Low density lipoprotein particle size and core cholesteryl ester physical state affect the proton NMR magnetic environment of fatty acid methylene and methyl nuclei.
Recent studies using proton NMR to quantify plasma lipoprotein concentrations have shown that lipoprotein particle size affects the chemical shift of methyl and methylene resonances. The purpose of this study was to investigate the interrelationship of low density lipoprotein (LDL) size and cholesteryl ester (CE) physical state on the chemical shift of methyl and methylene protons. LDL were isolated from the plasma of nonhuman primates fed diets containing lard or fish oil to result in a wide range of LDL particle sizes and CE core transition temperatures for NMR analysis. The proportion of nuclei in the fluid state in LDL at different temperatures, measured as proton NMR peak intensity, paralleled the melting profile of LDL CE determined by differential scanning calorimetry. At 37 degrees C the linewidths of several resolvable fatty acyl resonances, but not the choline methyl resonance, were significantly less in the fish oil LDL, indicating a less restrictive environment along the fatty acyl chain. The (CH2)n and t-CH3 resonances demonstrated an upfield shift (i.e., smaller chemical shift) with increasing temperature for both diet groups, but the peaks for fish oil LDL were always upfield from those of the lard LDL regardless of temperature. No change in the chemical shift of the choline CH3 resonance was observed as a function of dietary fat or temperature. Above the transition temperature for LDL CE there was a significant positive correlation between LDL size and the chemical shift of the (CH2)n and t-CH3 resonances. This relationship was not statistically significant for the same LDL below the Ce transition temperature. We conclude that LDL CE must be fully melted to obtain accurate results for LDL particle size distribution as well as mass quantification using portion NMR spectroscopy.