Nanomechanical characterization of single phospholipid bilayer in ripple phase with PF-QNM AFM.

Topography and nanomechanical properties of the supported 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) single bilayer in the ripple phase were investigated by atomic force microscopy with the use of PeakForce Quantitative Nanomechanical Mapping (PF-QNM) mode under liquid conditions. The DMPC single bilayer was deposited on the mica surface by Langmuir-Blodgett and Langmuir-Schaefer techniques combined. Next, the sample was stored overnight at 4 °C under buffer solution (pH = 7.6) in the presence of tris(hydroxymethyl)aminomethane, (Tris), and then, AFM imaging was performed at 21 °C using the same buffer. The AFM topography images revealed the presence of a periodic structure with an average wavelength of 96.0 ± 7.8 nm and an average amplitude of 0.97 ± 0.03 nm. This macro-ripple structure was asymmetric and formed by long ripples composed of two stripes of different heights. The changes in topography were accompanied by local changes in the Young's modulus, which indicate that the nanomechanical properties of the bilayer are not evenly distributed, namely, the higher stripes are characterized by a lower Young's modulus than that calculated for lower stripes of the ripples. This indicates that the ripple phase is composed of phospholipid molecules of periodically changed orientation and conformation characteristic for fluid-like and gel-like state of DMPC.