Flexible on the outside: The functional structure of the transport protein in human blood, human serum albumin (HSA), was characterized by distance measurements with double electron–electron resonance (DEER) spectroscopy on spin‐labeled fatty acids that are bound to HSA. The functional protein structure derived has a more rigid inner core, while the surface of the protein shows much greater structural flexibility.

Intrinsically disordered proteins (IDPs) constitute a class of biologically active proteins that lack defined tertiary and often secondary structure. The IDP Osteopontin (OPN), a cytokine involved in metastasis of several types of cancer, is shown to simultaneously sample extended, random coil-like conformations and stable, cooperatively folded conformations. By a combination of two magnetic resonance methods, electron paramagnetic resonance and nuclear magnetic resonance spectroscopy, we demonstrate that the OPN ensemble exhibits not only characteristics of an extended and flexible polypeptide, as expected for an IDP, but also simultaneously those of globular proteins, in particular sigmoidal structural denaturation profiles. Both types of states, extended and cooperatively folded, are populated simultaneously by OPN in its apo state. The heterogeneity of the structural properties of IDPs is thus shown to even involve cooperative folding and unfolding events.

Two equivalents of the unsymmetrical Schiff base ligand (L(tBu))(-) (4-tert-butyl phenyl(pyrrolato-2-ylmethylene)amine) and MoCl(2)(NtBu)O(dme) (dme = 1,2-dimethoxyethane) gave a single stereoisomer of a mixed imido/oxido Mo(VI) complex 2(tBu). The stereochemistry of 2(tBu) was elucidated using X-ray diffraction, NMR spectroscopy, and DFT calculations. The complex is active in an oxygen atom transfer (OAT) reaction to trimethyl phosphane. The putative intermediate five-coordinate Mo(IV) imido complex coordinates a PMe(3) ligand, giving the six-coordinate imido phosphane Mo(IV) complex 5(tBu). The stereochemistry of 5(tBu) is different from that of 2(tBu) as shown by NMR spectroscopy, DFT calculations, and X-ray diffraction. Single-electron oxidation of 5(tBu) with ferrocenium hexafluorophosphate gave the stable cationic imido phosphane Mo(V) complex [5(tBu)](+) as the PF(6)(-) salt. EPR spectra of [5(tBu)](PF(6)) confirmed the presence of PMe(3) in the coordination sphere. Single-crystal X-ray diffraction analysis of [5(tBu)](PF(6)) revealed that electron transfer occurred under retention of the stereochemical configuration. The rate of OAT, the outcome of the electron transfer reaction, and the stabilities of the imido complexes presented here differ dramatically from those of analogous oxido complexes.

This article highlights the occurrence and nature of nanoscale inhomogeneities in thermoresponsive polymers and focuses on different experimental techniques for their observation and characterization. Such inhomogeneities can be regarded as nanoscopic domains of collapsed polymer segments (or of a small number of unimers), which provide a nonpolar, hydrophobic interior. Continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy on amphiphilic reporter molecules (spin probes) as an intrinsically local technique is particularly emphasized. In combination with different ensemble-averaging methods, it provides a holistic understanding of the often inhomogeneous nanoscale processes during the temperature-induced collapse of a thermoresponsive polymer.

The effect of several ionic liquids (ILs) on the solution structure of human serum albumin (HSA) is revealed by continuous wave electron paramagnetic resonance (EPR) spectroscopy and nanoscale distance measurements with double electron-electron resonance (DEER) spectroscopy. HSA, the most abundant protein in human blood, is able to bind and transport multiple fatty acids (FAs). Using spin-labeled FA, the uptake of the FA by the protein and their spatial distribution in the protein can be monitored. The FA distribution provides an indirect yet effective way to characterize the structure of the protein in solution. Addition of imidazolium-based ILs to an aqueous solution of HSA/FA conjugates is accompanied by significant destabilization and unfolding of the protein's tertiary structure. In contrast, HSA maintains its tertiary structure when choline dihydrogenphosphate (dhp) is added. The comparison of FA distance distributions in HSA with and without choline dhp surprisingly revealed that with this IL, the FA anchoring units are in better agreement with the crystallographic data. Furthermore, the FA entry point distribution appears widened and more asymmetric than in pure buffer. These results indicate that choline dhp as a cosolvent may selectively stabilize HSA conformations closer to the crystal structure out of the overall conformational ensemble.