[Effect of denaturing agents, ionic strength and 2-chloroethanol on the structure of spin-labelled apolipoprotein A-1...

Spin labeled derivatives of apolipoprotein A-1 from high density lipoproteins of human plasma were prepared by the action of 4-(2,4-dichloro-1,3,5-triazin-6-yl)amino- 2,2,6,6-tetramethylpiperidine-1-oxyl at pH 8.3 and 9.8. These preparations contained 2.3 +/- 0.5 and 7.2 +/- 1.0 of paramagnetic radicals per protein molecule, respectively. Their ESR spectra were essentially different, with the spectra of the second preparation indicating the intramolecular spin-spin and dipole-dipole interactions of radicals. In aqueous solutions of the derivatives, ionic strength, organic solvents, or denaturing agents affected the ESR spectra due to changes in the average degree of spin label immobilization and radical interaction. The existence of intramolecular hydrophobic contacts between the amphiphilic regions of the Apo A-1 polypeptide chain in aqueous solutions was thus suggested.