Squeezing lipids: NMR characterization of lipoprotein particles under pressure

Determining the particle size and number of lipoprotein components found in blood plasma (HDL, LDL and VLDL) has become an important clinical tool in diagnosing risk of cardiovascular disease. Proton (¹H) NMR spectroscopy methods to quantify lipoprotein particle subclasses have been advancing since NMR lineshape analysis of plasma samples was first proposed in the 1990's. NMR methods, including a more recent DOSY-based diffusion spectroscopy test, provide the foundation for the advanced lipoprotein tests, including Lipoprotein® and Liposcale® analyses available for clinical use to determine particle size and number. At the time of this submission, no NMR studies exist which explore physical parameters of individual lipoprotein fractions when they are deformed by pressure. This study reports ¹H NMR frequency shifts and T₂* measurements for the broad methyl peak attributed to terminal methyls (cholesteryl positions 26, 27 and terminal acyl methyl groups) in three primary lipoprotein fractions as a function of hydraulic pressure. This terminal CH₃ resonance shifted linearly upfield as a function of pressure for HDL and VLDL (observed slopes of -0.014 Hz/bar). The LDL terminal CH₃ resonance shows segmented behavior, with a shallow slope between 0-900 bar (-0.008 hz/bar) and a slope similar to HDL and VDL across the range from 1000 to 2400 bar (slope -0.016 Hz/bar). ¹H T₂* values measured for VLDL and HDL dropped linearly with increasing pressure. ¹H T₂* values for LDL demonstrated segmented behavior as a function of pressure. The unique behavior observed for LDL terminal CH₃ frequency and ¹H T₂* trends suggests an approximate pressure at which phase transition occurs.