the high-power operation.