Effects of Shear Stress on Antiferromagnetic Interactions within Nitroxide Dimers in Spin-Labeled Polypeptides and Their Conformational Analyses

The relationship between the strong antiferromagnetic interactions within nitroxide dimers and the conformational characteristics of spin-labeled polypeptides (PTPALG) prepared by solvent evaporation from lyotropic cholesteric liquid crystalline (Ch) solutions was investigated by using polarized electron spin resonance and infrared spectroscopy. PTPALG promoted the formation of antiferromagnetic nitroxide dimers through the Ch phase. By applying shear stress to the Ch solution, some of the strongest antiferromagnetic interactions reported for nitroxide-containing organic materials were detected within the nitroxide dimer. This represents the first example of controlling and immobilizing magnetic properties using shear stress. Based on the anisotropy of the g values and the spin susceptibility, it was suggested that the nitroxide moiety formed dimers in the direction perpendicular to the helical axis, indicating the formation of a dimer between the nearest neighboring interchains. This study promotes a superior understanding of charge transport in nonconjugated radical polymers and highlights the importance of macromolecular design.