Application of electron spin resonance to the study of the structure of human serum lipoproteins

Human serum high density (HDL) and low density (LDL) lipoproteins were spin labeled with isothiocyanate and maleimide derivatives of the nitroxide radical N-(1-oxyl-2, 2, 6, 6-tetramethyl-4-piperidine). The ESR spectra of the HDL and LDL derivatives demonstrated at least two kinds of protein binding sites involving amino groups, one with slight and another with strong constraint on the tumbling of the nitroxide radical. There was relatively more of the strongly constrained component in labeled HDL than in LDL. The strongly immobilized signal was due to the binding of lipid as was shown by the effect of delipidation.