Evidence for protein-associated lipids from deuterium nuclear magnetic resonance studies of rhodopsin-dimyristoylphosphatidylcholine recombinants.

The technique of deuterium magnetic resonance was used to study the orientational order of the perdeuterated acyl chains of dimyristoylphosphatidylcholine (DMPC-d54) reconstituted with rhodopsin between 0 and 23 degrees C. This range includes the gel to liquid crystalline phase transition of DMPC-d54 at 20 degrees C. Molar lipid/protein (L/P) ratios of L/P = infinity, 150, 50, 30, and 12 were investigated. Measurements of orientational order parameters showed that the addition of rhodopsin broadened the range of the gel to liquid crystalline transition for L/P = 150 and 50. No transition was observed for L/P = 30 and 12. Moment analysis and spectral subtraction both showed that the low temperature spectra for L/P greater than 30 had two components. One was a pure phospholipid gel phase spectrum and the other a spectrum attributed to lipids in protein aggregates. The intensity of the second component corresponded to 30 lipids/protein and its shape was the same as the temperature-independent shape observed for L/P = 30 and 12. No such decomposition into two components was possible in the liquid crystalline phase for L/P greater than 30. Extraction of the oriented 2H NMR spectrum from its powder spectrum showed that the presence of proteins does not modify the distribution of quadrupole splittings but does produce both homogeneous and inhomogeneous broadening. The latter may be related to the heterogeneity seen in the spectra of electron paramagnetic resonance spin labels above the gel to liquid crystalline transition.