Investigation of α-Synuclein Fibril Structure by Site-directed Spin Labeling

Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source.

A series of linear and branched high-spin di- and tri(cation radical)s (12•2+, 72•2+, 33•3+, 43•3+, and 93•3+) has been prepared by oxidation of the corresponding meta−para aniline oligomers. The oligo(cation radical)s are stable under ambient conditions. The formation and properties of the cation radicals has been studied in detail by cyclic voltammetry and UV−vis−near IR spectroscopy. ESR spectroscopy has provided the zero-field splittings, which are consistent with the topology of the molecules and the localization of the unpaired electrons. Variable temperature ESR measurements reveal that the signal intensity follows Curie's law, consistent with a low-energy high-spin state. The stability of the high-spin meta−para aniline oligomers and the possibility to extend these systems demonstrate that alternating meta and para aniline oligomers are promising building blocks for future polaronic ferromagnets.

/pdf/high-spin-cation-radicals-of-meta-para-aniline-oligomers-47ba6zg12k.pdf

High-Spin Cation Radicals of Meta−Para Aniline Oligomers

The dependence of the electron spin‐exchange energy upon temperature and solvent has been studied for the carbonate, oxalate, and succinate diesters of tetramethyl‐piperidinol‐oxyl An analysis is given of exchange effects on ESR line positions, intensities, and widths For the carbonate diester the exchange energy is constant at low temperatures As the temperature is raised this energy gradually decreases, passes through a minimum, and then increases exponentially Interpretation suggests that at low temperatures exchange occurs through an indirect process involving the core of sigma electrons, whereas at high temperatures a direct exchange process is more important

Spin Exchange in Nitroxide Biradicals

α-Aminoisobutyric acid (Aib) is a Cα-tetrasubstituted amino acid that strongly favors helical structure. Most of the conformational trends established for Aib-rich peptides have been determined by X-ray crystallography. Whether these conformational trends carry over to protic solvents is an open question. In order to develop a general strategy for probing the properties of peptides containing Cα-tetrasubstituted amino acids, the hexameric sequences Boc-TOAC-Alan-TOAC-Ala4-n-OtBu were synthesized where n = 0−3 and TOAC is a spin label Aib analog. The peptides were studied by electron spin resonance (ESR) in four alcohols:  MeOH, EtOH, TFE, and HFIP. Biradical J-coupling and dipolar interactions between the TOACs within each peptide were used to determine peptide geometry as a function of solvent. In MeOH, strong biradical interactions were observed consistent with the geometry of a 310-helix. The solvents displayed differing tendencies to support helical structure with the ranking MeOH > EtOH > TFE > HFIP....

ESR Characterization of Hexameric, Helical Peptides Using Double TOAC Spin Labeling

The factors affecting the electron resonance linewidths of a biradical, showing strong spin exchange, tumbling isotropically in solution are considered. For a biradioal with two equivalent magnetic nuclei the hyperfine lines should alternate in width provided the dominant relaxation mechanism is a modulation of the exchange interaction between the unpaired electrons. Certain nitroxide biradicals do indeed exhibit an alternating linewidth effect which we attribute to this mechanism.

G.R. Luckhurst

Papers

Alternating linewidths. A novel relaxation process in the electron resonance of biradicals