Human serum high-density lipoproteins (HDL) were examined before and after delipidation by the techniques of infrared spectroscopy, by circular dichroism, and by electron spin resonance. Comparisons were made with the techniques under identical conditions of the buffer system, protein concentration, and temperature. In D2O solution the infrared band associated with the amide I resonance of HDL and apoHDL had its major absorption at 1637-1640 cm-1 with shoulders at about 1630 and 1650 cm-1. Circular dichroic spectra were characteristic of a high helical content as previously described by others. The electron in resonance spectrum of HDL, after spin labeling with N-(1-oxy-2,2,6,6-tetra-methylpiperidinyl)maleimide contained signals associated with both weakly (narrow signal) and strongly (broad signal) constrained spin label. The strongly constrained signal was decreased in relative intensity at high temperatures, was increased in relative intensity at low temperatures and was practically abolished by delipidation. The change caused in the electron spin resonance spectrum by delipidation was qualitatively much greater than any detected by infrared or circular dichroism spectroscopy. It was concluded that the changes in the electron spin resonance spectrum consequent upon delipidation were more likely related to change in the local environment of the spin label rather than to large changes in the secondary structure of the molecule. The helical conformation, much of which was to be retained by apoHDL, appeared to exhibit its major infrared band associated with the amide I vibration at a significantly lower frequency than does the helical conformation in other proteins such as myoglobin.
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