Imaging and force measurement of LDL and HDL by AFM in air and liquid

• AFM measured the equivalent diameters of LDL (∼22.7 nm) and HDL (∼14.4 nm) in PBS.
• AFM detected the mean Young’s moduli of LDL (∼0.39 GPa) and HDL (∼0.47 GPa) in PBS.
• AFM detected the mean adhesive forces of LDL (∼0.19 nN) and HDL (∼0.15 nN) in PBS.
• AFM detected much larger sizes of Young’s moduli and adhesive forces of LDL/HDL in air.

The size and biomechanical properties of lipoproteins are tightly correlated with their structures/functions. While atomic force microscopy (AFM) has been used to image lipoproteins the force measurement of these nano-sized particles is missing. We detected that the sizes of LDL and HDL in liquid are close to the commonly known values. The Young’s modulus of LDL or HDL is ∼0.4 GPa which is similar to that of some viral capsids or nanovesicles but greatly larger than that of various liposomes. The adhesive force of LDL or HDL is small (∼200 pN). The comparison of AFM detection in air and liquid was also performed which is currently lacking. Our data may provide useful information for better understanding and AFM detection of lipoproteins.