Study on diffusion of dipalmitoylphosphatidylcholine (DPPC) - cholesterol multilayers using fluorescence correlation spectroscopy

Abstract

DPPC-Cholesterol mixture is one of widely studied model systems for ordered domains in cell mem-brane. It is known to exist in gel phase at low mole fraction of cholesterol and liquid ordered phase at high mole fraction of cholesterol under chain melting temperature of DPPC. Especially, gel to liquid ordered phase transition is considered as an important factor for lipid raft formation. However, low cholesterol fraction re-gion has been poorly studied due to its extremely slow dynamic property. Despite this difficulty, the study of dynamic process for DPPC + low mole fraction of cholesterol mixture would be needed to understand funda-mentals of cell membrane mechanics. Here, we report the diffusion process of DPPC-Cholesterol multilayer as a function of cholesterol mole fraction using fluorescence correlation spectroscopy (FCS). We found that there are two distinct diffusion modes at different cholesterol fraction region. First, fast anomalous diffusion was observed at low cholesterol fraction region (0~5% of cholesterol concentration). Mean square displacement (MSD) analysis also supports that anomalous diffusion is dominant at this region. Specifically, diffusion coefficient tends to increase two times as multilayer contains more cholesterol from 0 to 5%. However, over 7% of cholesterol fraction, slow Brownian diffusion was observed , and this is consistent with previous researches. We propose that boundary lines between DPPC subdomains are the origin of fast anomalous diffusion process at low cholesterol fraction region. Recently, people report that gel phase contains several subdomains induced by chain tilt. Because boundary lines contain disordered structure, Lissamine Rhodamine B-DPPE dyes would be partitioned into boundary line, rather than ordered DPPC subdomains. The fast diffusion may come from the disordered structure of boundary line, and anomalous character is caused by confined movement inside boundary line. In addition, we also correlate the inc...