Thermotropic Changes in the Surface Structure of Lipoprotein B from Human‐Plasma Low‐Density Lipoproteins

The lipoprotein B fraction of human‐plasma low‐density lipoproteins was labeled with (a) spin‐label analogues of stearic acid I(m/n), the N‐oxyl‐4′,4′‐dimethyloxazolidine derivatives of 5‐oxo‐, 12‐oxo‐, and 16‐oxostearic acid, I(12/3), I(5/10), and I(1/14), respectively, and (b) a spin‐label analog of maleimide (3‐maleimido‐2,2,5,5‐tetramethyl‐1‐pyrrolidinyloxyl) bound covalently to the protein moiety. Electron spin resonance spectra were measured in the temperature range from 4 °C to 50 °C. The evaluation of the spectra yielded the following results. The stearic acid spin labels I(m/n) show a reversible transition in their motional freedom at temperatures between 25 and 30 °C. This effect is stronger for labels I(1/14) and I(5/10) than for I(12/3). The polarity of their environment decreases in a sigmoidal way between 15 and 40 °C for labels I(12/3) and I(5/10). No polarity changes are observed with label I(1/14). Below the transition the motional and polarity characteristics of I(m/n) are dominated by lipidprotein interaction, whereas above 30 °C the spectral parameters approach the situation of a model system of phosphatidylcholine‐cholesterol. The protein‐bound spin label shows two spectral components arising from weakly and tightly immobilized binding sites, the ratio of the former to the latter increasing strongly between 25 and 30 °C concomitantly with a decrease in the separation of the two signals. The combined results indicate reversible changes in the fine structure involving both protein and polar lipids at the surface of lipoprotein B which correlate to the thermotropic transition of the apolar lipids in the particle core.